Mid-ocean ridges

The San Andreas Fault and a large fault off British Columbia are interpreted as examples of the recently proposed "transform faults." They are joined by a short, isolated length of oceanic ridge striking N20 degrees E, with an associated "window" of young crust. The displacement along these faults is estimated at 400 kilometers.

@article{doi101126science1503695482,
    author = "Wilson, J. Tuzo",
    title = "Transform Faults, Oceanic Ridges, and Magnetic Anomalies Southwest of Vancouver Island",
    year = "1965",
    journal = "Science",
    abstract = {The San Andreas Fault and a large fault off British Columbia are interpreted as examples of the recently proposed "transform faults." They are joined by a short, isolated length of oceanic ridge striking N20 degrees E, with an associated "window" of young crust. The displacement along these faults is estimated at 400 kilometers.},
    url = "https://doi.org/10.1126/science.150.3695.482",
    doi = "10.1126/science.150.3695.482",
    openalex = "W2120909089"
}

Stick-slip often accompanies frictional sliding in laboratory experi ments with geologic materials. Shallow focus earthquakes may represent stick slip during sliding along old or newly formed faults in the earth In such a situation, observed stress drops repre sent release of a small fraction of the stress supported by the rock surround ing the earthquake focus.

@article{doi101126science1533739990,
    author = "Brace, W. F. and Byerlee, J. D.",
    title = "Stick-Slip as a Mechanism for Earthquakes",
    year = "1966",
    journal = "Science",
    abstract = "Stick-slip often accompanies frictional sliding in laboratory experi ments with geologic materials. Shallow focus earthquakes may represent stick slip during sliding along old or newly formed faults in the earth In such a situation, observed stress drops repre sent release of a small fraction of the stress supported by the rock surround ing the earthquake focus.",
    url = "https://doi.org/10.1126/science.153.3739.990",
    doi = "10.1126/science.153.3739.990",
    openalex = "W1970664646"
}

It is suggested that the entire history of the ocean basins, in terms of oceanfloor spreading,is contained frozen in the oceanic crust. Variations in the intensity and polarity of Earth's magnetic field are considered to be recorded in the remanent magnetism of the igneous rocks as they solidified and cooled through the Curie temperature at the crest of an oceanic ridge, and subsequently spread away from it at a steady rate. The hypothesis is supported by the extreme linearity and continuity of oceanic magnetic anomalies and their symmetry about the axes of ridges. If the proposed reversal time scale for the last 4 million years is combined with the model, computed anomaly profiles show remarkably good agreement with those observed, and one can deduce rates of spreading for all active parts of the midoceanic ridge system for which magnetic profilesor surveys are available. The rates obtained are in exact agreement with those needed to account for continental drift. An exceptionally high rate of spreading (approximately 4.5 cm/year) in the South Pacific enables one to deduce by extrapolation considerable details of the reversal time scale back to 11.5 million years ago. Again, this scale can be applied to other parts of the ridge system. Thus one isled to the suggestion that the crest of the East Pacific Rise in the northeast Pacific has been overridden and modified by the westward drift of North America, with the production of the anomalous width and unique features of the American cordillera in the western United States. The oceanicmagnetic anomalies also indicate that there was a change in derection of crustal spreading in this region during Pliocene time from eastwest to southeast-northwest. A profile from the crest to the boundary of the East Pacific Rise, and the difference between axial-zone and flank anomalies over ridges, suggest increase in the frequency of reversal of Earth's magnetic field, together, possibly, with decrease in its intensity, approximately 25 million years ago. Within the framework of ocean-floor spreading, it is suggested that magnetic anomaliesmay indicate the nature of oceanic fracture zones and distinguish the parts of the ridge system that are actively spreading. Thus data derived during the past year lend remarkable support to thehypothesis that magnetic anomalies may reveal the history of the ocean basins.

@article{doi101126science15437551405,
    author = "Vine, F. J.",
    title = "Spreading of the Ocean Floor: New Evidence",
    year = "1966",
    journal = "Science",
    abstract = "It is suggested that the entire history of the ocean basins, in terms of oceanfloor spreading,is contained frozen in the oceanic crust. Variations in the intensity and polarity of Earth's magnetic field are considered to be recorded in the remanent magnetism of the igneous rocks as they solidified and cooled through the Curie temperature at the crest of an oceanic ridge, and subsequently spread away from it at a steady rate. The hypothesis is supported by the extreme linearity and continuity of oceanic magnetic anomalies and their symmetry about the axes of ridges. If the proposed reversal time scale for the last 4 million years is combined with the model, computed anomaly profiles show remarkably good agreement with those observed, and one can deduce rates of spreading for all active parts of the midoceanic ridge system for which magnetic profilesor surveys are available. The rates obtained are in exact agreement with those needed to account for continental drift. An exceptionally high rate of spreading (approximately 4.5 cm/year) in the South Pacific enables one to deduce by extrapolation considerable details of the reversal time scale back to 11.5 million years ago. Again, this scale can be applied to other parts of the ridge system. Thus one isled to the suggestion that the crest of the East Pacific Rise in the northeast Pacific has been overridden and modified by the westward drift of North America, with the production of the anomalous width and unique features of the American cordillera in the western United States. The oceanicmagnetic anomalies also indicate that there was a change in derection of crustal spreading in this region during Pliocene time from eastwest to southeast-northwest. A profile from the crest to the boundary of the East Pacific Rise, and the difference between axial-zone and flank anomalies over ridges, suggest increase in the frequency of reversal of Earth's magnetic field, together, possibly, with decrease in its intensity, approximately 25 million years ago. Within the framework of ocean-floor spreading, it is suggested that magnetic anomaliesmay indicate the nature of oceanic fracture zones and distinguish the parts of the ridge system that are actively spreading. Thus data derived during the past year lend remarkable support to thehypothesis that magnetic anomalies may reveal the history of the ocean basins.",
    url = "https://doi.org/10.1126/science.154.3755.1405",
    doi = "10.1126/science.154.3755.1405",
    openalex = "W2014144720",
    references = "doi1010160011747166910783, doi101038199947a0, doi101038201591a0, doi101038207343a0, doi101038207907a0, doi101098rsta19650020, doi101126science14436261537, doi101126science1543747349, doi101126science15437531164, doi101144transglas183559"
}

The mechanisms of 17 earthquakes on the mid-oceanic ridges and their continental extensions were investigated using data from the World-Wide Standardized Seismograph Network of the U. S. Coast and Geodetic Survey and from other long-period seismograph instruments. Mechanism solutions of high precision can now be obtained for a large number of earthquakes with magnitudes as small as 6 in many areas of the world. Less than 1% of the data used in this study are inconsistent with a quadrant distribution of first motions of the phases P and PKP; in many previous investigations 15 to 20% of the data were often inconsistent with the published solutions. Ten of the earthquakes that were studied occurred on fracture zones that intersect the crest of the mid-oceanic ridge. The mechanism of each of the shocks that is located on a fracture zone is characterized by a predominance of strike-slip motion on a steeply dipping plane; the strike of one of the nodal planes for P waves is nearly coincident with the strike of the fracture zone. The sense of strike-slip motion in each of the ten solutions is in agreement with that predicted for transform faults; it is opposite to that expected for a simple offset of the ridge crest along the various fracture zones. The spatial distribution of earthquakes along fracture zones also seems to rule out the hypothesis of simple offset. Two well documented solutions for earthquakes that are located on the mid-Atlantic ridge but that do not appear to be located on fracture zones are characterized by a predominance of normal faulting. The mechanisms of four earthquakes on extensions of the mid-oceanic ridge system—one near northern Siberia and three in East Africa—are also characterized by a predominance of normal faulting. The inferred axes of maximum tension for these six events are approximately perpendicular to the strike of the mid-oceanic ridge system. The results are in agreement with hypotheses of sea-floor growth at the crest of the mid-oceanic ridge system.

@article{doi101029jz072i008p02131,
    author = "Sykes, Lynn R.",
    title = "Mechanism of earthquakes and nature of faulting on the mid-oceanic ridges",
    year = "1967",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "The mechanisms of 17 earthquakes on the mid-oceanic ridges and their continental extensions were investigated using data from the World-Wide Standardized Seismograph Network of the U. S. Coast and Geodetic Survey and from other long-period seismograph instruments. Mechanism solutions of high precision can now be obtained for a large number of earthquakes with magnitudes as small as 6 in many areas of the world. Less than 1\% of the data used in this study are inconsistent with a quadrant distribution of first motions of the phases P and PKP; in many previous investigations 15 to 20\% of the data were often inconsistent with the published solutions. Ten of the earthquakes that were studied occurred on fracture zones that intersect the crest of the mid-oceanic ridge. The mechanism of each of the shocks that is located on a fracture zone is characterized by a predominance of strike-slip motion on a steeply dipping plane; the strike of one of the nodal planes for P waves is nearly coincident with the strike of the fracture zone. The sense of strike-slip motion in each of the ten solutions is in agreement with that predicted for transform faults; it is opposite to that expected for a simple offset of the ridge crest along the various fracture zones. The spatial distribution of earthquakes along fracture zones also seems to rule out the hypothesis of simple offset. Two well documented solutions for earthquakes that are located on the mid-Atlantic ridge but that do not appear to be located on fracture zones are characterized by a predominance of normal faulting. The mechanisms of four earthquakes on extensions of the mid-oceanic ridge system—one near northern Siberia and three in East Africa—are also characterized by a predominance of normal faulting. The inferred axes of maximum tension for these six events are approximately perpendicular to the strike of the mid-oceanic ridge system. The results are in agreement with hypotheses of sea-floor growth at the crest of the mid-oceanic ridge system.",
    url = "https://doi.org/10.1029/jz072i008p02131",
    doi = "10.1029/jz072i008p02131",
    openalex = "W1974493245",
    references = "doi1010160025322764900489, doi1010160040195164900101, doi101038190854a0, doi101038199947a0, doi101038207343a0, doi101126science15437531164, doi101126science15437551405, doi101130petrologic1962599, doi101130spe65p1, doi105408002213687121"
}

@article{sykes1967mechanism,
    author = "Sykes, Lynn R.",
    title = "Mechanism of earthquakes and nature of faulting on the mid-oceanic ridges",
    year = "1967",
    journal = "Journal of Geophysical Research",
    url = "https://doi.org/10.1029/jz072i008p02131",
    doi = "10.1029/jz072i008p02131",
    number = "8",
    openalex = "W1974493245",
    pages = "2131-2153",
    volume = "72",
    references = "doi1010160025322764900489, doi1010160040195164900101, doi101038190854a0, doi101038199947a0, doi101038207343a0, doi101126science15437531164, doi101126science15437551405, doi101130petrologic1962599, doi101130spe65p1, doi101785bssa0350040175, doi105408002213687121"
}

@article{sykes1967mechanism1,
    author = "Sykes, L. R",
    title = "Mechanism of earthquakes and nature of faulting on the mid- oceanic ridges",
    year = "1967",
    journal = "Journal of Geophysical Research, v. 72, p. 2131-2153",
    note = "talkorigins\_source = {true}; raw\_reference = {Sykes, L. R., 1967, Mechanism of earthquakes and nature of faulting on the mid- oceanic ridges: Journal of Geophysical Research, v. 72, p. 2131-2153.}"
}

@article{doi101029jb074i002p00632,
    author = "Banghar, A. R. and Sykes, Lynn R.",
    title = "Focal mechanisms of earthquakes in the Indian Ocean and adjacent regions",
    year = "1969",
    journal = "Journal of Geophysical Research Atmospheres",
    url = "https://doi.org/10.1029/jb074i002p00632",
    doi = "10.1029/jb074i002p00632",
    openalex = "W1987659329"
}

Field and experimental evidence are combined to deduce the mechanism of slip on shallow continental transcurrent faults, such as the San Andreas in California. Several lines of evidence portray the central section of the San Andreas fault as a very smooth and fiat surface, with a very low frictional strength in comparison to the breaking strength of intact rock. The Parkfield earthquake of June 27, 1966, and its aftershock and creep sequences are examined as a detailed example of fault slippage that includes both types, seismic and aseismic. It is shown from considerable number of field data that during the main shock a region from about 4 to 10 km in depth slipped approximately 30 cm. In response to this slippage, creep and aftershocks were generated. The creep and aftershocks are not directly interrelated, but they are microscopically identical processes of time-dependent brittle friction occurring in parallel in different regions. The creep occurred by time-dependent stable frictional sliding in the 4-km-thick surface layer; the aftershocks, by time-dependent stick-slip at the ends of the initial slipped zone. This model is in good agreement with laboratory results which show that slippage should occur by stable (aseismic) friction in the upper 4 km, by stick-slip accompanied by earthquakes from about 4 to 12 km, and by stable sliding or plastic friction below 12 km on the fault. One feature not observed in the laboratory is the episodic nature of creep. These episodes can be predicted with an accuracy of about I week.

@article{doi101029jb074i008p02049,
    author = "Scholz, Christian and Wyss, Max and Smith, Stewart W.",
    title = "Seismic and aseismic slip on the San Andreas Fault",
    year = "1969",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "Field and experimental evidence are combined to deduce the mechanism of slip on shallow continental transcurrent faults, such as the San Andreas in California. Several lines of evidence portray the central section of the San Andreas fault as a very smooth and fiat surface, with a very low frictional strength in comparison to the breaking strength of intact rock. The Parkfield earthquake of June 27, 1966, and its aftershock and creep sequences are examined as a detailed example of fault slippage that includes both types, seismic and aseismic. It is shown from considerable number of field data that during the main shock a region from about 4 to 10 km in depth slipped approximately 30 cm. In response to this slippage, creep and aftershocks were generated. The creep and aftershocks are not directly interrelated, but they are microscopically identical processes of time-dependent brittle friction occurring in parallel in different regions. The creep occurred by time-dependent stable frictional sliding in the 4-km-thick surface layer; the aftershocks, by time-dependent stick-slip at the ends of the initial slipped zone. This model is in good agreement with laboratory results which show that slippage should occur by stable (aseismic) friction in the upper 4 km, by stick-slip accompanied by earthquakes from about 4 to 12 km, and by stable sliding or plastic friction below 12 km on the fault. One feature not observed in the laboratory is the episodic nature of creep. These episodes can be predicted with an accuracy of about I week.",
    url = "https://doi.org/10.1029/jb074i008p02049",
    doi = "10.1029/jb074i008p02049",
    openalex = "W1994518237"
}

Seismic data strongly support recent theories of tectonics in which large plates of lithosphere move coherently with respect to one another as nearly rigid bodies, spreading apart at ocean ridges, sliding past one another at transform faults, and underthrusting at island arcs. Boundaries between adjacent plates of lithosphere are defined by belts of high seismic activity. Redetermination of more than 600 hypocenters in the Middle America region and previous studies in the Galapagos and Caribbean regions define the boundaries of two relatively small, nearly aseismic plates in the region of interest. The first, the Cocos plate, is bordered by the East Pacific rise, the Galapagos rift zone, the north-trending Panama fracture zone near 82° W., and the Middle America arc; the second, the Caribbean plate, underlies the Caribbean Sea and is bounded by the Middle America arc, the Cayman trough, the West Indies arc, and the seismic zone through northern South America. Focal mechanisms of 70 earthquakes in these regions were determined to ascertain the relative motion of these two plates with respect to the surrounding regions or plates. The results show underthrusting of the Cocos plate beneath Mexico and Guatemala in a northeasterly direction and beneath the rest of Central America in a more north-northeasterly direction. The Cocos plate is spreading away from the rest of the Pacific floor at the East Pacific rise and at the Galapagos rift zone. Motion is right-lateral strike-slip along the Panama fracture zone, a transform fault connecting the Galapagos rift zone and the Middle America arc. At the same time, the Caribbean plate is moving easterly with respect to the Americas plate, which is here taken to include both North and South America and the western Atlantic. Left-lateral strike-slip motion along steeply dipping fault planes is observed on the Cayman trough. The Americas plate is underthrusting the Caribbean in a westerly direction at the Lesser Antilles and near Puerto Rico. Unlike the Lesser Antilles, however, motion at present is not perpendicular to the Puerto Rico trench but instead is almost parallel to the trench along nearly horizontal fault planes. Computations of rates of motion indicate that underthrusting is at a higher rate in southeastern Mexico and Guatemala than in western Mexico and that the Caribbean is moving at a lower rate relative to North America than is the Cocos plate.

@article{doi101130001676061969801639totcam20co2,
    author = "Molnár, Péter and Sykes, Lynn R.",
    title = "Tectonics of the Caribbean and Middle America Regions from Focal Mechanisms and Seismicity",
    year = "1969",
    journal = "Geological Society of America Bulletin",
    abstract = "Seismic data strongly support recent theories of tectonics in which large plates of lithosphere move coherently with respect to one another as nearly rigid bodies, spreading apart at ocean ridges, sliding past one another at transform faults, and underthrusting at island arcs. Boundaries between adjacent plates of lithosphere are defined by belts of high seismic activity. Redetermination of more than 600 hypocenters in the Middle America region and previous studies in the Galapagos and Caribbean regions define the boundaries of two relatively small, nearly aseismic plates in the region of interest. The first, the Cocos plate, is bordered by the East Pacific rise, the Galapagos rift zone, the north-trending Panama fracture zone near 82° W., and the Middle America arc; the second, the Caribbean plate, underlies the Caribbean Sea and is bounded by the Middle America arc, the Cayman trough, the West Indies arc, and the seismic zone through northern South America. Focal mechanisms of 70 earthquakes in these regions were determined to ascertain the relative motion of these two plates with respect to the surrounding regions or plates. The results show underthrusting of the Cocos plate beneath Mexico and Guatemala in a northeasterly direction and beneath the rest of Central America in a more north-northeasterly direction. The Cocos plate is spreading away from the rest of the Pacific floor at the East Pacific rise and at the Galapagos rift zone. Motion is right-lateral strike-slip along the Panama fracture zone, a transform fault connecting the Galapagos rift zone and the Middle America arc. At the same time, the Caribbean plate is moving easterly with respect to the Americas plate, which is here taken to include both North and South America and the western Atlantic. Left-lateral strike-slip motion along steeply dipping fault planes is observed on the Cayman trough. The Americas plate is underthrusting the Caribbean in a westerly direction at the Lesser Antilles and near Puerto Rico. Unlike the Lesser Antilles, however, motion at present is not perpendicular to the Puerto Rico trench but instead is almost parallel to the trench along nearly horizontal fault planes. Computations of rates of motion indicate that underthrusting is at a higher rate in southeastern Mexico and Guatemala than in western Mexico and that the Caribbean is moving at a lower rate relative to North America than is the Cocos plate.",
    url = "https://doi.org/10.1130/0016-7606(1969)80[1639:totcam]2.0.co;2",
    doi = "10.1130/0016-7606(1969)80[1639:totcam]2.0.co;2",
    openalex = "W1991156767"
}

The epicenters of about 900 earthquakes in the Indian Ocean, Africa, and adjacent areas that occurred from 1950 to 1966 were relocated by computer. These epicenters delineate many transform faults, ridge crests, and triple junctions much more precisely than was done previously. A series of great NNW-striking transform faults south of Tasmania successively offset the mid-ocean ridge a total of about 1500 km. The distribution of epicenters and of two focal mechanism solutions for the southwest brance of the mid-Indian ridge are indicative of sea-floor spreading with ridge crests oriented approximately WNW and transform faults NNE. Previous reports of sediment thicknesses near this branch of the ridge are in good accord with the inferred pattern of spreading ridge crests and transform faults. Likewise, failures to detect measurable sea-floor spreading along this branch can be ascribed to the unfortunate orientation of the few published profiles nearly parallel to ridge crests. An incipient tectonic feature, possibly a nascent island arc, may be associated with a seismic zone in the northeast Indian Ocean between Ceylon and Australia. The large width of the zone of shallow shocks, the relative abundance of earthquakes larger than magnitude 7, and the absence of observed topographic features associated with the seismic zone are indicative of a nascent tectonic feature that may be related to a decrease or change in the relative motion between the Indian-Australian plate and-the Eurasian plate, possibly as a result of continental collision. Other interpretations of the tectonics of this unusual zone are also discussed. Regardless of its tectonic significance, this zone deserves further study since it is the most seismically active region in the oceans that has not been identified as either a ridge, an arc, or a transform fault. The results of the relocations and other pertinent data are given in a separate appendix, which is available on microfiche along with the entire article. Order from the American Geophysical Union, Suite 435, 2100 Pennsylvania Ave., N.W., Washington, D.C. 20037. Document J 70-003; $1.00. Payment must accompany order.

@article{doi101029jb075i026p05041,
    author = "Sykes, Lynn R.",
    title = "Seismicity of the Indian Ocean and a possible nascent island arc between Ceylon and Australia",
    year = "1970",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "The epicenters of about 900 earthquakes in the Indian Ocean, Africa, and adjacent areas that occurred from 1950 to 1966 were relocated by computer. These epicenters delineate many transform faults, ridge crests, and triple junctions much more precisely than was done previously. A series of great NNW-striking transform faults south of Tasmania successively offset the mid-ocean ridge a total of about 1500 km. The distribution of epicenters and of two focal mechanism solutions for the southwest brance of the mid-Indian ridge are indicative of sea-floor spreading with ridge crests oriented approximately WNW and transform faults NNE. Previous reports of sediment thicknesses near this branch of the ridge are in good accord with the inferred pattern of spreading ridge crests and transform faults. Likewise, failures to detect measurable sea-floor spreading along this branch can be ascribed to the unfortunate orientation of the few published profiles nearly parallel to ridge crests. An incipient tectonic feature, possibly a nascent island arc, may be associated with a seismic zone in the northeast Indian Ocean between Ceylon and Australia. The large width of the zone of shallow shocks, the relative abundance of earthquakes larger than magnitude 7, and the absence of observed topographic features associated with the seismic zone are indicative of a nascent tectonic feature that may be related to a decrease or change in the relative motion between the Indian-Australian plate and-the Eurasian plate, possibly as a result of continental collision. Other interpretations of the tectonics of this unusual zone are also discussed. Regardless of its tectonic significance, this zone deserves further study since it is the most seismically active region in the oceans that has not been identified as either a ridge, an arc, or a transform fault. The results of the relocations and other pertinent data are given in a separate appendix, which is available on microfiche along with the entire article. Order from the American Geophysical Union, Suite 435, 2100 Pennsylvania Ave., N.W., Washington, D.C. 20037. Document J 70-003; $1.00. Payment must accompany order.",
    url = "https://doi.org/10.1029/jb075i026p05041",
    doi = "10.1029/jb075i026p05041",
    openalex = "W2134766173"
}

A region‐by‐region analysis of 204 reliable focal‐mechanism solutions for deep and intermediate‐depth earthquakes strongly supports the idea that portions of the lithosphere that descend into the mantle are slablike stress guides that align the earthquake‐generating stresses parallel to the inclined seismic zones. At intermediate depths extensional stresses parallel to the dip of the zone are predominant in zones characterized either by gaps in the seismicity as a function of depth or by an absence of deep earthquakes. Compressional stresses parallel to the dip of the zone are prevalent everywhere the zone exists below about 300 km. These results indicate that the lithosphere sinks into the asthenosphere under its own weight but encounters resistance to its downward motion below about 300 km. Additional results indicate contortions and disruptions of the descending slabs; however, stresses attributable to simple bending of the plates do not seem to be important in the generation of subcrustal earthquakes. This summary, intended to be comprehensive, includes nearly all solutions obtainable from the World‐Wide Standardized Seismograph Network (WWSSN) for the period 1962 through part of 1968 plus a selection of reliable solutions of pre‐1962 events, and it includes data from nearly every region in the world where earthquakes occur in the mantle. The double‐couple or shear dislocation model of the source mechanism is adequate for all the data.

@article{doi101029rg009i001p00103,
    author = "Isacks, Bryan L. and Molnár, Péter",
    title = "Distribution of stresses in the descending lithosphere from a global survey of focal‐mechanism solutions of mantle earthquakes",
    year = "1971",
    journal = "Reviews of Geophysics",
    abstract = "A region‐by‐region analysis of 204 reliable focal‐mechanism solutions for deep and intermediate‐depth earthquakes strongly supports the idea that portions of the lithosphere that descend into the mantle are slablike stress guides that align the earthquake‐generating stresses parallel to the inclined seismic zones. At intermediate depths extensional stresses parallel to the dip of the zone are predominant in zones characterized either by gaps in the seismicity as a function of depth or by an absence of deep earthquakes. Compressional stresses parallel to the dip of the zone are prevalent everywhere the zone exists below about 300 km. These results indicate that the lithosphere sinks into the asthenosphere under its own weight but encounters resistance to its downward motion below about 300 km. Additional results indicate contortions and disruptions of the descending slabs; however, stresses attributable to simple bending of the plates do not seem to be important in the generation of subcrustal earthquakes. This summary, intended to be comprehensive, includes nearly all solutions obtainable from the World‐Wide Standardized Seismograph Network (WWSSN) for the period 1962 through part of 1968 plus a selection of reliable solutions of pre‐1962 events, and it includes data from nearly every region in the world where earthquakes occur in the mantle. The double‐couple or shear dislocation model of the source mechanism is adequate for all the data.",
    url = "https://doi.org/10.1029/rg009i001p00103",
    doi = "10.1029/rg009i001p00103",
    openalex = "W2127454332",
    references = "doi101029jb073i006p01959, doi101029jb073i012p03661, doi101029jb073i018p05855, doi101029jb073i022p07089, doi101029jz070i016p03965, doi1010382161276a0, doi101038224125a0, doi101038226239a0, doi101111j1365246x1969tb00259x, doi101130001676061969801639totcam20co2, doi101785bssa0590010369, doi1023071790758, doi102307211302, openalexw623436458"
}

This chapter contains sections titled: Introduction Previous Mechanism Solutions for Earthquakes on the Mid-Oceanic Ridges Analysis of Data Presentation of Data for the Mid-Oceanic Ridges Extensions of the Mid-Oceanic Ridges East Pacific Rise Comparison of Inferred Mechanisms with those Deduced by Other Investigators Conclusions and Discussion References

@misc{doi1010029781118782149ch1,
    author = "Sykes, Lynn R.",
    title = "Mechanism of Earthquakes and Nature of Faulting on the Mid‐Oceanic Ridges",
    year = "1972",
    booktitle = "Collected reprint series",
    abstract = "This chapter contains sections titled: Introduction Previous Mechanism Solutions for Earthquakes on the Mid-Oceanic Ridges Analysis of Data Presentation of Data for the Mid-Oceanic Ridges Extensions of the Mid-Oceanic Ridges East Pacific Rise Comparison of Inferred Mechanisms with those Deduced by Other Investigators Conclusions and Discussion References",
    url = "https://doi.org/10.1002/9781118782149.ch1",
    doi = "10.1002/9781118782149.ch1",
    openalex = "W4249400877",
    references = "doi1010160025322764900489, doi1010160040195164900101, doi101038190854a0, doi101038199947a0, doi101038207343a0, doi101126science15437531164, doi101126science15437551405, doi101130spe65p1, doi101785bssa0350040175, doi105408002213687121"
}

@misc{sykes1972mechanism,
    author = "Sykes, Lynn R.",
    title = "Mechanism of Earthquakes and Nature of Faulting on the Mid‐Oceanic Ridges",
    year = "1972",
    booktitle = "Collected Reprint Series",
    url = "https://doi.org/10.1002/9781118782149.ch1",
    doi = "10.1002/9781118782149.ch1",
    openalex = "W4249400877",
    pages = "2131-2153",
    references = "doi1010160025322764900489, doi1010160040195164900101, doi101038190854a0, doi101038199947a0, doi101038207343a0, doi101126science15437531164, doi101126science15437551405, doi101130spe65p1, doi101785bssa0350040175, doi105408002213687121"
}

Physical factors likely to affect the genesis of the various fault rocks—frictional properties, temperature, effective stress normal to the fault and differential stress—are examined in relation to the energy budget of fault zones, the main velocity modes of faulting and the type of faulting, whether thrust, wrench, or normal. In a conceptual model of a major fault zone cutting crystalline quartzo-feldspathic crust, a zone of elastico-frictional (EF) behaviour generating random-fabric fault rocks (gouge—breccia—cataclasite series—pseudotachylyte) overlies a region where quasi-plastic (QP) processes of rock deformation operate in ductile shear zones with the production of mylonite series rocks possessing strong tectonite fabrics. In some cases, fault rocks developed by transient seismic faulting can be distinguished from those generated by slow aseismic shear. Random-fabric fault rocks may form as a result of seismic faulting within the ductile shear zones from time to time, but tend to be obliterated by continued shearing. Resistance to shear within the fault zone reaches a peak value (greatest for thrusts and least for normal faults) around the EF/QP transition level, which for normal geothermal gradients and an adequate supply of water, occurs at depths of 10–15 km.

@article{doi101144gsjgs13330191,
    author = "Sibson, Richard H.",
    title = "Fault rocks and fault mechanisms",
    year = "1977",
    journal = "Journal of the Geological Society",
    abstract = "Physical factors likely to affect the genesis of the various fault rocks—frictional properties, temperature, effective stress normal to the fault and differential stress—are examined in relation to the energy budget of fault zones, the main velocity modes of faulting and the type of faulting, whether thrust, wrench, or normal. In a conceptual model of a major fault zone cutting crystalline quartzo-feldspathic crust, a zone of elastico-frictional (EF) behaviour generating random-fabric fault rocks (gouge—breccia—cataclasite series—pseudotachylyte) overlies a region where quasi-plastic (QP) processes of rock deformation operate in ductile shear zones with the production of mylonite series rocks possessing strong tectonite fabrics. In some cases, fault rocks developed by transient seismic faulting can be distinguished from those generated by slow aseismic shear. Random-fabric fault rocks may form as a result of seismic faulting within the ductile shear zones from time to time, but tend to be obliterated by continued shearing. Resistance to shear within the fault zone reaches a peak value (greatest for thrusts and least for normal faults) around the EF/QP transition level, which for normal geothermal gradients and an adequate supply of water, occurs at depths of 10–15 km.",
    url = "https://doi.org/10.1144/gsjgs.133.3.0191",
    doi = "10.1144/gsjgs.133.3.0191",
    openalex = "W2155128667",
    references = "doi1010160040195164900101, doi101098rsta19760079, doi101111j1365246x1967tb06218x, doi101144transed83387, doi105408002213687121"
}

Closely spaced heat flow surveys at four sites on the flanks of the Central Indian Ridge and the Southeast Indian Ridge delineate a pattern of oscillatory heat flow which can only result from cellular convection of oceanic bottom water through the oceanic crust and overlying sediment. These cells have a wavelength of 5 to 10 kilometers and are presently active in sea floor 18 x 10(6), 25 x 10(6), and 45 x 10(6) years old of the Crozet Basin and in sea floor 55 x 10(6) years old of the Madagascar Basin. The precise measurement of nonlinear temperature profiles makes it possible to calculate the conductive and convective heat transfer components through the sea floor. Even in the oldest sites, geothermal convection is still a major component of heat transfer through both the crust and sedimentary layers. These observations coupled with the results of earlier oceanwide geothermal studies indicate that more than one-third of the entire surface area of the world's ocean floor contains presently active geothermal convection that is cellular in plan form.

@article{doi101126science2044395828,
    author = "Anderson, Roger N. and Hobart, Michael A. and Langseth, Marcus G.",
    title = "Geothermal Convection Through Oceanic Crust and Sediments in the Indian Ocean",
    year = "1979",
    journal = "Science",
    abstract = "Closely spaced heat flow surveys at four sites on the flanks of the Central Indian Ridge and the Southeast Indian Ridge delineate a pattern of oscillatory heat flow which can only result from cellular convection of oceanic bottom water through the oceanic crust and overlying sediment. These cells have a wavelength of 5 to 10 kilometers and are presently active in sea floor 18 x 10(6), 25 x 10(6), and 45 x 10(6) years old of the Crozet Basin and in sea floor 55 x 10(6) years old of the Madagascar Basin. The precise measurement of nonlinear temperature profiles makes it possible to calculate the conductive and convective heat transfer components through the sea floor. Even in the oldest sites, geothermal convection is still a major component of heat transfer through both the crust and sedimentary layers. These observations coupled with the results of earlier oceanwide geothermal studies indicate that more than one-third of the entire surface area of the world's ocean floor contains presently active geothermal convection that is cellular in plan form.",
    url = "https://doi.org/10.1126/science.204.4395.828",
    doi = "10.1126/science.204.4395.828",
    openalex = "W2038213979"
}

Focal mechanisms of intraplate earthquakes provide the only means at present by which to characterize the long‐wavelength tectonic stress field in oceanic lithosphere. Stress orientations inferred from focal mechanisms may not accurately reflect the state of stress in the epicentral area, however, or the measured stresses may be dominated by local rather than regional sources. To establish a data set with which to study these possibilities, a comprehensive catalog of 159 oceanic intraplate earthquakes has been compiled for events since 1963 with m b 4.7 or larger. Focal mechanisms are available for approximately one quarter of the events, and several new mechanisms are presented here. For a representative subset of this catalog (83 events), the bathymetry and tectonic history of the epicentral areas have been assembled, and the earthquakes have been rated according to their association with (1) a preexisting fault zone, which might decouple the P axis of the focal mechanism from the true orientation of maximum compressive stress, and (2) large bathymetric relief, which might be a source of large local stresses. Oceanic intraplate earthquakes are commonly found in association with zones of previous weakness (usually fracture zones), but they do not show any particular association with large bathymetric features. In the central Indian Ocean there are enough focal mechanisms available to establish a well‐defined NW‐SE orientation for P axes and presumably for the direction of greatest compressive stress. The consistency of the P axes of these widely varying mechanisms in the presence of the Ninetyeast Ridge, a site of major intraplate deformation and large bathymetric relief, is remarkable. A possible explanation is that in the presence of a large number of preexisting faults with a range of orientations, slip occurs on those faults which have large resolved shear stresses from the regional stress field. In such an instance the P axis of focal mechanisms will tend to show a consistent alignment with the true direction of maximum stress.

@article{doi101029jb085ib10p05389,
    author = "Bergman, Eric and Solomon, Sean C.",
    title = "Oceanic intraplate earthquakes: Implications for local and regional intraplate stress",
    year = "1980",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "Focal mechanisms of intraplate earthquakes provide the only means at present by which to characterize the long‐wavelength tectonic stress field in oceanic lithosphere. Stress orientations inferred from focal mechanisms may not accurately reflect the state of stress in the epicentral area, however, or the measured stresses may be dominated by local rather than regional sources. To establish a data set with which to study these possibilities, a comprehensive catalog of 159 oceanic intraplate earthquakes has been compiled for events since 1963 with m b 4.7 or larger. Focal mechanisms are available for approximately one quarter of the events, and several new mechanisms are presented here. For a representative subset of this catalog (83 events), the bathymetry and tectonic history of the epicentral areas have been assembled, and the earthquakes have been rated according to their association with (1) a preexisting fault zone, which might decouple the P axis of the focal mechanism from the true orientation of maximum compressive stress, and (2) large bathymetric relief, which might be a source of large local stresses. Oceanic intraplate earthquakes are commonly found in association with zones of previous weakness (usually fracture zones), but they do not show any particular association with large bathymetric features. In the central Indian Ocean there are enough focal mechanisms available to establish a well‐defined NW‐SE orientation for P axes and presumably for the direction of greatest compressive stress. The consistency of the P axes of these widely varying mechanisms in the presence of the Ninetyeast Ridge, a site of major intraplate deformation and large bathymetric relief, is remarkable. A possible explanation is that in the presence of a large number of preexisting faults with a range of orientations, slip occurs on those faults which have large resolved shear stresses from the regional stress field. In such an instance the P axis of focal mechanisms will tend to show a consistent alignment with the true direction of maximum stress.",
    url = "https://doi.org/10.1029/jb085ib10p05389",
    doi = "10.1029/jb085ib10p05389",
    openalex = "W1999221527"
}

The principal objective of this paper is to present a simple and self‐consistent review of the basic physical processes controlling heat loss from the earth. To accomplish this objective, we give a short summary of the oceanic and continental data and compare and contrast the respective mechanisms of heat loss. In the oceans we concentrate on the effect of hydrothermal circulation, and on the continents we consider in some detail a model relating surface heat flow to varying depth scales for the distribution of potassium, thorium, and uranium. From this comparison we conclude that the range in possible geotherms at depths below 100 to 150 km under continents and oceans overlaps and that the thermal structure beneath an old stable continent is indistinguishable from that beneath an ocean were it at equilibrium. Oceans and continents are part of the same thermal system. Both have an upper rigid mechanical layer where heat loss is by conduction and a lower thermal boundary layer where convection is dominant. The simple conductive definition of the plate thickness is an oversimplification. The observed distribution of area versus age in the ocean allows us to investigate the dominant mechanism of heat loss which is plate creation. This distribution and an understanding of the heat flow through oceans and continents can be used to calculate the heat loss of the earth. This heat loss is 10 13 cal/s (4.2 × 10 13 W) of which more than 60% results from the creation of oceanic plate. The relation between area and age of the oceans is coupled to the ridge and subducting slab forces that contribute to the driving mechanism for plate motions. These forces are self‐regulating and maintain the rate of plate generation required to achieve a balance between heat loss and heat generation.

@article{doi101029jb086ib12p11535,
    author = "Sclater, John G. and Parsons, B. and Jaupart, Claude",
    title = "Oceans and continents: Similarities and differences in the mechanisms of heat loss",
    year = "1981",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "The principal objective of this paper is to present a simple and self‐consistent review of the basic physical processes controlling heat loss from the earth. To accomplish this objective, we give a short summary of the oceanic and continental data and compare and contrast the respective mechanisms of heat loss. In the oceans we concentrate on the effect of hydrothermal circulation, and on the continents we consider in some detail a model relating surface heat flow to varying depth scales for the distribution of potassium, thorium, and uranium. From this comparison we conclude that the range in possible geotherms at depths below 100 to 150 km under continents and oceans overlaps and that the thermal structure beneath an old stable continent is indistinguishable from that beneath an ocean were it at equilibrium. Oceans and continents are part of the same thermal system. Both have an upper rigid mechanical layer where heat loss is by conduction and a lower thermal boundary layer where convection is dominant. The simple conductive definition of the plate thickness is an oversimplification. The observed distribution of area versus age in the ocean allows us to investigate the dominant mechanism of heat loss which is plate creation. This distribution and an understanding of the heat flow through oceans and continents can be used to calculate the heat loss of the earth. This heat loss is 10 13 cal/s (4.2 × 10 13 W) of which more than 60\% results from the creation of oceanic plate. The relation between area and age of the oceans is coupled to the ridge and subducting slab forces that contribute to the driving mechanism for plate motions. These forces are self‐regulating and maintain the rate of plate generation required to achieve a balance between heat loss and heat generation.",
    url = "https://doi.org/10.1029/jb086ib12p11535",
    doi = "10.1029/jb086ib12p11535",
    openalex = "W2018021437",
    references = "doi101098rsta19680031"
}

A first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics. As we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth's surface, it is becoming increasingly necessary to study the tectonic, volcanic, and hydro­ thermal processes within the spreading center plate boundary zone. All ocean­ ic crust bears the imprint of these processes. This review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid-ocean ridges with reference to associated volcanic and hydrothermal processes. It draws heavily on recent studies that use deeply towed instrument packages, multi-beam bathymetric mapping, ocean bottom instruments, and ALVIN (e.g. the Famous, AMAR, RISE, and Galapagos expeditions). We begin with a review of the large-scale structure of spreading centers. We then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones. Next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity, as well as the generation of magnetic anomaly stripes and their implications for crustal generation. One of our findings is that the initial two-dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

@article{doi101146annurevea10050182001103,
    author = "Macdonald, Ken C.",
    title = "Mid-Ocean Ridges: Fine Scale Tectonic, Volcanic and Hydrothermal Processes Within the Plate Boundary Zone",
    year = "1982",
    journal = "Annual Review of Earth and Planetary Sciences",
    abstract = "A first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics. As we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth's surface, it is becoming increasingly necessary to study the tectonic, volcanic, and hydro­ thermal processes within the spreading center plate boundary zone. All ocean­ ic crust bears the imprint of these processes. This review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid-ocean ridges with reference to associated volcanic and hydrothermal processes. It draws heavily on recent studies that use deeply towed instrument packages, multi-beam bathymetric mapping, ocean bottom instruments, and ALVIN (e.g. the Famous, AMAR, RISE, and Galapagos expeditions). We begin with a review of the large-scale structure of spreading centers. We then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones. Next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity, as well as the generation of magnetic anomaly stripes and their implications for crustal generation. One of our findings is that the initial two-dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations",
    url = "https://doi.org/10.1146/annurev.ea.10.050182.001103",
    doi = "10.1146/annurev.ea.10.050182.001103",
    openalex = "W2068641796",
    references = "doi101007bf00285656, doi1010160012821x80901636, doi1010160012821x81900418, doi101029jb081i014p02490, doi101029jb084ib10p05407, doi101029rg013i001p00057, doi101111j1365246x1970tb06087x, doi101126science20343851073, doi10113000167606197788507matoti20co2, sykes1967mechanism"
}

abstract Models of fault zones in continental crust, based on the analysis of rock deformation textures, suggest that the depth of seismic activity is controlled by the passage from a pressure-sensitive, dominantly frictional regime to strongly temperature-dependent, quasi-plastic mylonitization at greenschist and higher grades of metamorphism. Sufficient knowledge now exists concerning the frictional and rheological properties of quartz-bearing rocks to construct crude strength-depth curves for different geotherms. In such models, shear resistance peaks sharply at the inferred seismic-aseismic transition. The maximum depth of microseismic activity in various heat flow provinces of the conterminous United States generally correlates well with the frictional to quasi-plastic transition modeled for the different geotherms. Larger earthquakes (M L > 5.5) also tend to nucleate near the base of the seismogenic zone. This region is postulated to have the highest concentration of distortional strain energy for stress levels at failure, and can be regarded as the prime asperity in crustal fault zones.

@article{doi101785bssa0720010151,
    author = "Sibson, Richard H.",
    title = "Fault zone models, heat flow, and the depth distribution of earthquakes in the continental crust of the United States",
    year = "1982",
    journal = "Bulletin of the Seismological Society of America",
    abstract = "abstract Models of fault zones in continental crust, based on the analysis of rock deformation textures, suggest that the depth of seismic activity is controlled by the passage from a pressure-sensitive, dominantly frictional regime to strongly temperature-dependent, quasi-plastic mylonitization at greenschist and higher grades of metamorphism. Sufficient knowledge now exists concerning the frictional and rheological properties of quartz-bearing rocks to construct crude strength-depth curves for different geotherms. In such models, shear resistance peaks sharply at the inferred seismic-aseismic transition. The maximum depth of microseismic activity in various heat flow provinces of the conterminous United States generally correlates well with the frictional to quasi-plastic transition modeled for the different geotherms. Larger earthquakes (M L > 5.5) also tend to nucleate near the base of the seismogenic zone. This region is postulated to have the highest concentration of distortional strain energy for stress levels at failure, and can be regarded as the prime asperity in crustal fault zones.",
    url = "https://doi.org/10.1785/bssa0720010151",
    doi = "10.1785/bssa0720010151",
    openalex = "W2309907070"
}

Paleoseismological data for the Wasatch and San Andreas fault zones have led to the formulation of the characteristic earthquake model, which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum. Analysis of scarp‐derived colluvium in trench exposures across the Wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes. At all of the sites studied, the displacement per event has been consistently large; measured values range from 1.6 to 2.6 m, and the average is about 2 m. On the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry, six major segments are recognized along the Wasatch fault. On the basis of the most likely number of surface faulting events (18) that have occurred on segments of the Wasatch fault zone during the past 8000 years, an average recurrence interval of 400–666 years with a preferred average of 444 years is calculated for the entire zone. Geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south‐central segment of the San Andreas fault suggest that the M 8 1857 earthquake is a characteristic earthquake for this segment. Comparisons of earthquake recurrence relationships on both the Wasatch and San Andreas faults based on historical seismicity data and geologic data show that a linear (constant b value) extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes. Only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes. The characteristic earthquake appears to be a fundamental aspect of the behavior of the Wasatch and San Andreas faults and may apply to many other faults as well.

@article{doi101029jb089ib07p05681,
    author = "Schwartz, David P. and Coppersmith, Kevin J.",
    title = "Fault behavior and characteristic earthquakes: Examples from the Wasatch and San Andreas Fault Zones",
    year = "1984",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "Paleoseismological data for the Wasatch and San Andreas fault zones have led to the formulation of the characteristic earthquake model, which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum. Analysis of scarp‐derived colluvium in trench exposures across the Wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes. At all of the sites studied, the displacement per event has been consistently large; measured values range from 1.6 to 2.6 m, and the average is about 2 m. On the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry, six major segments are recognized along the Wasatch fault. On the basis of the most likely number of surface faulting events (18) that have occurred on segments of the Wasatch fault zone during the past 8000 years, an average recurrence interval of 400–666 years with a preferred average of 444 years is calculated for the entire zone. Geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south‐central segment of the San Andreas fault suggest that the M 8 1857 earthquake is a characteristic earthquake for this segment. Comparisons of earthquake recurrence relationships on both the Wasatch and San Andreas faults based on historical seismicity data and geologic data show that a linear (constant b value) extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes. Only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes. The characteristic earthquake appears to be a fundamental aspect of the behavior of the Wasatch and San Andreas faults and may apply to many other faults as well.",
    url = "https://doi.org/10.1029/jb089ib07p05681",
    doi = "10.1029/jb089ib07p05681",
    openalex = "W2079238116"
}

We report hypocenters and focal mechanisms of microearthquakes located by an ocean bottom seismic network deployed in the median valley of the Mid‐Atlantic Ridge near 23°N during a 3 week period in early 1982. The network consisted of seven ocean‐bottom hydrophones and three three‐component ocean bottom seismometers. The instrument coordinates were acoustically determined to within 25 m at the 1 standard deviation level of confidence. The hypocentral parameters of the 26 largest microearthquakes are reported; 18 of these events have epicenters arid focal depths which are resolvable to within ±1 km formal error at the 95% confidence level. Microearthquakes occur beneath the inner floor of the median valley and have focal depths generally between 5 and 8 km beneath the seafloor. Composite fault plane solutions for two spatially related groups of microearthquakes beneath the inner floor indicate normal faulting along fault planes that dip at angles of 30° or more; these solutions are similar to the mechanisms of nearby large earthquakes. Microearthquakes also occur beneath the steep eastern inner rift mountains. The rift mountain earthquakes have nominal focal depths of 5–7 km and epicenters as distant as 10–15 km from the center of the median valley, but these hypocenters have larger uncertainties because of the possible effects of large topographic relief and associated lateral heterogeneity in velocity structure. We interpret the depth distribution and source mechanisms of these microearthquakes to indicate that this segment of ridge axis is undergoing brittle failure under extension to a depth of at least 7–8 km. We infer that the entire crustal column has been cooled to temperatures within the brittle field of behavior and that significant time has elapsed since the most recent episode of volcanic or shallow magmatic activity along this ridge segment.

@article{doi101029jb090ib07p05443,
    author = "Toomey, D. R. and Solomon, Sean C. and Purdy, G. M. and Murray, M. H.",
    title = "Microearthquakes beneath the Median Valley of the Mid‐Atlantic Ridge near 23°N: Hypocenters and focal mechanisms",
    year = "1985",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "We report hypocenters and focal mechanisms of microearthquakes located by an ocean bottom seismic network deployed in the median valley of the Mid‐Atlantic Ridge near 23°N during a 3 week period in early 1982. The network consisted of seven ocean‐bottom hydrophones and three three‐component ocean bottom seismometers. The instrument coordinates were acoustically determined to within 25 m at the 1 standard deviation level of confidence. The hypocentral parameters of the 26 largest microearthquakes are reported; 18 of these events have epicenters arid focal depths which are resolvable to within ±1 km formal error at the 95\% confidence level. Microearthquakes occur beneath the inner floor of the median valley and have focal depths generally between 5 and 8 km beneath the seafloor. Composite fault plane solutions for two spatially related groups of microearthquakes beneath the inner floor indicate normal faulting along fault planes that dip at angles of 30° or more; these solutions are similar to the mechanisms of nearby large earthquakes. Microearthquakes also occur beneath the steep eastern inner rift mountains. The rift mountain earthquakes have nominal focal depths of 5–7 km and epicenters as distant as 10–15 km from the center of the median valley, but these hypocenters have larger uncertainties because of the possible effects of large topographic relief and associated lateral heterogeneity in velocity structure. We interpret the depth distribution and source mechanisms of these microearthquakes to indicate that this segment of ridge axis is undergoing brittle failure under extension to a depth of at least 7–8 km. We infer that the entire crustal column has been cooled to temperatures within the brittle field of behavior and that significant time has elapsed since the most recent episode of volcanic or shallow magmatic activity along this ridge segment.",
    url = "https://doi.org/10.1029/jb090ib07p05443",
    doi = "10.1029/jb090ib07p05443",
    openalex = "W2118300017"
}

We have determined the source mechanisms (double‐couple orientation, moment, centroid depth, source time function) of 14 earthquakes on the northern Mid‐Atlantic Ridge (0°–72°N) from an inversion of long‐period P and SH waveforms. The earthquakes are all characterized by nearly pure normal faulting on fault planes that dip at about 45° and strike parallel to the local trend of the ridge axis. Moments range from 3 to 15×10 24 dyn cm, and the source time functions are all of simple form. The P and S waveforms for all earthquakes can be well matched using conventional values for t * (1 and 4 s, respectively). These earthquakes are all very shallow; centroid depths range between 1.2 and 3.1 km beneath the seafloor. The P waves from these earthquakes show strong water column reverberations, suggesting that fault rupture extended to the seafloor. The predominant period of these reverberations constrains the water depth in the epicentral region. On the basis of estimated water depth and epicentral location, all of these earthquakes can be shown to have occurred beneath the inner floor of the median valley. The centroid depths do not show a correlation with either spreading rate or seismic moment. Under the assumption that the centroid depth marks the mean depth of fault slip, earthquake faulting extended to depths of 2–6 km for these events.

@article{doi101029jb091ib01p00579,
    author = "Huang, Paul Y. and Solomon, Sean C. and Bergman, Eric and Nábělek, J.",
    title = "Focal depths and mechanism of Mid‐Atlantic Ridge earthquakes from body waveform inversion",
    year = "1986",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "We have determined the source mechanisms (double‐couple orientation, moment, centroid depth, source time function) of 14 earthquakes on the northern Mid‐Atlantic Ridge (0°–72°N) from an inversion of long‐period P and SH waveforms. The earthquakes are all characterized by nearly pure normal faulting on fault planes that dip at about 45° and strike parallel to the local trend of the ridge axis. Moments range from 3 to 15×10 24 dyn cm, and the source time functions are all of simple form. The P and S waveforms for all earthquakes can be well matched using conventional values for t * (1 and 4 s, respectively). These earthquakes are all very shallow; centroid depths range between 1.2 and 3.1 km beneath the seafloor. The P waves from these earthquakes show strong water column reverberations, suggesting that fault rupture extended to the seafloor. The predominant period of these reverberations constrains the water depth in the epicentral region. On the basis of estimated water depth and epicentral location, all of these earthquakes can be shown to have occurred beneath the inner floor of the median valley. The centroid depths do not show a correlation with either spreading rate or seismic moment. Under the assumption that the centroid depth marks the mean depth of fault slip, earthquake faulting extended to depths of 2–6 km for these events.",
    url = "https://doi.org/10.1029/jb091ib01p00579",
    doi = "10.1029/jb091ib01p00579",
    openalex = "W2070406202",
    references = "doi1010160040195181901311, doi101029jb073i018p05855, doi101029jb083ib11p05331, doi101029jb084ib11p06140, doi101029jb091ib14p13993, doi101029jz067i013p05279, doi101029me004p0001, doi101111j1365246x1958tb00033x, doi10150830000033586, doi101785bssa0650051073, openalexw1579868249, sykes1967mechanism"
}

As part of a global study of the source characteristics and tectonic implications of large earthquakes on mid‐ocean ridges, we report on the focal depths and mechanisms of the six largest earthquakes that have occurred in the last 20 years on the Arctic mid‐ocean ridge system. For each earthquake we invert the long‐period P and SH waveforms to estimate the parameters of the best fitting point source, including seismic moment, centroid depth, double‐couple source orientation, and source time function. Three of the earthquakes occurred on the oceanic spreading center in the Eurasian Basin, along ridge segments spreading at half rates of 4–6 mm/yr. These events have mechanisms very similar to those of ridge crest earthquakes on the Mid‐Atlantic Ridge: almost pure normal faulting on planes that dip at approximately 45° and strike parallel to the rift axis, moments of 4–5 × 10 24 dyn cm, centroid depths of 1–2 km beneath the seafloor, and water depths (inferred from the predominant period of water column reverberations) appropriate to epicentral locations within the median valley. The remaining three earthquakes, also characterized by normal faulting, are associated with the continuation of the divergent plate boundary (2–3 mm/yr half rate) onto the continental shelf of the Laptev Sea, where the crust becomes transitional in nature. One of the largest known spreading center earthquakes (August 25, 1964, M 0 = 1 × 10 26 dyn cm) occurred where the oceanic ridge intersects the outer edge of the continental slope. Waveform inversion for this event can resolve unilateral rupture from north to south (landward) along a fault at least 30 km in length. The preferred centroid depth is 5 km beneath the seafloor in crustal material with an unusually low shear velocity, but a centroid depth as great as 15 km cannot be ruled out. Two earthquakes beneath the continental shelf have significantly greater centroid depths (10–20 km) than mid‐ocean ridge earthquakes, indicating a thicker brittle regime and a cooler thermal structure than are typical of oceanic spreading centers. The tectonic environment of these events is more representative of rifted continental lithosphere than of a mid‐ocean ridge.

@article{doi101029jb091ib14p13993,
    author = "Jemsek, John P. and Bergman, Eric and Nábělek, J. and Solomon, Sean C.",
    title = "Focal depths and mechanisms of large earthquakes on the Arctic Mid‐Ocean Ridge System",
    year = "1986",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "As part of a global study of the source characteristics and tectonic implications of large earthquakes on mid‐ocean ridges, we report on the focal depths and mechanisms of the six largest earthquakes that have occurred in the last 20 years on the Arctic mid‐ocean ridge system. For each earthquake we invert the long‐period P and SH waveforms to estimate the parameters of the best fitting point source, including seismic moment, centroid depth, double‐couple source orientation, and source time function. Three of the earthquakes occurred on the oceanic spreading center in the Eurasian Basin, along ridge segments spreading at half rates of 4–6 mm/yr. These events have mechanisms very similar to those of ridge crest earthquakes on the Mid‐Atlantic Ridge: almost pure normal faulting on planes that dip at approximately 45° and strike parallel to the rift axis, moments of 4–5 × 10 24 dyn cm, centroid depths of 1–2 km beneath the seafloor, and water depths (inferred from the predominant period of water column reverberations) appropriate to epicentral locations within the median valley. The remaining three earthquakes, also characterized by normal faulting, are associated with the continuation of the divergent plate boundary (2–3 mm/yr half rate) onto the continental shelf of the Laptev Sea, where the crust becomes transitional in nature. One of the largest known spreading center earthquakes (August 25, 1964, M 0 = 1 × 10 26 dyn cm) occurred where the oceanic ridge intersects the outer edge of the continental slope. Waveform inversion for this event can resolve unilateral rupture from north to south (landward) along a fault at least 30 km in length. The preferred centroid depth is 5 km beneath the seafloor in crustal material with an unusually low shear velocity, but a centroid depth as great as 15 km cannot be ruled out. Two earthquakes beneath the continental shelf have significantly greater centroid depths (10–20 km) than mid‐ocean ridge earthquakes, indicating a thicker brittle regime and a cooler thermal structure than are typical of oceanic spreading centers. The tectonic environment of these events is more representative of rifted continental lithosphere than of a mid‐ocean ridge.",
    url = "https://doi.org/10.1029/jb091ib14p13993",
    doi = "10.1029/jb091ib14p13993",
    openalex = "W1975000436",
    references = "doi101007bf00300398, doi1010160012821x78900717, doi101029jb073i018p05855, doi101029jb083ib11p05331, doi101029jb084ib03p01071, doi101029jb088ib05p04183, doi101029jb090ib08p06709, doi10113000167606197283619ssitna20co2, openalexw1579868249, sykes1967mechanism"
}

We have determined the centroid depths and source mechanisms of 12 large earthquakes on transform faults of the northern Mid‐Atlantic Ridge from an inversion of long‐period body waveforms. The earthquakes occurred on the Gibbs, Oceanographer, Hayes, Kane, 15°20′, and Vema transforms. We have also estimated the depth extent of faulting during each earthquake from the centroid depth and the fault width. For five of the transforms, earthquake centroid depths lie in the range 7–10 km beneath the seafloor, and the maximum depth of seismic faulting is 14–20 km. On the basis of a comparison with a simple thermal model for transform faults, this maximum depth of seismic behavior corresponds to a nominal temperature of 900° ± 100°C. In contrast, the nominal temperature limiting the maximum depth of faulting during oceanic intraplate earthquakes with strike‐slip mechanisms is 700° ± 100°C. The difference in these limiting temperatures may be attributed to the different strain rates characterizing intraplate and transform fault environments. Three large earthquakes on the 15°20′ transform have shallower centroid depths of 4–5 km and a maximum depth of seismic faulting of 10 km, corresponding to a limiting temperature of 600°C. The shallower extent of seismic behavior along the 15°20′ transform may be related to a recent episode of extension across the transform associated with the northward migration of the triple junction among North American, South American, and African plates to its present position near the transform. The source mechanisms for all events in this study display the strike‐slip motion expected for transform fault earthquakes; slip vector azimuths agree to within 2°–3° of the local strike of the zone of active faulting. The only anomalies in mechanism were for two earthquakes near the western end of the Vema transform which occurred on significantly nonvertical fault planes. Secondary faulting, occurring either precursory to or near the end of the main episode of strike‐slip rupture, was observed for five of the 12 earthquakes. For three events the secondary faulting was characterized by reverse motion on fault planes striking oblique to the trend of the transform. In all three cases the site of secondary reverse faulting is near a compressional jog in the current trace of the active transform fault zone. We find no evidence to support the conclusions of Engeln, Wiens, and Stein that oceanic transform faults in general are either hotter than expected from simple thermal models or weaker than normal oceanic lithosphere.

@article{doi101029jb093ib08p09027,
    author = "Bergman, Eric and Solomon, Sean C.",
    title = "Transform fault earthquakes in the North Atlantic: Source mechanisms and depth of faulting",
    year = "1988",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "We have determined the centroid depths and source mechanisms of 12 large earthquakes on transform faults of the northern Mid‐Atlantic Ridge from an inversion of long‐period body waveforms. The earthquakes occurred on the Gibbs, Oceanographer, Hayes, Kane, 15°20′, and Vema transforms. We have also estimated the depth extent of faulting during each earthquake from the centroid depth and the fault width. For five of the transforms, earthquake centroid depths lie in the range 7–10 km beneath the seafloor, and the maximum depth of seismic faulting is 14–20 km. On the basis of a comparison with a simple thermal model for transform faults, this maximum depth of seismic behavior corresponds to a nominal temperature of 900° ± 100°C. In contrast, the nominal temperature limiting the maximum depth of faulting during oceanic intraplate earthquakes with strike‐slip mechanisms is 700° ± 100°C. The difference in these limiting temperatures may be attributed to the different strain rates characterizing intraplate and transform fault environments. Three large earthquakes on the 15°20′ transform have shallower centroid depths of 4–5 km and a maximum depth of seismic faulting of 10 km, corresponding to a limiting temperature of 600°C. The shallower extent of seismic behavior along the 15°20′ transform may be related to a recent episode of extension across the transform associated with the northward migration of the triple junction among North American, South American, and African plates to its present position near the transform. The source mechanisms for all events in this study display the strike‐slip motion expected for transform fault earthquakes; slip vector azimuths agree to within 2°–3° of the local strike of the zone of active faulting. The only anomalies in mechanism were for two earthquakes near the western end of the Vema transform which occurred on significantly nonvertical fault planes. Secondary faulting, occurring either precursory to or near the end of the main episode of strike‐slip rupture, was observed for five of the 12 earthquakes. For three events the secondary faulting was characterized by reverse motion on fault planes striking oblique to the trend of the transform. In all three cases the site of secondary reverse faulting is near a compressional jog in the current trace of the active transform fault zone. We find no evidence to support the conclusions of Engeln, Wiens, and Stein that oceanic transform faults in general are either hotter than expected from simple thermal models or weaker than normal oceanic lithosphere.",
    url = "https://doi.org/10.1029/jb093ib08p09027",
    doi = "10.1029/jb093ib08p09027",
    openalex = "W2163562760",
    references = "doi1010160031920181900467, doi101029jb082i005p00803, doi101029jb083ib11p05331, doi101029jb085ib11p06248, doi101029jb088ib05p04183, doi101029jb091ib01p00579, doi101029jb091ib14p13993, doi101111j1365246x1979tb02567x, doi101130dnaggnam351, doi101785bssa0650051073, doi101785bssa0720010151, openalexw1579868249"
}

A R Y Fault plane solutions of earthquakes within and on the margins of the Tibetan Plateau show diverse styles of faulting and deformation, with thrust faulting and crustal shortening normal to the margins of the plateau and with normal and strike-slip faulting resulting in roughly east-west crustal extension within the plateau. The direction of overthrusting of the Himalaya onto the Indian Shield is radially outward, varying from southwest in the western Himalaya to south-southeast in the east. Assuming that the Indian Shield behaves rigidly, this requires a west-northwest divergence of western Tibet from southeastern Tibet at a rate of 18f 9 mm yr-', comparable with the rate of convergence at the Himalaya. Fault plane solutions of earthquakes in the southern portion of the Tibet Plateau consistently show large components of normal faulting on roughly north-striking planes and corroborate such extension. Within the high plateau, where elevations exceed 5000m, normal and strike-slip faulting occur so that an overall east-southeast-west-northwest extension of the region (at about 10 mm yr-') is partitioned into roughly equal parts of crustal thinning and north-northeast-southsouthwest crustal shortening (about 5 mm yr-I). In general, strike-slip faulting characterizes solutions for earthquakes within eastern Tibet, where mean elevations drop below 4500-5000 m, but the orientations of the strike-slip faults vary across the region. In central Tibet, left-lateral slip occurs on planes trending roughly northeast, but for earthquakes farther east, the orientations of that plane become progressively east-west and then southeast. This variation in orientation implies a rotation of material along curved left-lateral shear zones. Thus, the eastward extrusion of Tibet appears to be facilitated not only by rapid left-lateral shear, but also by large clockwise rotations of the material in eastern Tibet. The rate of eastward extrusion of material in eastern Tibet, relative to the Tarim Basin to its north, is roughly 30-40 mm yr-'. Fault plane solutions of earthquakes in the northern and eastern margins of Tibet show large components of thrust faulting, with the P-axes, oriented radially outward from the plateau and approximately perpendicular to the regional topographic contours of the plateau. The orientation of this crustal shortening is northeast-southwest on the northeastern margin, east-west on the eastern margin, and northwest-southeast in the Longmenshan on the southeastern margin. Thus, at least some of the extrusion of eastern Tibet out of India's northward path into Asia is absorbed by crustal shortening on the margins of the plateau. The variation from normal faulting in the high Tibetan Plateau, where elevations exceed 5000 m, to dominantly strike-slip faulting farther east where elevations are lower, and then to thrust faulting on the margins of the plateau, where elevations drop below 3000 m, surely results, at least in part, from a decrease in the value of the vertical stress: the magnitude of the east-west compressive stress need not vary across the plateau.

@article{doi101111j1365246x1989tb02020x,
    author = "Molnár, Péter and Lyon‐Caen, H.",
    title = "Fault plane solutions of earthquakes and active tectonics of the Tibetan Plateau and its margins",
    year = "1989",
    journal = "Geophysical Journal International",
    abstract = "A R Y Fault plane solutions of earthquakes within and on the margins of the Tibetan Plateau show diverse styles of faulting and deformation, with thrust faulting and crustal shortening normal to the margins of the plateau and with normal and strike-slip faulting resulting in roughly east-west crustal extension within the plateau. The direction of overthrusting of the Himalaya onto the Indian Shield is radially outward, varying from southwest in the western Himalaya to south-southeast in the east. Assuming that the Indian Shield behaves rigidly, this requires a west-northwest divergence of western Tibet from southeastern Tibet at a rate of 18f 9 mm yr-', comparable with the rate of convergence at the Himalaya. Fault plane solutions of earthquakes in the southern portion of the Tibet Plateau consistently show large components of normal faulting on roughly north-striking planes and corroborate such extension. Within the high plateau, where elevations exceed 5000m, normal and strike-slip faulting occur so that an overall east-southeast-west-northwest extension of the region (at about 10 mm yr-') is partitioned into roughly equal parts of crustal thinning and north-northeast-southsouthwest crustal shortening (about 5 mm yr-I). In general, strike-slip faulting characterizes solutions for earthquakes within eastern Tibet, where mean elevations drop below 4500-5000 m, but the orientations of the strike-slip faults vary across the region. In central Tibet, left-lateral slip occurs on planes trending roughly northeast, but for earthquakes farther east, the orientations of that plane become progressively east-west and then southeast. This variation in orientation implies a rotation of material along curved left-lateral shear zones. Thus, the eastward extrusion of Tibet appears to be facilitated not only by rapid left-lateral shear, but also by large clockwise rotations of the material in eastern Tibet. The rate of eastward extrusion of material in eastern Tibet, relative to the Tarim Basin to its north, is roughly 30-40 mm yr-'. Fault plane solutions of earthquakes in the northern and eastern margins of Tibet show large components of thrust faulting, with the P-axes, oriented radially outward from the plateau and approximately perpendicular to the regional topographic contours of the plateau. The orientation of this crustal shortening is northeast-southwest on the northeastern margin, east-west on the eastern margin, and northwest-southeast in the Longmenshan on the southeastern margin. Thus, at least some of the extrusion of eastern Tibet out of India's northward path into Asia is absorbed by crustal shortening on the margins of the plateau. The variation from normal faulting in the high Tibetan Plateau, where elevations exceed 5000 m, to dominantly strike-slip faulting farther east where elevations are lower, and then to thrust faulting on the margins of the plateau, where elevations drop below 3000 m, surely results, at least in part, from a decrease in the value of the vertical stress: the magnitude of the east-west compressive stress need not vary across the plateau.",
    url = "https://doi.org/10.1111/j.1365-246x.1989.tb02020.x",
    doi = "10.1111/j.1365-246x.1989.tb02020.x",
    openalex = "W2151474583",
    references = "doi101029jz067i013p05279"
}

Summary The SW Indian and American-Antarctic Ridges are two of the world’s slowest spreading ocean ridges (less than 1 cm a −1), making them the low end-members for rate of ocean ridge magma supply. Two-thirds of the rocks dredged at the numerous large offset transforms along the ridges are residual mantle peridotites. Gabbroic rocks, however, representing layer 3 and possible palaeo-magma chambers are rare. This suggests a highly segmented crustal structure, with anomalously thin crust near fracture zones that may consist of only a thin veneer of pillow basalt erupted over mantle peridotite. The dredged peridotites underwent high degrees of melting, spanning the range believed to produce abyssal basalt. Their depleted compositions show that the melt was almost entirely removed. At the same time, the spatially associated basalts have a large range of compositions, similar to those from the rift valleys, requiring extensive shallow-level fractional crystallization. Since there is little evidence for magma chambers at these fracture zones, it is concluded that melts formed in the underlying mantle flowed laterally through the mantle beneath the crust towards a magmatic centre at the mid-point of an adjacent ridge segment. Magma was then subsequently intruded down the rift valley fissure system from the magmatic centre to erupt onto the fracture zone floor. Alternatively, the melt was drained from a mantle diapir beneath the midpoint of a ridge segment, prior to lateral flow of the residual peridotite beneath the ridge axis to the fracture zone. These processes suggest behaviour of the partially molten layer beneath ocean ridges analogous to Rayleigh-Taylor fluid instability, where a light less viscous fluid layer floating upwards in a denser medium goes unstable and drains at regularly spaced points into protrusions which rise rapidly to the surface. Evidence for such dynamically driven non-uniform melt flow in the mantle is seen in locally-abundant plagioclase peridotites, where the plagioclase crystallized from impregnated trapped melt. These rocks can contain up to 30% trapped melt, contrasting sharply with the typical abyssal peridotite which contains virtually none. Basalts erupted along these ridges provide a classic case of trace- and major-element decoupling during magma genesis. Despite trace-element and isotopic diversity, basalts from individual ridge segments were derived from primary magmas with similar major-element compositions. These observations can be explained if melt flows locally through the depleted mantle at the end of melting towards the midpoint of a ridge segment. This would cause melts originating at different points in an initially heterogeneous mantle to migrate through and equilibrate with the same section of mantle immediately prior to segregation—which, for the most part, would homogenize the melt’s major-element compositions. However, by virtue of the lever rule, this would have little effect on critical incompatible-trace-element or isotopic ratios of the migrating melts because of the very low incompatible-trace-element content of residual peridotite. Ocean ridges, then, appear to be marked by strings of regularly spaced volcanic centres overlying instability points in the partially molten upwelling asthenosphere much as has been postulated for arc volcanism and early continental rifting. Unlike arcs, the asthenosphere upwells to the base of the crust and the magmatic centres undergo continuous extension. Thus, large volcanoes are not constructed, and instead, ribbons of basaltic crust form parallel to the spreading direction. This is most evident at the SW Indian and American-Antarctic Ridges because of their highly attenuated magma supply. Where the magma supply is more robust and the magma chambers are correspondingly larger, the chambers may merge and eliminate the surficial morphological and chemical expression of punctuated magmatism at ocean ridges.

@article{doi101144gslsp19890420106,
    author = "Dick, H. J.",
    title = "Abyssal peridotites, very slow spreading ridges and ocean ridge magmatism",
    year = "1989",
    journal = "Geological Society London Special Publications",
    abstract = "Summary The SW Indian and American-Antarctic Ridges are two of the world’s slowest spreading ocean ridges (less than 1 cm a −1), making them the low end-members for rate of ocean ridge magma supply. Two-thirds of the rocks dredged at the numerous large offset transforms along the ridges are residual mantle peridotites. Gabbroic rocks, however, representing layer 3 and possible palaeo-magma chambers are rare. This suggests a highly segmented crustal structure, with anomalously thin crust near fracture zones that may consist of only a thin veneer of pillow basalt erupted over mantle peridotite. The dredged peridotites underwent high degrees of melting, spanning the range believed to produce abyssal basalt. Their depleted compositions show that the melt was almost entirely removed. At the same time, the spatially associated basalts have a large range of compositions, similar to those from the rift valleys, requiring extensive shallow-level fractional crystallization. Since there is little evidence for magma chambers at these fracture zones, it is concluded that melts formed in the underlying mantle flowed laterally through the mantle beneath the crust towards a magmatic centre at the mid-point of an adjacent ridge segment. Magma was then subsequently intruded down the rift valley fissure system from the magmatic centre to erupt onto the fracture zone floor. Alternatively, the melt was drained from a mantle diapir beneath the midpoint of a ridge segment, prior to lateral flow of the residual peridotite beneath the ridge axis to the fracture zone. These processes suggest behaviour of the partially molten layer beneath ocean ridges analogous to Rayleigh-Taylor fluid instability, where a light less viscous fluid layer floating upwards in a denser medium goes unstable and drains at regularly spaced points into protrusions which rise rapidly to the surface. Evidence for such dynamically driven non-uniform melt flow in the mantle is seen in locally-abundant plagioclase peridotites, where the plagioclase crystallized from impregnated trapped melt. These rocks can contain up to 30\% trapped melt, contrasting sharply with the typical abyssal peridotite which contains virtually none. Basalts erupted along these ridges provide a classic case of trace- and major-element decoupling during magma genesis. Despite trace-element and isotopic diversity, basalts from individual ridge segments were derived from primary magmas with similar major-element compositions. These observations can be explained if melt flows locally through the depleted mantle at the end of melting towards the midpoint of a ridge segment. This would cause melts originating at different points in an initially heterogeneous mantle to migrate through and equilibrate with the same section of mantle immediately prior to segregation—which, for the most part, would homogenize the melt’s major-element compositions. However, by virtue of the lever rule, this would have little effect on critical incompatible-trace-element or isotopic ratios of the migrating melts because of the very low incompatible-trace-element content of residual peridotite. Ocean ridges, then, appear to be marked by strings of regularly spaced volcanic centres overlying instability points in the partially molten upwelling asthenosphere much as has been postulated for arc volcanism and early continental rifting. Unlike arcs, the asthenosphere upwells to the base of the crust and the magmatic centres undergo continuous extension. Thus, large volcanoes are not constructed, and instead, ribbons of basaltic crust form parallel to the spreading direction. This is most evident at the SW Indian and American-Antarctic Ridges because of their highly attenuated magma supply. Where the magma supply is more robust and the magma chambers are correspondingly larger, the chambers may merge and eliminate the surficial morphological and chemical expression of punctuated magmatism at ocean ridges.",
    url = "https://doi.org/10.1144/gsl.sp.1989.042.01.06",
    doi = "10.1144/gsl.sp.1989.042.01.06",
    openalex = "W2080973789",
    references = "doi101007bf00300398, doi101086625580"
}

The third edition of this classic treatise presents a wealth of new topics and new observations. These include slow earthquake phenomena; friction of phyllosilicates, and at high sliding velocities; fault structures; relative roles of strong and seismogenic versus weak and creeping faults; dynamic triggering of earthquakes; oceanic earthquakes; megathrust earthquakes in subduction zones; deep earthquakes; and new observations of earthquake precursory phenomena.

@article{doi105860choice281579,
    author = "Scholz, C. H., (Christopher H.)",
    title = "The mechanics of earthquakes and faulting",
    year = "1990",
    journal = "Choice Reviews Online",
    abstract = "The third edition of this classic treatise presents a wealth of new topics and new observations. These include slow earthquake phenomena; friction of phyllosilicates, and at high sliding velocities; fault structures; relative roles of strong and seismogenic versus weak and creeping faults; dynamic triggering of earthquakes; oceanic earthquakes; megathrust earthquakes in subduction zones; deep earthquakes; and new observations of earthquake precursory phenomena.",
    url = "https://doi.org/10.5860/choice.28-1579",
    doi = "10.5860/choice.28-1579",
    openalex = "W2110448165",
    references = "doi101007bf00876528, doi1010160022509660900132, doi1010160040195183901488, doi1010160191814184900014, doi1010160191814188900570, doi101016s0065215608701212, doi10102992jb00132, doi101029jb073i018p05855, doi101029jb075i014p02625, doi101029jb075i026p04997, doi101029jb076i026p06414, doi101029jb082i020p02981, doi101029jb083ib11p05331, doi101029jb085ib11p06248, doi101029jb088ib02p01153, doi101029jb088ib05p04183, doi101029jb089ib06p04344, doi101029jb091ib12p12587, doi101029jb092ib06p04798, doi101029jb093ib08p09027, doi101029jz070i016p03965, doi101029jz072i008p02131, doi101029me001, doi101029rg009i001p00103, doi101029rg016i004p00621, doi101029rg018i001p00269, doi101029tc007i003p00663, doi101038207343a0, doi101038284135a0, doi101038334058a0, doi10106311721448, doi101098rspa19570133, doi101098rspa19660242, doi101098rsta19210006, doi101103physreva38364, doi101103physrevlett59381, doi101111j1365246x1975tb00631x, doi101111j1365246x1990tb06579x, doi10111513601206, doi101126science19142331230, doi101130001676061977881667dawtmo20co2, doi101144transed83387, doi101785bssa0350040175, sykes1967mechanism"
}

This paper discusses the geological and geophysical data available on mid‐ocean ridges with outcrops of serpentinized mantle peridotites, with the objective of better constraining the modes of emplacement of these rocks in the seafloor. Ridges with serpentinized peridotites outcrops are in most cases characterized by slow‐spreading rates, and in every case by deep axial valleys. Such deep axial valleys are thought, based on geophysical constraints and on mechanical modelling results, to characterize ridges with a thick axial lithosphere. A predictable effect of a thick axial lithosphere is that it should prevent magmas from pooling at crustal depths in a long‐lasting magma chamber: gabbroïc magmas should instead form shortlived dike or sill‐like intrusions. Samples from axial outcrops of serpentinized peridotites are often cut by dikelets of evolved gabbros which are interpreted as apophyses of such dike and sill‐like intrusions. This observation leads to a discontinuous magmatic crust model, in which mantle‐derived peridotites form screens for numerous gabbroïc intrusions. This discontinuous magmatic crust is expected to form in magma‐poor ridge regions, where there is not enough magma to produce a 4‐to 7‐km‐thick magmatic crust, and where the uppermost kilometers of oceanic lithosphere therefore have to be at least partially made of tectonically uplifted mantle material. Because the dimensions of individual mantle‐derived ultramafic screens may be smaller than seismic experiments detection limits, the discontinuous magmatic crust model discussed in this paper may produce a layer 3‐type seismic signature, even without extensive serpentinization of its ultramafic component. It therefore provides an alternative to Hess's [1962] serpentinite layer 3 model, for the geological interpretation of seismic data from oceanic areas with frequent outcrops of deep crustal and mantle‐derived rocks.

@article{doi10102992jb02221,
    author = "Cannat, Mathilde",
    title = "Emplacement of mantle rocks in the seafloor at mid‐ocean ridges",
    year = "1993",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "This paper discusses the geological and geophysical data available on mid‐ocean ridges with outcrops of serpentinized mantle peridotites, with the objective of better constraining the modes of emplacement of these rocks in the seafloor. Ridges with serpentinized peridotites outcrops are in most cases characterized by slow‐spreading rates, and in every case by deep axial valleys. Such deep axial valleys are thought, based on geophysical constraints and on mechanical modelling results, to characterize ridges with a thick axial lithosphere. A predictable effect of a thick axial lithosphere is that it should prevent magmas from pooling at crustal depths in a long‐lasting magma chamber: gabbroïc magmas should instead form shortlived dike or sill‐like intrusions. Samples from axial outcrops of serpentinized peridotites are often cut by dikelets of evolved gabbros which are interpreted as apophyses of such dike and sill‐like intrusions. This observation leads to a discontinuous magmatic crust model, in which mantle‐derived peridotites form screens for numerous gabbroïc intrusions. This discontinuous magmatic crust is expected to form in magma‐poor ridge regions, where there is not enough magma to produce a 4‐to 7‐km‐thick magmatic crust, and where the uppermost kilometers of oceanic lithosphere therefore have to be at least partially made of tectonically uplifted mantle material. Because the dimensions of individual mantle‐derived ultramafic screens may be smaller than seismic experiments detection limits, the discontinuous magmatic crust model discussed in this paper may produce a layer 3‐type seismic signature, even without extensive serpentinization of its ultramafic component. It therefore provides an alternative to Hess's [1962] serpentinite layer 3 model, for the geological interpretation of seismic data from oceanic areas with frequent outcrops of deep crustal and mantle‐derived rocks.",
    url = "https://doi.org/10.1029/92jb02221",
    doi = "10.1029/92jb02221",
    openalex = "W2126671525",
    references = "doi101007bf00300398, doi101007bf00310065, doi10102991jb02508, doi101029jb073i014p04741, doi101029jb091ib01p00579, doi101029jb092ib08p08089, doi101029rg021i006p01458, doi101038326035a0, doi101038344627a0, doi101130petrologic1962599, doi101144gslsp19890420106, doi101146annurevea10050182001103"
}

The contribution of extensional faulting to seafloor spreading along the East Pacific Rise (EPR) axis near 3°S and between 13°N and 15°N is calculated using data on the displacement and length distributions of faults obtained from side scan sonar and bathymetric data. It is found that faulting may account for of the order of 5–10% of the total spreading rate, which is comparable to a previous estimate from the EPR near 19°S. Given the paucity of normal faulting earthquakes on the EPR axis, a maximum estimate of the seismic moment release shows that seismicity can account for only 1% of the strain due to faulting. This result leads us to conclude that most of the slip on active faults must be occurring by stable sliding. Laboratory observations of the stability of frictional sliding show that increasing normal stress promotes unstable sliding, while increasing temperature promotes stable sliding. By applying a simple frictional model to mid‐ocean ridge faults it is shown that at fast spreading ridges (≥90 mm/yr) the seismic portion of a fault (W s) is a small proportion of the total downdip width of the fault (W ƒ). The ratio W s / W ƒ interpreted as the seismic coupling coefficient X, and in this case X ≈ 0. In contrast, at slow spreading rates (≤40 mm/yr), W s ≈ W ƒ, and therefore X ≈ 1, which is consistent with the occurrence of large‐magnitude earthquakes (m b = 5.0 to 6.0) occurring, for example, along the Mid‐Atlantic Ridge axis.

@article{doi10102993jb01567,
    author = "Cowie, P. A. and Scholz, Christopher H. and Edwards, Margo H. and Malinverno, Alberto",
    title = "Fault strain and seismic coupling on mid‐ocean ridges",
    year = "1993",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "The contribution of extensional faulting to seafloor spreading along the East Pacific Rise (EPR) axis near 3°S and between 13°N and 15°N is calculated using data on the displacement and length distributions of faults obtained from side scan sonar and bathymetric data. It is found that faulting may account for of the order of 5–10\% of the total spreading rate, which is comparable to a previous estimate from the EPR near 19°S. Given the paucity of normal faulting earthquakes on the EPR axis, a maximum estimate of the seismic moment release shows that seismicity can account for only 1\% of the strain due to faulting. This result leads us to conclude that most of the slip on active faults must be occurring by stable sliding. Laboratory observations of the stability of frictional sliding show that increasing normal stress promotes unstable sliding, while increasing temperature promotes stable sliding. By applying a simple frictional model to mid‐ocean ridge faults it is shown that at fast spreading ridges (≥90 mm/yr) the seismic portion of a fault (W s) is a small proportion of the total downdip width of the fault (W ƒ). The ratio W s / W ƒ interpreted as the seismic coupling coefficient X, and in this case X ≈ 0. In contrast, at slow spreading rates (≤40 mm/yr), W s ≈ W ƒ, and therefore X ≈ 1, which is consistent with the occurrence of large‐magnitude earthquakes (m b = 5.0 to 6.0) occurring, for example, along the Mid‐Atlantic Ridge axis.",
    url = "https://doi.org/10.1029/93jb01567",
    doi = "10.1029/93jb01567",
    openalex = "W2043371807",
    references = "doi101038334058a0"
}

We have conducted a comparative study of the tectonic morphology of young seafloor using SeaMARC II side scan sonar surveys of the intermediate spreading Ecuador Rift, the fast spreading East Pacific Rise (EPR) (8°30′–10°N), and the super fast spreading EPR (18°–19°S). We find that characteristics of fault populations are not only a function of spreading rate but also vary along axis within individual ridge segments (i.e., with proximity to large‐ and short‐offset discontinuities). We also find that fault azimuths can be used to examine plate kinematics on a finer scale than can be obtained using magnetic data alone. Most of the variation in fault populations with spreading rate can be explained by an inverse relationship between spreading rate and thickness of the brittle layer. For example, regions of super fast spreading are characterized by the largest numbers of short faults, the smallest average fault spacing and throw, and the highest fault density. In addition, clusters of short, closely spaced antithetic faults subsidiary to long master inward dipping faults are common within the super fast spreading area, presumably the result of a thinner, weaker brittle layer. Faults facing away from the ridge axis occur in increasing numbers with increasing spreading rate such that few outward facing faults are found at slow to intermediate rates and approximately equal numbers of inward and outward facing faults are observed at the fastest rates. Rapid thickening of the brittle layer with distance from the ridge may account for the predominance of inward facing faults at slower spreading rates. Outward facing faults at all spreading rates have shorter mean lengths and lower vertical offsets. These differences may reflect the shorter time outward facing faults are active owing to increasing strength of the lithosphere with distance from the ridge. Fault lengths and spacings in all areas approximate exponential distributions. The extensional strain represented by fault populations is calculated from the displacement and length distributions of faults, and strain estimates of ∼4% are obtained for each area. Assuming that fault spacing reflects fracture depth extent where faults initiate, we infer a brittle layer thickness of ∼1 km when faulting begins. Fault populations are examined for ridge segment scale variations in amagmatic extension. We see evidence for greater amagmatic extension associated with long‐term reduced magma supply along the eastern third of the Ecuador Rift. Evidence for local increased brittle extension is also found within 15 km of transform faults. Discordant zones left by overlapping spreading centers (OSCs) are characterized by low fault abundances. At OSCs, discrete events of ridge tip propagation may accommodate extension taken up elsewhere along the ridge by normal faulting. Fault azimuths do appear to be useful indicators of plate motion. Within the EPR 8°30′–10°N area, fault trends record a recent change in Pacific‐Cocos plate motion (3°–6° at ∼1 m.y.) consistent with magnetic anomaly and fault lineation data from elsewhere along the northern EPR. Within the Ecuador Rift, fault azimuths scatter within 3° of predicted trends and are consistent with constant spreading about one pole for the past 1.5 m.y.

@article{doi10102993jb02971,
    author = "Carbotte, S. M. and Macdonald, Ken C.",
    title = "Comparison of seafloor tectonic fabric at intermediate, fast, and super fast spreading ridges: Influence of spreading rate, plate motions, and ridge segmentation on fault patterns",
    year = "1994",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "We have conducted a comparative study of the tectonic morphology of young seafloor using SeaMARC II side scan sonar surveys of the intermediate spreading Ecuador Rift, the fast spreading East Pacific Rise (EPR) (8°30′–10°N), and the super fast spreading EPR (18°–19°S). We find that characteristics of fault populations are not only a function of spreading rate but also vary along axis within individual ridge segments (i.e., with proximity to large‐ and short‐offset discontinuities). We also find that fault azimuths can be used to examine plate kinematics on a finer scale than can be obtained using magnetic data alone. Most of the variation in fault populations with spreading rate can be explained by an inverse relationship between spreading rate and thickness of the brittle layer. For example, regions of super fast spreading are characterized by the largest numbers of short faults, the smallest average fault spacing and throw, and the highest fault density. In addition, clusters of short, closely spaced antithetic faults subsidiary to long master inward dipping faults are common within the super fast spreading area, presumably the result of a thinner, weaker brittle layer. Faults facing away from the ridge axis occur in increasing numbers with increasing spreading rate such that few outward facing faults are found at slow to intermediate rates and approximately equal numbers of inward and outward facing faults are observed at the fastest rates. Rapid thickening of the brittle layer with distance from the ridge may account for the predominance of inward facing faults at slower spreading rates. Outward facing faults at all spreading rates have shorter mean lengths and lower vertical offsets. These differences may reflect the shorter time outward facing faults are active owing to increasing strength of the lithosphere with distance from the ridge. Fault lengths and spacings in all areas approximate exponential distributions. The extensional strain represented by fault populations is calculated from the displacement and length distributions of faults, and strain estimates of ∼4\% are obtained for each area. Assuming that fault spacing reflects fracture depth extent where faults initiate, we infer a brittle layer thickness of ∼1 km when faulting begins. Fault populations are examined for ridge segment scale variations in amagmatic extension. We see evidence for greater amagmatic extension associated with long‐term reduced magma supply along the eastern third of the Ecuador Rift. Evidence for local increased brittle extension is also found within 15 km of transform faults. Discordant zones left by overlapping spreading centers (OSCs) are characterized by low fault abundances. At OSCs, discrete events of ridge tip propagation may accommodate extension taken up elsewhere along the ridge by normal faulting. Fault azimuths do appear to be useful indicators of plate motion. Within the EPR 8°30′–10°N area, fault trends record a recent change in Pacific‐Cocos plate motion (3°–6° at ∼1 m.y.) consistent with magnetic anomaly and fault lineation data from elsewhere along the northern EPR. Within the Ecuador Rift, fault azimuths scatter within 3° of predicted trends and are consistent with constant spreading about one pole for the past 1.5 m.y.",
    url = "https://doi.org/10.1029/93jb02971",
    doi = "10.1029/93jb02971",
    openalex = "W1980193353",
    references = "doi101038334058a0"
}

First‐order (transform) and second‐order ridge‐axis discontinuities create a fundamental segmentation of the lithosphere along mid‐ocean ridges, and in slow spreading crust they commonly are associated with exposure of subvolcanic crust and upper mantle. We analyzed available morphological, gravity, and rock sample data from the Atlantic Ocean to determine whether consistent structural patterns occur at these discontinuities and to constrain the processes that control the patterns. The results show that along their older, inside‐corner sides, both first‐and second‐order discontinuities are characterized by thinned crust and/or mantle exposures as well as by irregular fault patterns and a paucity of volcanic features. Crust on young, outside‐corner sides of discontinuities has more normal thickness, regular fault patterns, and common volcanic forms. These patterns are consistent with tectonic thinning of crust at inside corners by low‐angle detachment faults as previously suggested for transform discontinuities by Dick et al. [1981] and Karson [1990]. Volcanic upper crust accretes in the hanging wall of the detachment, is stripped from the inside‐corner footwall, and is carried to the outside comer. Gravity and morphological data suggest that detachment faulting is a relatively continuous, long‐lived process in crust spreading at <25–30 mm/yr, that it rnay be intermittent at intermediate rates of 25–40 mm/yr, and that it is unlikely to occur at faster rates. Detachment surfaces are dissected by later, high‐angle faults formed during crustal uplift into the rift mountains; these faults can cut through the entire crust and may be the kinds of faults imaged by seismic reflection profiling over Cretaceous North Atlantic crust. Off‐axis variations in gravity anomalies indicate that slow spreading crust experiences cyclic magmatic/amagmatic extension and that a typical cycle is about 2 m.y. long. During magmatic phases the footwall of the detachment fault probably exposes lower crustal gabbros, although these rocks locally may have an unconformable volcanic carapace. During amagmatic extension the detachment may dip steeply through the crust, providing a mechanism whereby upper mantle ultramafic rocks can be exhumed very rapidly, perhaps in as little as 0.5 m.y. Together, detachment faulting and cyclic magmatic/amagmatic extension create strongly heterogeneous lithosphere both along and across isochrons in slow spreading ocean crust.

@article{doi10102994jb00338,
    author = "Tucholke, Brian E. and Lin, Jian",
    title = "A geological model for the structure of ridge segments in slow spreading ocean crust",
    year = "1994",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "First‐order (transform) and second‐order ridge‐axis discontinuities create a fundamental segmentation of the lithosphere along mid‐ocean ridges, and in slow spreading crust they commonly are associated with exposure of subvolcanic crust and upper mantle. We analyzed available morphological, gravity, and rock sample data from the Atlantic Ocean to determine whether consistent structural patterns occur at these discontinuities and to constrain the processes that control the patterns. The results show that along their older, inside‐corner sides, both first‐and second‐order discontinuities are characterized by thinned crust and/or mantle exposures as well as by irregular fault patterns and a paucity of volcanic features. Crust on young, outside‐corner sides of discontinuities has more normal thickness, regular fault patterns, and common volcanic forms. These patterns are consistent with tectonic thinning of crust at inside corners by low‐angle detachment faults as previously suggested for transform discontinuities by Dick et al. [1981] and Karson [1990]. Volcanic upper crust accretes in the hanging wall of the detachment, is stripped from the inside‐corner footwall, and is carried to the outside comer. Gravity and morphological data suggest that detachment faulting is a relatively continuous, long‐lived process in crust spreading at <25–30 mm/yr, that it rnay be intermittent at intermediate rates of 25–40 mm/yr, and that it is unlikely to occur at faster rates. Detachment surfaces are dissected by later, high‐angle faults formed during crustal uplift into the rift mountains; these faults can cut through the entire crust and may be the kinds of faults imaged by seismic reflection profiling over Cretaceous North Atlantic crust. Off‐axis variations in gravity anomalies indicate that slow spreading crust experiences cyclic magmatic/amagmatic extension and that a typical cycle is about 2 m.y. long. During magmatic phases the footwall of the detachment fault probably exposes lower crustal gabbros, although these rocks locally may have an unconformable volcanic carapace. During amagmatic extension the detachment may dip steeply through the crust, providing a mechanism whereby upper mantle ultramafic rocks can be exhumed very rapidly, perhaps in as little as 0.5 m.y. Together, detachment faulting and cyclic magmatic/amagmatic extension create strongly heterogeneous lithosphere both along and across isochrons in slow spreading ocean crust.",
    url = "https://doi.org/10.1029/94jb00338",
    doi = "10.1029/94jb00338",
    openalex = "W2006712012",
    references = "doi101007bf00300398, doi10102992jb02221"
}

We postulate that fluctuations in magmatic activity at mid‐oceanic ridges perturb the horizontal least principal stress across rift‐bounding normal faults, leading to alternating phases of magmatic accretion, which increases valley width, and tectonic extension, which results in the growth of inner rift wall topography. Fine‐scale bathymetrie surveys and earthquake fault plane solutions show that active normal faults at slow‐spreading ridges are moderately dipping (approximately 45°) planar features throughout the seismogenic oceanic lithosphere. A simple quantitative model that includes flexural deformation of a 10‐km‐thick elastic plate by slippage on 45° dipping normal faults can match the bathymetrie profiles across several slow‐spreading ridge segments. Comparison among dip distributions of normal‐faulting earthquakes at mid‐ocean ridges, in the trench‐outer rise region, and on continents suggests that most events from these three tectonic environments initiated at dips close to 45°, raising unanswered questions about the mechanical conditions under which the faults originated.

@article{doi10102994jb02593,
    author = "Thatcher, Wayne and Hill, David P.",
    title = "A simple model for the fault‐generated morphology of slow‐spreading mid‐oceanic ridges",
    year = "1995",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "We postulate that fluctuations in magmatic activity at mid‐oceanic ridges perturb the horizontal least principal stress across rift‐bounding normal faults, leading to alternating phases of magmatic accretion, which increases valley width, and tectonic extension, which results in the growth of inner rift wall topography. Fine‐scale bathymetrie surveys and earthquake fault plane solutions show that active normal faults at slow‐spreading ridges are moderately dipping (approximately 45°) planar features throughout the seismogenic oceanic lithosphere. A simple quantitative model that includes flexural deformation of a 10‐km‐thick elastic plate by slippage on 45° dipping normal faults can match the bathymetrie profiles across several slow‐spreading ridge segments. Comparison among dip distributions of normal‐faulting earthquakes at mid‐ocean ridges, in the trench‐outer rise region, and on continents suggests that most events from these three tectonic environments initiated at dips close to 45°, raising unanswered questions about the mechanical conditions under which the faults originated.",
    url = "https://doi.org/10.1029/94jb02593",
    doi = "10.1029/94jb02593",
    openalex = "W2100163176",
    references = "doi101007bf00369150, doi101007bf01204232, doi1010160148906289919852, doi1010160191814189900333, doi101017cbo9780511735349, doi10102993jb01565, doi101029jb091ib14p13993, doi101029jb093ib11p13421, doi101029tc007i005p00959, doi101111j1365246x1989tb02020x, doi101111j1365246x1991tb03906x"
}

The objective of this Thesis was to interpret the structural development of slowspreading ridge segments by: 1) delineating the nature, magnitude, and relative importance of primary tectonic and volcanic processes that control crustal morphology, 2) investigating the spatial and temporal variability of these processes, and 3) examining how rheological variations in the lithosphere control its structural configuration. To that end, this Thesis provides detailed documentation of faults and volcanoes (seamounts) at the Mid-Atlantic Ridge from 2525'N to 2710'N and extending from zero-age crust at the ridge axis to -29 Ma crust on the ridge flank. This information was used to analyze the evolution of ocean crust from initial formation in the rift valley to degradation by aging processes on the ridge flank. Accumulation of sediments affects the seafloor morphological expression of ocean crustal structure, and sediment thicknesses were also mapped to facilitate study of the morphological record of crustal accretion and tectonism. In addition, deformation conditions in the lithosphere were analyzed by study of microstructure and geothermometry of abyssal peridotite mylonites recovered from fault zones at slow-spreading ridges.

@book{doi10157519125693,
    author = "Jaroslow, Gary E.",
    title = "The geological record of oceanic crustal accretion and tectonism at slow-spreading ridges",
    year = "1996",
    abstract = "The objective of this Thesis was to interpret the structural development of slowspreading ridge segments by: 1) delineating the nature, magnitude, and relative importance of primary tectonic and volcanic processes that control crustal morphology, 2) investigating the spatial and temporal variability of these processes, and 3) examining how rheological variations in the lithosphere control its structural configuration. To that end, this Thesis provides detailed documentation of faults and volcanoes (seamounts) at the Mid-Atlantic Ridge from 2525'N to 2710'N and extending from zero-age crust at the ridge axis to -29 Ma crust on the ridge flank. This information was used to analyze the evolution of ocean crust from initial formation in the rift valley to degradation by aging processes on the ridge flank. Accumulation of sediments affects the seafloor morphological expression of ocean crustal structure, and sediment thicknesses were also mapped to facilitate study of the morphological record of crustal accretion and tectonism. In addition, deformation conditions in the lithosphere were analyzed by study of microstructure and geothermometry of abyssal peridotite mylonites recovered from fault zones at slow-spreading ridges.",
    url = "https://doi.org/10.1575/1912/5693",
    doi = "10.1575/1912/5693",
    openalex = "W1482970135",
    references = "doi1010029781118782149ch1, doi1010160040195187903489, doi101016019181419290053y, doi101029jb082i005p00803, doi101029jb085ib11p06248, doi101029jb088ib05p04183, doi101038326035a0, doi101086627339, doi101126science2605109771, doi101130001676061970812181htfoda20co2, doi101144gslsp19890420106, sykes1972mechanism"
}

In a study of geological and geophysical data from the Mid‐Atlantic Ridge, we have identified 17 large, domed edifices (megamullions) that have surfaces corrugated by distinctive mullion structure and that are developed within inside‐corner tectonic settings at ends of spreading segments. The edifices have elevated residual gravity anomalies, and limited sampling has recovered gabbros and serpentinites, suggesting that they expose extensive cross sections of the oceanic crust and upper mantle. Oceanic megamullions are comparable to continental metamorphic core complexes in scale and structure, and they may originate by similar processes. The megamullions are interpreted to be rotated footwall blocks of low‐angle detachment faults, and they provide the best evidence to date for the common development and longevity (∼1–2 m.y.) of such faults in ocean crust. Prolonged slip on a detachment fault probably occurs when a spreading segment experiences a lengthy phase of relatively amagmatic extension. During these periods it is easier to maintain slip on an existing fault at the segment end than it is to break a new fault in the strong rift‐valley lithosphere; slip on the detachment fault probably is facilitated by fault weakening related to deep lithospheric changes in deformation mechanism and mantle serpentinization. At the segment center, minor, episodic magmatism may continue to weaken the axial lithosphere and thus sustain inward jumping of faults. A detachment fault will be terminated when magmatism becomes robust enough to reach the segment end, weaken the axial lithosphere, and promote inward fault jumps there. This mechanism may be generally important in controlling the longevity of normal faults at segment ends and thus in accounting for variable and intermittent development of inside‐corner highs.

@article{doi10102998jb00167,
    author = "Tucholke, Brian E. and Lin, Jian and Kleinrock, Martin C.",
    title = "Megamullions and mullion structure defining oceanic metamorphic core complexes on the Mid‐Atlantic Ridge",
    year = "1998",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "In a study of geological and geophysical data from the Mid‐Atlantic Ridge, we have identified 17 large, domed edifices (megamullions) that have surfaces corrugated by distinctive mullion structure and that are developed within inside‐corner tectonic settings at ends of spreading segments. The edifices have elevated residual gravity anomalies, and limited sampling has recovered gabbros and serpentinites, suggesting that they expose extensive cross sections of the oceanic crust and upper mantle. Oceanic megamullions are comparable to continental metamorphic core complexes in scale and structure, and they may originate by similar processes. The megamullions are interpreted to be rotated footwall blocks of low‐angle detachment faults, and they provide the best evidence to date for the common development and longevity (∼1–2 m.y.) of such faults in ocean crust. Prolonged slip on a detachment fault probably occurs when a spreading segment experiences a lengthy phase of relatively amagmatic extension. During these periods it is easier to maintain slip on an existing fault at the segment end than it is to break a new fault in the strong rift‐valley lithosphere; slip on the detachment fault probably is facilitated by fault weakening related to deep lithospheric changes in deformation mechanism and mantle serpentinization. At the segment center, minor, episodic magmatism may continue to weaken the axial lithosphere and thus sustain inward jumping of faults. A detachment fault will be terminated when magmatism becomes robust enough to reach the segment end, weaken the axial lithosphere, and promote inward fault jumps there. This mechanism may be generally important in controlling the longevity of normal faults at segment ends and thus in accounting for variable and intermittent development of inside‐corner highs.",
    url = "https://doi.org/10.1029/98jb00167",
    doi = "10.1029/98jb00167",
    openalex = "W2097274806",
    references = "doi102973dsdpproc431401979"
}

A sequence of large interplate earthquakes from 1952 to 1965 along the Aleutian arc and Kurile-Kamchatka trench released accumulated stresses along nearly the entire northern portion of the Pacific Plate boundary. The postseismic stress evolution across the northern Pacific and Arctic basins, calculated from a viscoelastic coupling model with an asthenospheric viscosity of 5 x 10(17) pascal seconds, is consistent with triggering of oceanic intraplate earthquakes, temporal patterns in seismicity at remote plate boundaries, and space-based geodetic measurements of anomalous velocity over an area 7000 by 7000 kilometers square during the 30-year period after the sequence.

@article{doi101126science28053671245,
    author = "Pollitz, Fred F. and Bürgmann, Roland and Romanowicz, Barbara",
    title = "Viscosity of Oceanic Asthenosphere Inferred from Remote Triggering of Earthquakes",
    year = "1998",
    journal = "Science",
    abstract = "A sequence of large interplate earthquakes from 1952 to 1965 along the Aleutian arc and Kurile-Kamchatka trench released accumulated stresses along nearly the entire northern portion of the Pacific Plate boundary. The postseismic stress evolution across the northern Pacific and Arctic basins, calculated from a viscoelastic coupling model with an asthenospheric viscosity of 5 x 10(17) pascal seconds, is consistent with triggering of oceanic intraplate earthquakes, temporal patterns in seismicity at remote plate boundaries, and space-based geodetic measurements of anomalous velocity over an area 7000 by 7000 kilometers square during the 30-year period after the sequence.",
    url = "https://doi.org/10.1126/science.280.5367.1245",
    doi = "10.1126/science.280.5367.1245",
    openalex = "W1979854288",
    references = "doi101007bf00875969, doi10102994gl02118, doi10102994jb01405, doi10102997jb00514, doi10102997jb01277, doi101029jb084ib05p02348, doi101029jb085ib10p05389, doi101029jb091ib14p13993, doi101038359123a0, doi101111j1365246x1982tb05994x, doi101785bssa0840030935"
}

We present a crust and mantle velocity structure for the West Iberia passive continental margin derived from a 320‐km‐long wide‐angle seismic profile acquired in the southern Iberia Abyssal Plain. We observe a 170‐km‐wide ocean‐continent transition zone which includes a pair of overlapping peridotite ridges and is bounded by oceanic crust and landward by fault‐bounded blocks of continental crust. The profile lies ∼40 km south of the transect sampled by Ocean Drilling Program (ODP) Legs 149 and 173. The transition zone structure can be divided into an upper layer, 2–4 km thick with velocities of between 4.5 and 7.0 km s −1 and generally a high‐velocity‐gradient (1 s −1), and a lower layer up to 4 km thick with a velocity of ∼7.6 km s −1 and a low‐velocity‐gradient. A weak Moho reflection in this zone was seen only on wide‐angle profiles at an offset of ∼30 km. The upper layer has a distinctly lower velocity than thinned continental crust adjacent to the continental slope. Conversely, the lower layer has too high a velocity to be magmatically intruded or underplated lower continental crust. On the coincident seismic reflection profile, fault‐bounded crustal blocks, identified in unequivocal extended continental crust, are not observed in the transition zone. The upper layer has velocity bounds and gradient similar to oceanic layer 2 observed west of the peridotite ridges, but no oceanic layer 3 velocity structure is present. While magnetic anomalies have been identified within the transition zone, they have not been modeled successfully as seafloor spreading magnetic anomalies, nor do they generally form long linear margin‐parallel features. Finally, ODP boreholes, ∼40 km north of our profile and within the interpreted transition zone, have recovered up to 140‐m‐thick sections of serpentinite and serpentinized peridotites with little evidence of mafic igneous material. We conclude that the transition zone cannot be dominantly composed of either extended continental crust or oceanic crust. Although current melting models predict a considerably thicker crust of decompression melt products, we interpret this region as exposed upper mantle peridotite with little or no synrift extrusive material and limited amounts of synrift material intruded within the serpentinized peridotite.

@article{doi1010291999jb900301,
    author = "Dean, S. M. and Minshull, T. A. and Whitmarsh, R. B. and Louden, Keith E.",
    title = "Deep structure of the ocean‐continent transition in the southern Iberia Abyssal Plain from seismic refraction profiles: The IAM‐9 transect at 40°20′N",
    year = "2000",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "We present a crust and mantle velocity structure for the West Iberia passive continental margin derived from a 320‐km‐long wide‐angle seismic profile acquired in the southern Iberia Abyssal Plain. We observe a 170‐km‐wide ocean‐continent transition zone which includes a pair of overlapping peridotite ridges and is bounded by oceanic crust and landward by fault‐bounded blocks of continental crust. The profile lies ∼40 km south of the transect sampled by Ocean Drilling Program (ODP) Legs 149 and 173. The transition zone structure can be divided into an upper layer, 2–4 km thick with velocities of between 4.5 and 7.0 km s −1 and generally a high‐velocity‐gradient (1 s −1), and a lower layer up to 4 km thick with a velocity of ∼7.6 km s −1 and a low‐velocity‐gradient. A weak Moho reflection in this zone was seen only on wide‐angle profiles at an offset of ∼30 km. The upper layer has a distinctly lower velocity than thinned continental crust adjacent to the continental slope. Conversely, the lower layer has too high a velocity to be magmatically intruded or underplated lower continental crust. On the coincident seismic reflection profile, fault‐bounded crustal blocks, identified in unequivocal extended continental crust, are not observed in the transition zone. The upper layer has velocity bounds and gradient similar to oceanic layer 2 observed west of the peridotite ridges, but no oceanic layer 3 velocity structure is present. While magnetic anomalies have been identified within the transition zone, they have not been modeled successfully as seafloor spreading magnetic anomalies, nor do they generally form long linear margin‐parallel features. Finally, ODP boreholes, ∼40 km north of our profile and within the interpreted transition zone, have recovered up to 140‐m‐thick sections of serpentinite and serpentinized peridotites with little evidence of mafic igneous material. We conclude that the transition zone cannot be dominantly composed of either extended continental crust or oceanic crust. Although current melting models predict a considerably thicker crust of decompression melt products, we interpret this region as exposed upper mantle peridotite with little or no synrift extrusive material and limited amounts of synrift material intruded within the serpentinized peridotite.",
    url = "https://doi.org/10.1029/1999jb900301",
    doi = "10.1029/1999jb900301",
    openalex = "W2044000131",
    references = "doi1010160012821x94900825, doi1010160025322771900533, doi10102992jb01749, doi10102994jb01889, doi10102995jb00259, doi10102996jb03223, doi101029jb094ib06p07685, doi101111j1365246x1992tb00836x, doi1011211381747, doi101146annurevea10050182001103, doi102973odpprocsr1492491996"
}

Our understanding of earthquakes and faulting processes has developed significantly since publication of the successful first edition of this book in 1990. This revised edition, first published in 2002, was therefore thoroughly up-dated whilst maintaining and developing the two major themes of the first edition. The first of these themes is the connection between fault and earthquake mechanics, including fault scaling laws, the nature of fault populations, and how these result from the processes of fault growth and interaction. The second major theme is the central role of the rate-state friction laws in earthquake mechanics, which provide a unifying framework within which a wide range of faulting phenomena can be interpreted. With the inclusion of two chapters explaining brittle fracture and rock friction from first principles, this book is written at a level which will appeal to graduate students and research scientists in the fields of seismology, physics, geology, geodesy and rock mechanics

@book{doi101017cbo9780511818516,
    author = "Scholz, Christopher H.",
    title = "The Mechanics of Earthquakes and Faulting",
    year = "2002",
    booktitle = "Cambridge University Press eBooks",
    abstract = "Our understanding of earthquakes and faulting processes has developed significantly since publication of the successful first edition of this book in 1990. This revised edition, first published in 2002, was therefore thoroughly up-dated whilst maintaining and developing the two major themes of the first edition. The first of these themes is the connection between fault and earthquake mechanics, including fault scaling laws, the nature of fault populations, and how these result from the processes of fault growth and interaction. The second major theme is the central role of the rate-state friction laws in earthquake mechanics, which provide a unifying framework within which a wide range of faulting phenomena can be interpreted. With the inclusion of two chapters explaining brittle fracture and rock friction from first principles, this book is written at a level which will appeal to graduate students and research scientists in the fields of seismology, physics, geology, geodesy and rock mechanics",
    url = "https://doi.org/10.1017/cbo9780511818516",
    doi = "10.1017/cbo9780511818516",
    openalex = "W4302565032"
}

We present the results of a series of 3‐D boundary element calculations to investigate the effects of oceanic transform faults on stress state and fault development at adjacent mid‐ocean ridge spreading centers. We find that the time‐averaged strength of transform faults is low, and that on time scales longer than a typical earthquake cycle transform faults behave as zones of significant weakness. Specifically, mechanical coupling of only ∼5% best explains the observed patterns of strike‐slip and oblique normal faulting near a ridge‐transform intersection. On time scales shorter than a typical earthquake cycle, transient “locked” periods can produce anomalous reverse faulting similar to that observed at the inside corner (IC) of several slow‐spreading ridge segments. Furthermore, we predict that extensional stresses will be suppressed at the IC due to the shear along the transform resisting ridge‐normal extension. This implies that an alternative mechanism is necessary to explain the preferential normal fault growth and enhanced microseismicity observed at many ICs.

@article{doi1010292002gl015612,
    author = "Behn, M. D. and Lin, Jian and Zuber, M. T.",
    title = "Evidence for weak oceanic transform faults",
    year = "2002",
    journal = "Geophysical Research Letters",
    abstract = "We present the results of a series of 3‐D boundary element calculations to investigate the effects of oceanic transform faults on stress state and fault development at adjacent mid‐ocean ridge spreading centers. We find that the time‐averaged strength of transform faults is low, and that on time scales longer than a typical earthquake cycle transform faults behave as zones of significant weakness. Specifically, mechanical coupling of only ∼5\% best explains the observed patterns of strike‐slip and oblique normal faulting near a ridge‐transform intersection. On time scales shorter than a typical earthquake cycle, transient “locked” periods can produce anomalous reverse faulting similar to that observed at the inside corner (IC) of several slow‐spreading ridge segments. Furthermore, we predict that extensional stresses will be suppressed at the IC due to the shear along the transform resisting ridge‐normal extension. This implies that an alternative mechanism is necessary to explain the preferential normal fault growth and enhanced microseismicity observed at many ICs.",
    url = "https://doi.org/10.1029/2002gl015612",
    doi = "10.1029/2002gl015612",
    openalex = "W2165102708",
    references = "doi10102994jb02593"
}

Large, sustained well water level changes (>10 cm) in response to distant (more than hundreds of kilometers) earthquakes have proven enigmatic for over 30 years. Here we use high sampling rates at a well near Grants Pass, Oregon, to perform the first simultaneous analysis of both the dynamic response of water level and sustained changes, or steps. We observe a factor of 40 increase in the ratio of water level amplitude to seismic wave ground velocity during a sudden coseismic step. On the basis of this observation we propose a new model for coseismic pore pressure steps in which a temporary barrier deposited by groundwater flow is entrained and removed by the more rapid flow induced by the seismic waves. In hydrothermal areas, this mechanism could lead to 4 × 10 −2 MPa pressure changes and triggered seismicity.

@article{doi1010292002jb002321,
    author = "Brodsky, E. E. and Roeloffs, Evelyn and Woodcock, Douglas and Gall, I. and Manga, Michael",
    title = "A mechanism for sustained groundwater pressure changes induced by distant earthquakes",
    year = "2003",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "Large, sustained well water level changes (>10 cm) in response to distant (more than hundreds of kilometers) earthquakes have proven enigmatic for over 30 years. Here we use high sampling rates at a well near Grants Pass, Oregon, to perform the first simultaneous analysis of both the dynamic response of water level and sustained changes, or steps. We observe a factor of 40 increase in the ratio of water level amplitude to seismic wave ground velocity during a sudden coseismic step. On the basis of this observation we propose a new model for coseismic pore pressure steps in which a temporary barrier deposited by groundwater flow is entrained and removed by the more rapid flow induced by the seismic waves. In hydrothermal areas, this mechanism could lead to 4 × 10 −2 MPa pressure changes and triggered seismicity.",
    url = "https://doi.org/10.1029/2002jb002321",
    doi = "10.1029/2002jb002321",
    openalex = "W2110237356"
}

@article{doi101038nature03358,
    author = "Buck, W. Roger and Lavier, L. L. and Poliakov, Alexei N. B.",
    title = "Modes of faulting at mid-ocean ridges",
    year = "2005",
    journal = "Nature",
    url = "https://doi.org/10.1038/nature03358",
    doi = "10.1038/nature03358",
    openalex = "W2086471046",
    references = "doi10102992jb02650, doi10102994jb00338, doi10102994jb02593, doi10102998jb00167, doi101029gm091, doi101038326035a0, doi101038385329a0, doi101038nature01704, doi101038nature02128, doi101126science1067361, doi105860choice284546"
}

The region of the Mid‐Atlantic Ridge (MAR) between the Fifteen‐Twenty and Marathon fracture zones displays the topographic characteristics of prevalent and vigorous tectonic extension. Normal faults show large amounts of rotation, dome‐shaped corrugated detachment surfaces (core complexes) intersect the seafloor at the edge of the inner valley floor, and extinct core complexes cover the seafloor off‐axis. We have identified 45 potential core complexes in this region whose locations are scattered everywhere along two segments (13° and 15°N segments). Steep outward‐facing slopes suggest that the footwalls of many of the normal faults in these two segments have rotated by more than 30°. The rotation occurs very close to the ridge axis (as much as 20° within 5 km of the volcanic axis) and is complete by ∼1 My, producing distinctive linear ridges with roughly symmetrical slopes. This morphology is very different from linear abyssal hill faults formed at the 14°N magmatic segment, which display a smaller amount of rotation (typically <15°). We suggest that the severe rotation of faults is diagnostic of a region undergoing large amounts of tectonic extension on single faults. If faults are long‐lived, a dome‐shaped corrugated surface develops in front of the ridges and lower crustal and upper mantle rocks are exposed to form a core complex. A single ridge segment can have several active core complexes, some less than 25 km apart that are separated by swales. We present two models for multiple core complex formation: a continuous model in which a single detachment surface extends along axis to include all of the core complexes and swales, and a discontinuous model in which local detachment faults form the core complexes and magmatic spreading forms the intervening swales. Either model can explain the observed morphology.

@article{doi1010292007gc001699,
    author = "Smith, Deborah K. and Escartı́n, J. and Schouten, Hans and Cann, J. R.",
    title = "Fault rotation and core complex formation: Significant processes in seafloor formation at slow‐spreading mid‐ocean ridges (Mid‐Atlantic Ridge, 13°–15°N)",
    year = "2008",
    journal = "Geochemistry Geophysics Geosystems",
    abstract = "The region of the Mid‐Atlantic Ridge (MAR) between the Fifteen‐Twenty and Marathon fracture zones displays the topographic characteristics of prevalent and vigorous tectonic extension. Normal faults show large amounts of rotation, dome‐shaped corrugated detachment surfaces (core complexes) intersect the seafloor at the edge of the inner valley floor, and extinct core complexes cover the seafloor off‐axis. We have identified 45 potential core complexes in this region whose locations are scattered everywhere along two segments (13° and 15°N segments). Steep outward‐facing slopes suggest that the footwalls of many of the normal faults in these two segments have rotated by more than 30°. The rotation occurs very close to the ridge axis (as much as 20° within 5 km of the volcanic axis) and is complete by ∼1 My, producing distinctive linear ridges with roughly symmetrical slopes. This morphology is very different from linear abyssal hill faults formed at the 14°N magmatic segment, which display a smaller amount of rotation (typically <15°). We suggest that the severe rotation of faults is diagnostic of a region undergoing large amounts of tectonic extension on single faults. If faults are long‐lived, a dome‐shaped corrugated surface develops in front of the ridges and lower crustal and upper mantle rocks are exposed to form a core complex. A single ridge segment can have several active core complexes, some less than 25 km apart that are separated by swales. We present two models for multiple core complex formation: a continuous model in which a single detachment surface extends along axis to include all of the core complexes and swales, and a discontinuous model in which local detachment faults form the core complexes and magmatic spreading forms the intervening swales. Either model can explain the observed morphology.",
    url = "https://doi.org/10.1029/2007gc001699",
    doi = "10.1029/2007gc001699",
    openalex = "W2169552584",
    references = "doi101038nature03358"
}

The Blanco Transform Fault Zone (BTFZ) forms the ∼350 km long Pacific–Juan de Fuca plate boundary between the Gorda and Juan de Fuca ridges. Nearby broadband seismic networks provide a unique framework for a detailed, long‐term seismotectonic study of an entire oceanic transform fault (OTF) system. We use regional waveforms to determine 129 earthquake source parameters; combined with 28 Harvard moment tensors, they represent the largest waveform derived OTF source parameter data set. Joint epicenter determination removes the northeasterly routine location bias. Projecting seismicity onto the BTFZ, we determine along‐fault seismic slip rate variations. Earthquake source parameters and morphology indicate several transform segments separated by extensional step overs. The eastern segment from Gorda Ridge to Gorda Depression is a pull‐apart basin. The longest transform (∼150 km) following Blanco Ridge from the Gorda to Cascadia depression is seismically very active, seismically fully coupled, has a wider seismic zone (∼9 km) than other BTFZ transform segments and accommodates the largest (M w 6.4–6.5) BTFZ earthquakes. Interpretation of Cascadia Depression as spreading ridge is supported by plate motion parallel normal faulting T axes. Spreading is currently tectonic; 9 km deep earthquakes indicate a deep source for intermittent intrusives and rapid postemplacement cooling. A short transform connects to the pull‐apart Surveyor Depression. Widely spread seismicity along the western BTFZ reflects complex morphology indicating ongoing plate boundary reorganization along short, narrow width subparallel faults. Seismic coupling is low in extensional (≤15%) compared to transform areas (35–100%), implying different mechanical properties. Centroid depth variations are consistent with seismic slip cutoff near 600°C.

@article{doi1010292007jb005213,
    author = "Braunmiller, Jochen and Nábělek, J.",
    title = "Segmentation of the Blanco Transform Fault Zone from earthquake analysis: Complex tectonics of an oceanic transform fault",
    year = "2008",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "The Blanco Transform Fault Zone (BTFZ) forms the ∼350 km long Pacific–Juan de Fuca plate boundary between the Gorda and Juan de Fuca ridges. Nearby broadband seismic networks provide a unique framework for a detailed, long‐term seismotectonic study of an entire oceanic transform fault (OTF) system. We use regional waveforms to determine 129 earthquake source parameters; combined with 28 Harvard moment tensors, they represent the largest waveform derived OTF source parameter data set. Joint epicenter determination removes the northeasterly routine location bias. Projecting seismicity onto the BTFZ, we determine along‐fault seismic slip rate variations. Earthquake source parameters and morphology indicate several transform segments separated by extensional step overs. The eastern segment from Gorda Ridge to Gorda Depression is a pull‐apart basin. The longest transform (∼150 km) following Blanco Ridge from the Gorda to Cascadia depression is seismically very active, seismically fully coupled, has a wider seismic zone (∼9 km) than other BTFZ transform segments and accommodates the largest (M w 6.4–6.5) BTFZ earthquakes. Interpretation of Cascadia Depression as spreading ridge is supported by plate motion parallel normal faulting T axes. Spreading is currently tectonic; 9 km deep earthquakes indicate a deep source for intermittent intrusives and rapid postemplacement cooling. A short transform connects to the pull‐apart Surveyor Depression. Widely spread seismicity along the western BTFZ reflects complex morphology indicating ongoing plate boundary reorganization along short, narrow width subparallel faults. Seismic coupling is low in extensional (≤15\%) compared to transform areas (35–100\%), implying different mechanical properties. Centroid depth variations are consistent with seismic slip cutoff near 600°C.",
    url = "https://doi.org/10.1029/2007jb005213",
    doi = "10.1029/2007jb005213",
    openalex = "W2170344966",
    references = "doi1010160012825281900441, doi101017cbo9780511807442, doi101017cbo9780511818516, doi10102995eo00198, doi101029jb073i002p00777, doi101029jb084ib05p02348, doi101029jb091ib14p13993, doi101111j1365246x1969tb00259x, doi105860choice281579, openalexw1579868249, openalexw3041301201"
}

Normal faults are ubiquitous on mid-ocean ridges and are expected to develop increasing offset with reduced spreading rate as the proportion of tectonic extension increases. Numerous long-lived detachment faults that form megamullions with large-scale corrugations have been identifi ed on magma-poor mid-ocean ridges, but recent studies suggest, counterintuitively, that they may be associated with elevated magmatism. We present numerical models and geological data to show that these detachments occur when ~30%-50% of total extension is accommodated by magmatic accretion and that there is signifi cant magmatic accretion in the fault footwalls. Under these low-melt conditions, magmatism may focus unevenly along the spreading axis to create an irregular brittle-plastic transition where detachments root, thus explaining the origin of the enigmatic corrugations. Morphological and compositional characteristics of the oceanic lithosphere suggested by this study provide important new constraints to assess the distribution of magmatic versus tectonic extension along mid-ocean ridges.

@article{doi101130g24639a1,
    author = "Tucholke, Brian E. and Behn, M. D. and Buck, W. Roger and Lin, Jian",
    title = "Role of melt supply in oceanic detachment faulting and formation of megamullions",
    year = "2008",
    journal = "Geology",
    abstract = "Normal faults are ubiquitous on mid-ocean ridges and are expected to develop increasing offset with reduced spreading rate as the proportion of tectonic extension increases. Numerous long-lived detachment faults that form megamullions with large-scale corrugations have been identifi ed on magma-poor mid-ocean ridges, but recent studies suggest, counterintuitively, that they may be associated with elevated magmatism. We present numerical models and geological data to show that these detachments occur when \textasciitilde 30\%-50\% of total extension is accommodated by magmatic accretion and that there is signifi cant magmatic accretion in the fault footwalls. Under these low-melt conditions, magmatism may focus unevenly along the spreading axis to create an irregular brittle-plastic transition where detachments root, thus explaining the origin of the enigmatic corrugations. Morphological and compositional characteristics of the oceanic lithosphere suggested by this study provide important new constraints to assess the distribution of magmatic versus tectonic extension along mid-ocean ridges.",
    url = "https://doi.org/10.1130/g24639a.1",
    doi = "10.1130/g24639a.1",
    openalex = "W2076461491",
    references = "doi10102994jb02593, doi101038nature03358"
}

Mid‐ocean ridge transform faults (RTFs) have thermal structures that vary systematically with tectonic parameters, resulting in predictable seismic characteristics and clear seismic cycles. We develop a scaling relation for repeat time, t R, of the largest expected earthquake, M C: t R = μ −1 Δ σ 2/3 C Mc 1/3 A T 1/4 V − 1, where μ is the shear modulus, Δ σ is the stress drop, C Mc is a constant, A T is the area above 600°C, and V is the slip rate. We identify repeating M C earthquakes by measuring differential arrival times of first orbit Rayleigh waves to determine centroid offsets between pairs of events. Comparing our observations of t R (5–14 years for earthquakes on Gofar and Blanco RTFs) with predictions from our scaling relation, we can constrain RTF stress drops. Specific tests of this scaling relation are proposed for earthquakes on Blanco, Gofar, Discovery, and Clipperton RTFs, which are all expected to have large ruptures in the next few years.

@article{doi1010292009gl040115,
    author = "Boettcher, M. S. and McGuire, J. J.",
    title = "Scaling relations for seismic cycles on mid‐ocean ridge transform faults",
    year = "2009",
    journal = "Geophysical Research Letters",
    abstract = "Mid‐ocean ridge transform faults (RTFs) have thermal structures that vary systematically with tectonic parameters, resulting in predictable seismic characteristics and clear seismic cycles. We develop a scaling relation for repeat time, t R, of the largest expected earthquake, M C: t R = μ −1 Δ σ 2/3 C Mc 1/3 A T 1/4 V − 1, where μ is the shear modulus, Δ σ is the stress drop, C Mc is a constant, A T is the area above 600°C, and V is the slip rate. We identify repeating M C earthquakes by measuring differential arrival times of first orbit Rayleigh waves to determine centroid offsets between pairs of events. Comparing our observations of t R (5–14 years for earthquakes on Gofar and Blanco RTFs) with predictions from our scaling relation, we can constrain RTF stress drops. Specific tests of this scaling relation are proposed for earthquakes on Blanco, Gofar, Discovery, and Clipperton RTFs, which are all expected to have large ruptures in the next few years.",
    url = "https://doi.org/10.1029/2009gl040115",
    doi = "10.1029/2009gl040115",
    openalex = "W1980208200",
    references = "doi1010292007jb005213"
}

A R Y There have been many searches for evidence of tidal triggering in earthquake catalogues. With the exception of volcanically active regions, the more rigorous studies in continental settings tend to find no correlation or only a very weak correlation. In the oceans, the effect of loading by the ocean tides can increase tidal stresses by about an order of magnitude over continental settings. In recent years, several studies have reported evidence of tidal triggering in oceanic regions and such observations can represent a useful constraint on models of earthquake rupture. In this paper, I systematically search for a link between ocean tide height and the incidence of earthquakes in the Northeast Pacific Ocean, a region of high-amplitude open ocean tides. The focal mechanisms of most of the earthquakes in these catalogues are unknown but it can be shown that tidal stresses will in most instances promote failure at low tides. I investigate three declustered data sets comprising (1) earthquakes from 1980 to 2007 on the Juan de Fuca plate and in the Queen Charlotte Fault region from land based catalogues;

@article{doi101111j1365246x200904319x,
    author = "Wilcock, William S. D.",
    title = "Tidal triggering of earthquakes in the Northeast Pacific Ocean",
    year = "2009",
    journal = "Geophysical Journal International",
    abstract = "A R Y There have been many searches for evidence of tidal triggering in earthquake catalogues. With the exception of volcanically active regions, the more rigorous studies in continental settings tend to find no correlation or only a very weak correlation. In the oceans, the effect of loading by the ocean tides can increase tidal stresses by about an order of magnitude over continental settings. In recent years, several studies have reported evidence of tidal triggering in oceanic regions and such observations can represent a useful constraint on models of earthquake rupture. In this paper, I systematically search for a link between ocean tide height and the incidence of earthquakes in the Northeast Pacific Ocean, a region of high-amplitude open ocean tides. The focal mechanisms of most of the earthquakes in these catalogues are unknown but it can be shown that tidal stresses will in most instances promote failure at low tides. I investigate three declustered data sets comprising (1) earthquakes from 1980 to 2007 on the Juan de Fuca plate and in the Queen Charlotte Fault region from land based catalogues;",
    url = "https://doi.org/10.1111/j.1365-246x.2009.04319.x",
    doi = "10.1111/j.1365-246x.2009.04319.x",
    openalex = "W2106277649",
    references = "doi1010292007jb005213"
}

Although fault and magmatic processes have achieved plate spreading at mid-ocean ridges throughout Earth's history, discrete rifting episodes have rarely been observed. This paper synthesizes ongoing seismic, structural, space-based geodetic, and petrologic studies from the subaerial Red Sea rift in Ethiopia where a major rifting episode commenced in September 2005. Our aims are to determine the length and timescales of magmatism and faulting, the partitioning of strain between faulting and magmatism, and their implications for the maintenance of along-axis segmentation. Most of the magma for the initial and subsequent 12 intrusions was sourced from the center of the Dabbahu-Manda Hararo rift segment. Strain is accommodated primarily by axial dike intrusions fed from mid-segment magma chamber(s). These findings show that episodic (approximate century interval), rapid opening of discrete rift segments is the primary mechanism of plate boundary deformation. The scale (∼65 km × 8 km) and intensity of crustal deformation (∼6 m), as well as the volume of intrusive and extrusive magmatism (>3 km 3), provokes a re-evaluation of seismic and volcanic hazards in subaerial rift zones.

@article{doi101146annurevearth040809152333,
    author = "Ebinger, C. J. and Ayele, Atalay and Keir, Derek and Rowland, J. V. and Yirgu, Gezahegn and Wright, Tim and Belachew, M. and Hamling, Ian",
    title = "Length and Timescales of Rift Faulting and Magma Intrusion: The Afar Rifting Cycle from 2005 to Present",
    year = "2010",
    journal = "Annual Review of Earth and Planetary Sciences",
    abstract = "Although fault and magmatic processes have achieved plate spreading at mid-ocean ridges throughout Earth's history, discrete rifting episodes have rarely been observed. This paper synthesizes ongoing seismic, structural, space-based geodetic, and petrologic studies from the subaerial Red Sea rift in Ethiopia where a major rifting episode commenced in September 2005. Our aims are to determine the length and timescales of magmatism and faulting, the partitioning of strain between faulting and magmatism, and their implications for the maintenance of along-axis segmentation. Most of the magma for the initial and subsequent 12 intrusions was sourced from the center of the Dabbahu-Manda Hararo rift segment. Strain is accommodated primarily by axial dike intrusions fed from mid-segment magma chamber(s). These findings show that episodic (approximate century interval), rapid opening of discrete rift segments is the primary mechanism of plate boundary deformation. The scale (∼65 km × 8 km) and intensity of crustal deformation (∼6 m), as well as the volume of intrusive and extrusive magmatism (>3 km 3), provokes a re-evaluation of seismic and volcanic hazards in subaerial rift zones.",
    url = "https://doi.org/10.1146/annurev-earth-040809-152333",
    doi = "10.1146/annurev-earth-040809-152333",
    openalex = "W2162046601",
    references = "doi10102994jb02593, doi101038334058a0"
}

This chapter contains sections titled: Introduction Rheology of the Oceanic Lithosphere The Thermal Structure of Oceanic Lithosphere Flexure and the Elastic Properties of the Lithosphere The Thickness of the Seismogenic Zone The Median Valley and the Axial High Morphology and Crustal Architecture of Ridge Segments Lithological Structure of Mid-Ocean Ridges Faulting at Mid-Ocean Ridges Summary of Observations: Rheological Structure of Slow and Fast-Spreading Ridges Conclusions

@incollection{doi101029148gm03,
    author = "Searle, R. C. and Escartı́n, J.",
    title = "The Rheology and Morphology of Oceanic Lithosphere and Mid-Ocean Ridges",
    year = "2011",
    booktitle = "Geophysical monograph",
    abstract = "This chapter contains sections titled: Introduction Rheology of the Oceanic Lithosphere The Thermal Structure of Oceanic Lithosphere Flexure and the Elastic Properties of the Lithosphere The Thickness of the Seismogenic Zone The Median Valley and the Axial High Morphology and Crustal Architecture of Ridge Segments Lithological Structure of Mid-Ocean Ridges Faulting at Mid-Ocean Ridges Summary of Observations: Rheological Structure of Slow and Fast-Spreading Ridges Conclusions",
    url = "https://doi.org/10.1029/148gm03",
    doi = "10.1029/148gm03",
    openalex = "W1554533974",
    references = "doi10157519125693"
}

This chapter contains sections titled: Introduction Definitions Data Spreading Ridges Oceanic Transform Faults Discussion

@incollection{doi101029gd030p0203,
    author = "Bird, Peter and Kagan, Y. Y. and Jackson, D. D.",
    title = "Plate Tectonics and Earthquake Potential of Spreading Ridges and Oceanic Transform Faults",
    year = "2011",
    booktitle = "Geodynamics series/Geodynamic series",
    abstract = "This chapter contains sections titled: Introduction Definitions Data Spreading Ridges Oceanic Transform Faults Discussion",
    url = "https://doi.org/10.1029/gd030p0203",
    doi = "10.1029/gd030p0203",
    openalex = "W1496733841",
    references = "doi101029jb091ib01p00579"
}

This chapter contains sections titled: Introduction Magmatism at Mid-Ocean Ridges Recent Eruptive Events and Associated Structures Analytical Dating Techniques for Young Lavas Compositional Variations of Mid-Ocean Ridge Basalts Discussion and Conclusions

@incollection{doi101029gm106p0059,
    author = "Perfit, M. R. and Chadwick, William W.",
    title = "Magmatism at Mid-Ocean Ridges: Constraints from Volcanological and Geochemical Investigations",
    year = "2011",
    booktitle = "Geophysical monograph",
    abstract = "This chapter contains sections titled: Introduction Magmatism at Mid-Ocean Ridges Recent Eruptive Events and Associated Structures Analytical Dating Techniques for Young Lavas Compositional Variations of Mid-Ocean Ridge Basalts Discussion and Conclusions",
    url = "https://doi.org/10.1029/gm106p0059",
    doi = "10.1029/gm106p0059",
    openalex = "W1563674614",
    references = "doi101038334058a0"
}

Various seismicity patterns before major earthquakes have been reported in the literature. They include foreshocks (broad sense), preseismic quiescence, precursory swarms, and doughnut patterns. Although many earthquakes are preceded by all, or some, of these patterns, their detail differ significantly from event to event. In order to examine the details of seismicity patterns on as uniform a basis as possible, we made space-time plots of seismicity for many large earthquakes by using the NOAA and JMA catalogs. Among various seismicity patterns, preseismic quiescence appears most common, the case for the 1978 Oaxaca earthquake being the most prominent. Although the nature of other patterns varies from event to event, a common physical mechanism may be responsible for these patterns; details of the pattern are probably controlled by the tectonic environment (fault geometry, strain rate) and the heterogeneity of the fault plane. Here a simple asperity model is introduced to explain these seismicity patterns. In this model, a fault plane with an asperity is divided into a number of subfaults. The subfaults within the asperity are, on the average, stronger than those in the surrounding weak zone. As the tectonic stress increases, the subfaults in the weak zone break in the form of background small earthquakes. If the frequency distribution of the strength of the subfaults has a sharp peak, a precursory swarm occurs. By this time, most of the subfaults in the weak zone are broken and the fault plane becomes seismically quiet. As the tectonic stress increases further, eventually the asperity breaks and sympathetic displacement occurs on the entire fault zone in the form of the main shock. Foreshocks do or do not occur depending upon the distribution of the strength of the subfaults within the asperity. Since the spatio-temporal change in the stress on the fault plane is most likely to dictate the change in seismicity patterns, detailed analysis of seismicity patterns would provide a most direct clue to the state of stress in the fault zone. However, because of the large variation from event to event, seismicity pattern alone is not a definitive tool for earthquake prediction; measurements of other physical parameters such as the spectra, the mechanism and the wave forms of the background events should be made concurrently.

@incollection{doi101029me004p0001,
    author = "Kanamori, Hiroo",
    title = "The Nature of Seismicity Patterns Before Large Earthquakes",
    year = "2011",
    booktitle = "Maurice Ewing series",
    abstract = "Various seismicity patterns before major earthquakes have been reported in the literature. They include foreshocks (broad sense), preseismic quiescence, precursory swarms, and doughnut patterns. Although many earthquakes are preceded by all, or some, of these patterns, their detail differ significantly from event to event. In order to examine the details of seismicity patterns on as uniform a basis as possible, we made space-time plots of seismicity for many large earthquakes by using the NOAA and JMA catalogs. Among various seismicity patterns, preseismic quiescence appears most common, the case for the 1978 Oaxaca earthquake being the most prominent. Although the nature of other patterns varies from event to event, a common physical mechanism may be responsible for these patterns; details of the pattern are probably controlled by the tectonic environment (fault geometry, strain rate) and the heterogeneity of the fault plane. Here a simple asperity model is introduced to explain these seismicity patterns. In this model, a fault plane with an asperity is divided into a number of subfaults. The subfaults within the asperity are, on the average, stronger than those in the surrounding weak zone. As the tectonic stress increases, the subfaults in the weak zone break in the form of background small earthquakes. If the frequency distribution of the strength of the subfaults has a sharp peak, a precursory swarm occurs. By this time, most of the subfaults in the weak zone are broken and the fault plane becomes seismically quiet. As the tectonic stress increases further, eventually the asperity breaks and sympathetic displacement occurs on the entire fault zone in the form of the main shock. Foreshocks do or do not occur depending upon the distribution of the strength of the subfaults within the asperity. Since the spatio-temporal change in the stress on the fault plane is most likely to dictate the change in seismicity patterns, detailed analysis of seismicity patterns would provide a most direct clue to the state of stress in the fault zone. However, because of the large variation from event to event, seismicity pattern alone is not a definitive tool for earthquake prediction; measurements of other physical parameters such as the spectra, the mechanism and the wave forms of the background events should be made concurrently.",
    url = "https://doi.org/10.1029/me004p0001",
    doi = "10.1029/me004p0001",
    openalex = "W1547242768"
}

We use a three‐dimensional strike‐slip fault model in the framework of rate and state‐dependent friction to investigate earthquake behavior and scaling relations on oceanic transform faults (OTFs). Gabbro friction data under hydrothermal conditions are mapped onto OTFs using temperatures from (1) a half‐space cooling model, and (2) a thermal model that incorporates a visco‐plastic rheology, non‐Newtonian viscous flow and the effects of shear heating and hydrothermal circulation. Without introducing small‐scale frictional heterogeneities on the fault, our model predicts that an OTF segment can transition between seismic and aseismic slip over many earthquake cycles, consistent with the multimode hypothesis for OTF ruptures. The average seismic coupling coefficient χ is strongly dependent on the ratio of seismogenic zone width W to earthquake nucleation size h *; χ increases by four orders of magnitude as W / h * increases from ∼1 to 2. Specifically, the average χ = 0.15 ± 0.05 derived from global OTF earthquake catalogs can be reached at W / h * ≈ 1.2–1.7. Further, in all simulations the area of the largest earthquake rupture is less than the total seismogenic area and we predict a deficiency of large earthquakes on long transforms, which is also consistent with observations. To match these observations over this narrow range of W / h * requires an increase in the characteristic slip distance d c as the seismogenic zone becomes wider and normal stress is higher on long transforms. Earthquake magnitude and distribution on the Gofar and Romanche transforms are better predicted by simulations using the visco‐plastic model than the half‐space cooling model.

@article{doi1010292011jb009025,
    author = "Liu, Yajing and McGuire, J. J. and Behn, M. D.",
    title = "Frictional behavior of oceanic transform faults and its influence on earthquake characteristics",
    year = "2012",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "We use a three‐dimensional strike‐slip fault model in the framework of rate and state‐dependent friction to investigate earthquake behavior and scaling relations on oceanic transform faults (OTFs). Gabbro friction data under hydrothermal conditions are mapped onto OTFs using temperatures from (1) a half‐space cooling model, and (2) a thermal model that incorporates a visco‐plastic rheology, non‐Newtonian viscous flow and the effects of shear heating and hydrothermal circulation. Without introducing small‐scale frictional heterogeneities on the fault, our model predicts that an OTF segment can transition between seismic and aseismic slip over many earthquake cycles, consistent with the multimode hypothesis for OTF ruptures. The average seismic coupling coefficient χ is strongly dependent on the ratio of seismogenic zone width W to earthquake nucleation size h *; χ increases by four orders of magnitude as W / h * increases from ∼1 to 2. Specifically, the average χ = 0.15 ± 0.05 derived from global OTF earthquake catalogs can be reached at W / h * ≈ 1.2–1.7. Further, in all simulations the area of the largest earthquake rupture is less than the total seismogenic area and we predict a deficiency of large earthquakes on long transforms, which is also consistent with observations. To match these observations over this narrow range of W / h * requires an increase in the characteristic slip distance d c as the seismogenic zone becomes wider and normal stress is higher on long transforms. Earthquake magnitude and distribution on the Gofar and Romanche transforms are better predicted by simulations using the visco‐plastic model than the half‐space cooling model.",
    url = "https://doi.org/10.1029/2011jb009025",
    doi = "10.1029/2011jb009025",
    openalex = "W2054816855",
    references = "doi1010292007jb005213"
}

Core-complex formation driven by lithospheric extension is a first-order process of heat and mass transfer in the Earth. Core-complex structures have been recognized in the continents, at slow- and ultraslow-spreading mid-ocean ridges, and at continental rifted margins; in each of these settings, extension has driven the exhumation of deep crust and/or upper mantle. The style of extension and the magnitude of core-complex exhumation are determined fundamentally by rheology: (1) Coupling between brittle and ductile layers regulates fault patterns in the brittle layer; and (2) viscosity of the flowing layer is controlled dominantly by the synextension geotherm and the presence or absence of melt. The pressure-temperature-time-fluid-deformation history of core complexes, investigated via field- and modeling-based studies, reveals the magnitude, rate, and mechanisms of advection of heat and material from deep to shallow levels, as well as the consequences for the chemical and physical evolution of the lithosphere, including the role of core-complex development in crustal differentiation, global element cycles, and ore formation. In this review, we provide a survey of ∼40 yr of core-complex literature, discuss processes and questions relevant to the formation and evolution of core complexes in continental and oceanic settings, highlight the significance of core complexes for lithosphere dynamics, and propose a few possible directions for future research.

@article{doi101130b307541,
    author = "Whitney, Donna L. and Teyssier, Christian and Rey, Patrice and Buck, W. Roger",
    title = "Continental and oceanic core complexes",
    year = "2012",
    journal = "Geological Society of America Bulletin",
    abstract = "Core-complex formation driven by lithospheric extension is a first-order process of heat and mass transfer in the Earth. Core-complex structures have been recognized in the continents, at slow- and ultraslow-spreading mid-ocean ridges, and at continental rifted margins; in each of these settings, extension has driven the exhumation of deep crust and/or upper mantle. The style of extension and the magnitude of core-complex exhumation are determined fundamentally by rheology: (1) Coupling between brittle and ductile layers regulates fault patterns in the brittle layer; and (2) viscosity of the flowing layer is controlled dominantly by the synextension geotherm and the presence or absence of melt. The pressure-temperature-time-fluid-deformation history of core complexes, investigated via field- and modeling-based studies, reveals the magnitude, rate, and mechanisms of advection of heat and material from deep to shallow levels, as well as the consequences for the chemical and physical evolution of the lithosphere, including the role of core-complex development in crustal differentiation, global element cycles, and ore formation. In this review, we provide a survey of ∼40 yr of core-complex literature, discuss processes and questions relevant to the formation and evolution of core complexes in continental and oceanic settings, highlight the significance of core complexes for lithosphere dynamics, and propose a few possible directions for future research.",
    url = "https://doi.org/10.1130/b30754.1",
    doi = "10.1130/b30754.1",
    openalex = "W2071527329",
    references = "doi101007bf00300398, doi10102992jb02221, doi101038nature03358, doi101038nature07333, doi101130spe233p1"
}

Abstract The 2015 M w 7.1 earthquake on the Charlie‐Gibbs transform fault along the Mid‐Atlantic Ridge is the latest in a series of seven large earthquakes since 1923. We propose that these earthquakes form a pair of quasi‐repeating sequences with the largest magnitudes and longest repeat times for such sequences observed to date. We model teleseismic body waves and find that the 2015 earthquake ruptured a distinct segment of the transform from the previous 1998 earthquake. The two events display similarities to earthquakes in 1974 and 1967, respectively. We observe large oceanic transform earthquakes to exhibit characteristic slip behavior, initiating with small slip near the ridge, and propagating unilaterally to significant slip asperities nearer the center of the transform. These slip distributions combined with apparent segmentation support multimode slip behavior with fault slip accommodated both seismically during large earthquakes and aseismically in between.

@article{doi1010022016gl068802,
    author = "Aderhold, Kasey and Abercrombie, Rachel E.",
    title = "The 2015 M w 7.1 earthquake on the Charlie‐Gibbs transform fault: Repeating earthquakes and multimodal slip on a slow oceanic transform",
    year = "2016",
    journal = "Geophysical Research Letters",
    abstract = "Abstract The 2015 M w 7.1 earthquake on the Charlie‐Gibbs transform fault along the Mid‐Atlantic Ridge is the latest in a series of seven large earthquakes since 1923. We propose that these earthquakes form a pair of quasi‐repeating sequences with the largest magnitudes and longest repeat times for such sequences observed to date. We model teleseismic body waves and find that the 2015 earthquake ruptured a distinct segment of the transform from the previous 1998 earthquake. The two events display similarities to earthquakes in 1974 and 1967, respectively. We observe large oceanic transform earthquakes to exhibit characteristic slip behavior, initiating with small slip near the ridge, and propagating unilaterally to significant slip asperities nearer the center of the transform. These slip distributions combined with apparent segmentation support multimode slip behavior with fault slip accommodated both seismically during large earthquakes and aseismically in between.",
    url = "https://doi.org/10.1002/2016gl068802",
    doi = "10.1002/2016gl068802",
    openalex = "W2405965957",
    references = "doi1010292007jb005213"
}

Slow earthquakes represent an important conundrum in earthquake physics. While regular earthquakes are catastrophic events with rupture velocities governed by elastic wave speed, the processes that underlie slow fault slip phenomena, including recent discoveries of tremor, slow-slip and low-frequency earthquakes, are less understood. Theoretical models and sparse laboratory observations have provided insights, but the physics of slow fault rupture remain enigmatic. Here we report on laboratory observations that illuminate the mechanics of slow-slip phenomena. We show that a spectrum of slow-slip behaviours arises near the threshold between stable and unstable failure, and is governed by frictional dynamics via the interplay of fault frictional properties, effective normal stress and the elastic stiffness of the surrounding material. This generalizable frictional mechanism may act in concert with other hypothesized processes that damp dynamic ruptures, and is consistent with the broad range of geologic environments where slow earthquakes are observed.

@article{doi101038ncomms11104,
    author = "Leeman, J. R. and Saffer, D. M. and Scuderi, Marco Maria and Marone, Chris",
    title = "Laboratory observations of slow earthquakes and the spectrum of tectonic fault slip modes",
    year = "2016",
    journal = "Nature Communications",
    abstract = "Slow earthquakes represent an important conundrum in earthquake physics. While regular earthquakes are catastrophic events with rupture velocities governed by elastic wave speed, the processes that underlie slow fault slip phenomena, including recent discoveries of tremor, slow-slip and low-frequency earthquakes, are less understood. Theoretical models and sparse laboratory observations have provided insights, but the physics of slow fault rupture remain enigmatic. Here we report on laboratory observations that illuminate the mechanics of slow-slip phenomena. We show that a spectrum of slow-slip behaviours arises near the threshold between stable and unstable failure, and is governed by frictional dynamics via the interplay of fault frictional properties, effective normal stress and the elastic stiffness of the surrounding material. This generalizable frictional mechanism may act in concert with other hypothesized processes that damp dynamic ruptures, and is consistent with the broad range of geologic environments where slow earthquakes are observed.",
    url = "https://doi.org/10.1038/ncomms11104",
    doi = "10.1038/ncomms11104",
    openalex = "W2315582328",
    references = "doi1010292007jb004930"
}

This essential reference for graduate students and researchers provides a unified treatment of earthquakes and faulting as two aspects of brittle tectonics at different timescales. The intimate connection between the two is manifested in their scaling laws and populations, which evolve from fracture growth and interactions between fractures. The connection between faults and the seismicity generated is governed by the rate and state dependent friction laws - producing distinctive seismic styles of faulting and a gamut of earthquake phenomena including aftershocks, afterslip, earthquake triggering, and slow slip events. The third edition of this classic treatise presents a wealth of new topics and new observations. These include slow earthquake phenomena; friction of phyllosilicates, and at high sliding velocities; fault structures; relative roles of strong and seismogenic versus weak and creeping faults; dynamic triggering of earthquakes; oceanic earthquakes; megathrust earthquakes in subduction zones; deep earthquakes; and new observations of earthquake precursory phenomena.

@book{doi1010179781316681473,
    author = "Scholz, Christopher H.",
    title = "The Mechanics of Earthquakes and Faulting",
    year = "2018",
    booktitle = "Cambridge University Press eBooks",
    abstract = "This essential reference for graduate students and researchers provides a unified treatment of earthquakes and faulting as two aspects of brittle tectonics at different timescales. The intimate connection between the two is manifested in their scaling laws and populations, which evolve from fracture growth and interactions between fractures. The connection between faults and the seismicity generated is governed by the rate and state dependent friction laws - producing distinctive seismic styles of faulting and a gamut of earthquake phenomena including aftershocks, afterslip, earthquake triggering, and slow slip events. The third edition of this classic treatise presents a wealth of new topics and new observations. These include slow earthquake phenomena; friction of phyllosilicates, and at high sliding velocities; fault structures; relative roles of strong and seismogenic versus weak and creeping faults; dynamic triggering of earthquakes; oceanic earthquakes; megathrust earthquakes in subduction zones; deep earthquakes; and new observations of earthquake precursory phenomena.",
    url = "https://doi.org/10.1017/9781316681473",
    doi = "10.1017/9781316681473",
    openalex = "W4211212742",
    references = "doi1010160040195183901488, doi1010160191814184900014, doi1010160191814188900570, doi101016s0012821x03004242, doi101016s019181410200161x, doi1010291998rg900002, doi1010292005jb004051, doi1010292007jb004930, doi1010292007jb005213, doi10102992jb00132, doi10102992jb00517, doi10102995jb00862, doi10102996jb01651, doi101029jb076i026p06414, doi101029jb082i020p02981, doi101029jb088ib02p01153, doi101029jb089ib06p04344, doi101029jb091ib12p12587, doi101029jb092ib06p04798, doi101029jb093ib08p09027, doi101029jz070i016p03965, doi101029me001, doi101029rg016i004p00621, doi101029tc007i003p00663, doi101038284135a0, doi101038334058a0, doi101038nature03358, doi101038nature07333, doi101046j1365246x200201720x, doi101126science19142331230, doi101130001676061977881667dawtmo20co2, doi101144transed83387, doi101785bssa0350040175, openalexw191472345"
}

The strong tidal triggering of mid-ocean ridge earthquakes has remained unexplained because the earthquakes occur preferentially during low tide, when normal faulting earthquakes should be inhibited. Using Axial Volcano on the Juan de Fuca ridge as an example, we show that the axial magma chamber inflates/deflates in response to tidal stresses, producing Coulomb stresses on the faults that are opposite in sign to those produced by the tides. When the magma chamber’s bulk modulus is sufficiently low, the phase of tidal triggering is inverted. We find that the stress dependence of seismicity rate conforms to triggering theory over the entire tidal stress range. There is no triggering stress threshold and stress shadowing is just a continuous function of stress decrease. We find the viscous friction parameter A to be an order of magnitude smaller than laboratory measurements. The high tidal sensitivity at Axial Volcano results from the shallow earthquake depths.

@article{scholz2019the,
    author = "Scholz, Christopher H. and Tan, Yen Joe and Albino, Fabien",
    title = "The mechanism of tidal triggering of earthquakes at mid-ocean ridges",
    year = "2019",
    journal = "Nature Communications",
    abstract = "The strong tidal triggering of mid-ocean ridge earthquakes has remained unexplained because the earthquakes occur preferentially during low tide, when normal faulting earthquakes should be inhibited. Using Axial Volcano on the Juan de Fuca ridge as an example, we show that the axial magma chamber inflates/deflates in response to tidal stresses, producing Coulomb stresses on the faults that are opposite in sign to those produced by the tides. When the magma chamber’s bulk modulus is sufficiently low, the phase of tidal triggering is inverted. We find that the stress dependence of seismicity rate conforms to triggering theory over the entire tidal stress range. There is no triggering stress threshold and stress shadowing is just a continuous function of stress decrease. We find the viscous friction parameter A to be an order of magnitude smaller than laboratory measurements. The high tidal sensitivity at Axial Volcano results from the shallow earthquake depths.",
    url = "https://doi.org/10.1038/s41467-019-10605-2",
    doi = "10.1038/s41467-019-10605-2",
    number = "1",
    openalex = "W2902069436",
    volume = "10",
    references = "doi1010292002jb002321, doi1010292011rg000382, doi10102993jb02581, doi10102998jb00765, doi101029jb089ib06p04077, doi10103845144, doi10108800344885678r03, doi101126science26051141617, doi1011751520042620020190183eimobo20co2, doi1017850119990114"
}

@article{khutorskoi2020nature,
    author = "Khutorskoi, M. D. and Teveleva, E. A.",
    title = "Nature of Heat Flow Asymmetry on the Mid-Oceanic Ridges of the World Ocean",
    year = "2020",
    journal = "Oceanology",
    url = "https://doi.org/10.1134/s0001437020010142",
    doi = "10.1134/s0001437020010142",
    number = "1",
    openalex = "W3033921037",
    pages = "108-119",
    volume = "60",
    references = "doi101007978366205382916, doi101007bf00310065, doi101023a1004312623534, doi1010291999jb900195, doi101029jb082i005p00803, doi101029jb082i023p03391, doi101029jz068i014p04219, doi101126science2044395828, doi101134s0016852117010022, openalexw3085247966"
}

Abstract Oceanic transform faults accommodate plate motions through both seismic and aseismic slips. However, deformation partition and slip mode interaction at these faults remain elusive mainly limited by rare observations. We use 1‐year ocean bottom seismometer data collected in 2008 to detect and locate earthquakes at the westernmost Gofar transform fault. The ultra‐fast slipping rate of Gofar results in ∼30,000 earthquakes during the observational period, providing an excellent opportunity to investigate interrelations between the slip mode, seismicity, and fault architecture at an unprecedented resolution. Earthquake distribution indicates that the ∼100‐km‐long Gofar transform fault is distinctly segmented into five zones, including one zone contouring a M6 earthquake that was captured by the experiment. Further, a barrier zone east of the M6 earthquake hosted abundant foreshocks preceding the M6 event and halted its active seismicity afterward. The barrier zone has two layers of earthquakes at depth, and they responded to the M6 earthquake differently. Additionally, a zone connecting to the East Pacific Rise had quasi‐periodic earthquake swarms. The seismicity segmentation suggests that the Gofar fault has multiple slip modes occurring in adjacent fault patches. Spatiotemporal characteristics of the earthquakes suggest that complex fault architecture and fluid–rock interaction play primary roles in modulating the slip modes at Gofar, possibly involving multiple concurrent physical processes.

@article{doi1010292022jb024918,
    author = "Gong, J. and Fan, Wenyuan",
    title = "Seismicity, Fault Architecture, and Slip Mode of the Westernmost Gofar Transform Fault",
    year = "2022",
    journal = "Journal of Geophysical Research Solid Earth",
    abstract = "Abstract Oceanic transform faults accommodate plate motions through both seismic and aseismic slips. However, deformation partition and slip mode interaction at these faults remain elusive mainly limited by rare observations. We use 1‐year ocean bottom seismometer data collected in 2008 to detect and locate earthquakes at the westernmost Gofar transform fault. The ultra‐fast slipping rate of Gofar results in ∼30,000 earthquakes during the observational period, providing an excellent opportunity to investigate interrelations between the slip mode, seismicity, and fault architecture at an unprecedented resolution. Earthquake distribution indicates that the ∼100‐km‐long Gofar transform fault is distinctly segmented into five zones, including one zone contouring a M6 earthquake that was captured by the experiment. Further, a barrier zone east of the M6 earthquake hosted abundant foreshocks preceding the M6 event and halted its active seismicity afterward. The barrier zone has two layers of earthquakes at depth, and they responded to the M6 earthquake differently. Additionally, a zone connecting to the East Pacific Rise had quasi‐periodic earthquake swarms. The seismicity segmentation suggests that the Gofar fault has multiple slip modes occurring in adjacent fault patches. Spatiotemporal characteristics of the earthquakes suggest that complex fault architecture and fluid–rock interaction play primary roles in modulating the slip modes at Gofar, possibly involving multiple concurrent physical processes.",
    url = "https://doi.org/10.1029/2022jb024918",
    doi = "10.1029/2022jb024918",
    openalex = "W4307896081",
    references = "scholz2019the"
}

• Bedrock mass wasting on abyssal hills shapes scarp morphology from fault inception. • Earthquakes are a key driver of bedrock mass wasting on abyssal hills. • Ratio of mass removal efficiency to uplift rate is similar across spreading rates. • Global earthquake-driven mass flux for abyssal hills between 24 and 1428 m m 3 yr −1. Fault-bound abyssal hills form at mid-ocean ridges and cover ∼65 % of Earth's surface, but few studies have characterized the extent to which bedrock erosion controls their morphology. Here, we use bathymetry data to characterize the morphology of fault-bound abyssal hills on a global scale, and employ numerical modelling and seismicity catalogues to quantify how simultaneous rock uplift and bedrock erosion sculpt deep-ocean landscapes. By generating a global database on abyssal hill morphology, we show that most large abyssal-hill scarps (>100 m in height) within the near-axis zone of seismicity (i.e., <30 km from axis) have slopes between 10 and 30°, well below the expected range of underlying normal fault dips of 45–60°. We interpret this as a manifestation of efficient bedrock mass wasting on near-axis growing faults, a process that operates from fault inception. Using a non-linear topographic diffusion model to parameterise the effects of erosion, we find a balance between erosion and rock uplift that is similar for slow, intermediate, and fast spreading rates. We express the ratio of erosion to uplift as an inverse Peclet number that ranges between 0.06 and 0.82 for abyssal hills. We also calculate a global bedrock diffusivity for abyssal hills in the range 0.01–1.51 m 2 yr −1. These results imply that bedrock erosion is a significant process that sculpts abyssal hill morphology and reshapes the oceanic crust. Overall, this study provides a framework to incorporate bedrock mass wasting into future models of ocean-floor evolution and, more generally, to active extensional settings on Earth and beyond.

@article{doi101016jepsl2024119073,
    author = "Hughes, Alex and Olive, Jean‐Arthur and Malatesta, Luca C. and Escartı́n, J.",
    title = "Characterization of bedrock mass-wasting at fault-bound abyssal hills",
    year = "2024",
    journal = "Earth and Planetary Science Letters",
    abstract = "• Bedrock mass wasting on abyssal hills shapes scarp morphology from fault inception. • Earthquakes are a key driver of bedrock mass wasting on abyssal hills. • Ratio of mass removal efficiency to uplift rate is similar across spreading rates. • Global earthquake-driven mass flux for abyssal hills between 24 and 1428 m m 3 yr −1. Fault-bound abyssal hills form at mid-ocean ridges and cover ∼65 \% of Earth's surface, but few studies have characterized the extent to which bedrock erosion controls their morphology. Here, we use bathymetry data to characterize the morphology of fault-bound abyssal hills on a global scale, and employ numerical modelling and seismicity catalogues to quantify how simultaneous rock uplift and bedrock erosion sculpt deep-ocean landscapes. By generating a global database on abyssal hill morphology, we show that most large abyssal-hill scarps (>100 m in height) within the near-axis zone of seismicity (i.e., <30 km from axis) have slopes between 10 and 30°, well below the expected range of underlying normal fault dips of 45–60°. We interpret this as a manifestation of efficient bedrock mass wasting on near-axis growing faults, a process that operates from fault inception. Using a non-linear topographic diffusion model to parameterise the effects of erosion, we find a balance between erosion and rock uplift that is similar for slow, intermediate, and fast spreading rates. We express the ratio of erosion to uplift as an inverse Peclet number that ranges between 0.06 and 0.82 for abyssal hills. We also calculate a global bedrock diffusivity for abyssal hills in the range 0.01–1.51 m 2 yr −1. These results imply that bedrock erosion is a significant process that sculpts abyssal hill morphology and reshapes the oceanic crust. Overall, this study provides a framework to incorporate bedrock mass wasting into future models of ocean-floor evolution and, more generally, to active extensional settings on Earth and beyond.",
    url = "https://doi.org/10.1016/j.epsl.2024.119073",
    doi = "10.1016/j.epsl.2024.119073",
    openalex = "W4403854702",
    references = "doi101016jepsl201102005, doi101016s0267726199000123, doi1010291998wr900090, doi1010292001gc000252, doi1010292008gc002332, doi101029jb094ib10p13919, doi101038nature02128, doi101038nature03358, doi101038nature07333, doi101038s4158602407247w, doi101785gssrl68194"
}

Mid-ocean ridges (MORs) are quintessential sites of tectonic extension1-4, at which divergence between lithospheric plates shapes abyssal hills that cover about two-thirds of the Earth's surface5,6. Here we show that tectonic extension at the ridge axis can be partially undone by tectonic shortening across the ridge flanks. This process is evidenced by recent sequences of reverse-faulting earthquakes about 15 km off-axis at the Mid-Atlantic Ridge and Carlsberg Ridge. Using mechanical models, we show that shallow compression of the ridge flanks up to the brittle failure point is a natural consequence of lithosphere unbending away from the axial relief. Intrusion of magma-filled fractures, which manifests as migrating swarms of extensional seismicity along the ridge axis, can provide the small increment of compressive stress that triggers reverse-faulting earthquakes. Through bathymetric analyses, we further find that reverse reactivation of MOR normal faults is a widely occurring process that can reduce the amplitude of abyssal hills by as much as 50%, shortly after they form at the ridge axis. This 'unfaulting' mechanism exerts a first-order influence on the fabric of the global ocean floor and provides a physical explanation for reverse-faulting earthquakes in an extensional environment.

@article{doi101038s4158602407247w,
    author = "Olive, Jean-Arthur and Ekström, Göran and Buck, W Roger and Liu, Zhonglan and Escartín, Javier and Bickert, Manon",
    title = "Mid-ocean ridge unfaulting revealed by magmatic intrusions.",
    year = "2024",
    journal = "Nature",
    abstract = "Mid-ocean ridges (MORs) are quintessential sites of tectonic extension1-4, at which divergence between lithospheric plates shapes abyssal hills that cover about two-thirds of the Earth's surface5,6. Here we show that tectonic extension at the ridge axis can be partially undone by tectonic shortening across the ridge flanks. This process is evidenced by recent sequences of reverse-faulting earthquakes about 15 km off-axis at the Mid-Atlantic Ridge and Carlsberg Ridge. Using mechanical models, we show that shallow compression of the ridge flanks up to the brittle failure point is a natural consequence of lithosphere unbending away from the axial relief. Intrusion of magma-filled fractures, which manifests as migrating swarms of extensional seismicity along the ridge axis, can provide the small increment of compressive stress that triggers reverse-faulting earthquakes. Through bathymetric analyses, we further find that reverse reactivation of MOR normal faults is a widely occurring process that can reduce the amplitude of abyssal hills by as much as 50\%, shortly after they form at the ridge axis. This 'unfaulting' mechanism exerts a first-order influence on the fabric of the global ocean floor and provides a physical explanation for reverse-faulting earthquakes in an extensional environment.",
    url = "https://pubmed.ncbi.nlm.nih.gov/38600388/",
    doi = "10.1038/s41586-024-07247-w",
    openalex = "W4393350915",
    pmid = "38600388",
    references = "doi101007bf00538368, doi101016jpepi201204002, doi101017cbo9780511807442, doi1010292007jb004930, doi10102997jb02671, doi101029jb086ib04p02825, doi101038nature03358, doi101038nature07333, doi101146annurevea10050182001103, sykes1967mechanism"
}

Large-offset detachment faults are commonly observed at slow-spreading mid-ocean ridges (MORs), typically in areas with a moderate to low magma supply (e.g., 13&#186;20'N on the Mid-Atlantic Ridge). Detachments are also found at nearly amagmatic sections of ultraslow MORs (e.g., 64&#186;E on the Southwest Indian Ridge), where the seismogenic lithosphere is unusually thick (> 15 km). There, detachments of opposing polarity form in sequence and cross-cut each other in a "flip-flop" regime. Prior studies have shown that marked strength contrasts, resulting from reduced cohesion and/or friction in fault zones, promote stable detachments. Here we present 2-D thermo-mechanical models based on geological observations to examine how strength contrasts between fault zones and the adjacent lithosphere impact the modes of faulting at an ultraslow and nearly amagmatic ridge axis.We model the brittle lithosphere as a Mohr-Coulomb elasto-plastic material, where cohesion and friction diminish with increasing plastic strain. We explore a broad range of cohesion and friction contrasts between deformed and intact material. We also consider the influence of a strong, viscous lower lithosphere on the brittle deformation of the upper lithosphere by comparing simulations that use a dry olivine flow law with models where the brittle lithosphere sharply transitions into a low-viscosity asthenosphere. Fluid circulation in the shallow axial lithosphere is also considered, parameterizing both the cooling and the mechanical effect of hydrothermal circulation.Our simulations produce three distinct regimes: (1) sequential development of horsts bound by two active antithetic faults, (2) formation of intersecting &#8220;flip-flopping&#8221; detachments, (3) runaway detachments. The latter case describes models in which a single detachment remains active. In nature, this endmember case is not observed, probably because it results in an excessive migration of the detachment toward its hanging wall. We show that these 3 regimes transition over a narrow range of cohesion and friction contrasts between deformed and intact material (the contrast in friction coefficient over which our simulations transition from regimes 1 to 3 is only 0.1- 0.2). Distributed footwall damage produces antithetic proto-faults, but their ability to mature as major seafloor-breaching faults depends on the degree of rheological weakening. A stronger lower lithosphere promotes such distributed faulting and modifies the onset of the persistent detachment regime to greater strength contrasts. The impact of hydrostatic fluid pressure on tectonic styles is relatively minor compared to fault weakening.The results of these simulations are consistent with an analytical force balance model that compares the (localizing) loss of fault strength in the detachments to the (delocalizing) flexural force that develops in the surrounding lithosphere. Detachments persist when the magnitude of fault strength loss exceeds the maximum bending force. We find that runaway detachments require a total loss of integrated strength in excess of 1.5e12 N.m, equivalent in our models to a drop in friction coefficient by \textasciitilde 0.25&#8211;0.3 in fault zones. Thus, even a moderate frictional weakening, such as that allowed by the presence of lizardite in the fault zone (frictional strength of 0.45) enables large-offset (>15 km) faulting.

@misc{demont2025modes,
    author = "Demont, Antoine and Cannat, Mathilde and Olive, Jean-Arthur",
    title = "Modes of detachment faulting at slow and ultraslow mid-oceanic ridges",
    year = "2025",
    abstract = {Large-offset detachment faults are commonly observed at slow-spreading mid-ocean ridges (MORs), typically in areas with a moderate to low magma supply (e.g., 13\&\#186;20'N on the Mid-Atlantic Ridge). Detachments are also found at nearly amagmatic sections of ultraslow MORs (e.g., 64\&\#186;E on the Southwest Indian Ridge), where the seismogenic lithosphere is unusually thick (> 15 km). There, detachments of opposing polarity form in sequence and cross-cut each other in a "flip-flop" regime. Prior studies have shown that marked strength contrasts, resulting from reduced cohesion and/or friction in fault zones, promote stable detachments. Here we present 2-D thermo-mechanical models based on geological observations to examine how strength contrasts between fault zones and the adjacent lithosphere impact the modes of faulting at an ultraslow and nearly amagmatic ridge axis.We model the brittle lithosphere as a Mohr-Coulomb elasto-plastic material, where cohesion and friction diminish with increasing plastic strain. We explore a broad range of cohesion and friction contrasts between deformed and intact material. We also consider the influence of a strong, viscous lower lithosphere on the brittle deformation of the upper lithosphere by comparing simulations that use a dry olivine flow law with models where the brittle lithosphere sharply transitions into a low-viscosity asthenosphere. Fluid circulation in the shallow axial lithosphere is also considered, parameterizing both the cooling and the mechanical effect of hydrothermal circulation.Our simulations produce three distinct regimes: (1) sequential development of horsts bound by two active antithetic faults, (2) formation of intersecting \&\#8220;flip-flopping\&\#8221; detachments, (3) runaway detachments. The latter case describes models in which a single detachment remains active. In nature, this endmember case is not observed, probably because it results in an excessive migration of the detachment toward its hanging wall. We show that these 3 regimes transition over a narrow range of cohesion and friction contrasts between deformed and intact material (the contrast in friction coefficient over which our simulations transition from regimes 1 to 3 is only 0.1- 0.2). Distributed footwall damage produces antithetic proto-faults, but their ability to mature as major seafloor-breaching faults depends on the degree of rheological weakening. A stronger lower lithosphere promotes such distributed faulting and modifies the onset of the persistent detachment regime to greater strength contrasts. The impact of hydrostatic fluid pressure on tectonic styles is relatively minor compared to fault weakening.The results of these simulations are consistent with an analytical force balance model that compares the (localizing) loss of fault strength in the detachments to the (delocalizing) flexural force that develops in the surrounding lithosphere. Detachments persist when the magnitude of fault strength loss exceeds the maximum bending force. We find that runaway detachments require a total loss of integrated strength in excess of 1.5e12 N.m, equivalent in our models to a drop in friction coefficient by \textasciitilde 0.25\&\#8211;0.3 in fault zones. Thus, even a moderate frictional weakening, such as that allowed by the presence of lizardite in the fault zone (frictional strength of 0.45) enables large-offset (>15 km) faulting.},
    url = "https://doi.org/10.5194/egusphere-egu24-15593",
    doi = "10.5194/egusphere-egu24-15593",
    openalex = "W4392624399"
}

Abstract The most accepted theory for the formation of the Canada Basin is that it was created during 66° rotation of Arctic Alaska around the Euler pole located near the Mackenzie Delta sometime during the Mesozoic. Gravity and magnetic anomaly data are consistent with an extinct mid‐oceanic ridge (MOR) in the central basin. This extinct MOR is critical to understand the development of the Canada Basin, but it is not well mapped due to ice conditions and is buried by thick sediment. The objective of this study is to map the ridge structure and gather all the available data to establish the role of the MOR in the history of the Amerasia Basin. We acquired multichannel seismic reflection (MCS) from RV Sikuliaq in 2021 across the Canada Basin between ∼75.5°N to 77°N. We combined our MCS data with the data collected in 2007–2011 from the IB Louis S. St. Laurent to generate a basement map of the Canada Basin. On MCS profiles, we observed the rugged axial topography of the inferred fossil spreading ridge. The MCS profiles parallel to this feature reveal the unsignificant broken basement morphology, which suggests that this ridge is not surely segmented by transform faults. The basement map is consistent with the continuous linear feature we interpret as an extinct MOR. The ridge morphology and tectonic setting are similar to the ultra‐slow spreading Gakkel Ridge. If so, assuming a 1–2 cm/yr spreading rate, forming a 300 km wide strip of oceanic crust in the central basin would require between 15 and 30 My.

@article{doi1010292024gc012023,
    author = "Priyanto, W. S. and Coakley, B. J.",
    title = "Seismic Insight on Basement Structure of the Extinct Mid‐Oceanic Ridge in Canada Basin, Arctic Ocean",
    year = "2025",
    journal = "Geochemistry Geophysics Geosystems",
    abstract = "Abstract The most accepted theory for the formation of the Canada Basin is that it was created during 66° rotation of Arctic Alaska around the Euler pole located near the Mackenzie Delta sometime during the Mesozoic. Gravity and magnetic anomaly data are consistent with an extinct mid‐oceanic ridge (MOR) in the central basin. This extinct MOR is critical to understand the development of the Canada Basin, but it is not well mapped due to ice conditions and is buried by thick sediment. The objective of this study is to map the ridge structure and gather all the available data to establish the role of the MOR in the history of the Amerasia Basin. We acquired multichannel seismic reflection (MCS) from RV Sikuliaq in 2021 across the Canada Basin between ∼75.5°N to 77°N. We combined our MCS data with the data collected in 2007–2011 from the IB Louis S. St. Laurent to generate a basement map of the Canada Basin. On MCS profiles, we observed the rugged axial topography of the inferred fossil spreading ridge. The MCS profiles parallel to this feature reveal the unsignificant broken basement morphology, which suggests that this ridge is not surely segmented by transform faults. The basement map is consistent with the continuous linear feature we interpret as an extinct MOR. The ridge morphology and tectonic setting are similar to the ultra‐slow spreading Gakkel Ridge. If so, assuming a 1–2 cm/yr spreading rate, forming a 300 km wide strip of oceanic crust in the central basin would require between 15 and 30 My.",
    url = "https://doi.org/10.1029/2024gc012023",
    doi = "10.1029/2024gc012023",
    openalex = "W7108474827",
    references = "doi1010029781118782149ch1, doi101016jepsl201212013, doi101016jtecto201608005, doi10102992jb02221, doi101038nature01704, doi101038nature02128, doi101038nature05105, doi101038nature07333, doi101144m353, doi10119011442837, doi10119019781560801580, sykes1972mechanism"
}

The lavas–dykes transition in oceanic crust represents a critical boundary between two portions of the crust with different lithology, structure, porosity, permeability and other physical properties. We refined the structural features of the lavas–dykes transition in ODP/IODP Hole 1256D and DSDP/ODP Hole 504B drilled in in situ oceanic crust created at superfast- and intermediate-spreading rate in the Pacific Ocean. Both holes, although showing some differences in the thickness of lithological sections, have a comparable lavas–dykes transition zone characterized by a wide diffusion of veins, breccias, vein network and cataclasites, possibly with development of pseudotachylytes. Fracturing started with fissuring of basalt already in the ridge axis domain due to thermal contraction of lavas. Fissures triggered the circulation of seawater from above and hydrothermal fluids from below, enhancing alteration of the crust and the mechanical response of the basaltic crust near the lavas–dykes boundary. The high concentration of permeable structures including cataclasites in the lavas–dykes transition in the two holes favour interpreting this crustal interval as a (large-scale?) mechanically weak zone, possibly a fault zone at least for the intermediate-spreading ridge of Hole 504B. The lack of clear kinematic indicators prevents a conclusive interpretation. Other processes that may have led to fracturing include fluid overpressure and dyking. These processes are not mutually exclusive: intense fluid circulation concentrated in the transition zone is responsible for alteration inducing a dramatic weakening of the rock; conversely, rock fissuring due to thermal cracks, dyking and eventually faulting developed permeable structures that may channel the circulation of fluids. Cataclasites occurring at a shallower depth in Hole 1256D and at deeper depth in Hole 504B with respect to those described so far suggest that the extension of the lavas–dykes boundary should be revised by taking into consideration not only the change in lithology but also the structural features. In addition, our finding of chlorite and amphibole at shallower depths with respect to those described so far suggests that the alteration boundary across the lavas–dykes transition is thicker than previously thought, allowing the upflow of high-temperature hydrothermal fluids to shallower levels.

@article{doi101144jgs2024117,
    author = "Tartarotti, Paola and Crispini, Laura and Moroni, M. and Proscia, Alessandro and Tondo, Melissa",
    title = "Structural control on fluid circulation and alteration patterns in the lavas–dykes transition zone of the superfast- and intermediate-spreading oceanic crust, equatorial Pacific Ocean",
    year = "2025",
    journal = "Journal of the Geological Society",
    abstract = "The lavas–dykes transition in oceanic crust represents a critical boundary between two portions of the crust with different lithology, structure, porosity, permeability and other physical properties. We refined the structural features of the lavas–dykes transition in ODP/IODP Hole 1256D and DSDP/ODP Hole 504B drilled in in situ oceanic crust created at superfast- and intermediate-spreading rate in the Pacific Ocean. Both holes, although showing some differences in the thickness of lithological sections, have a comparable lavas–dykes transition zone characterized by a wide diffusion of veins, breccias, vein network and cataclasites, possibly with development of pseudotachylytes. Fracturing started with fissuring of basalt already in the ridge axis domain due to thermal contraction of lavas. Fissures triggered the circulation of seawater from above and hydrothermal fluids from below, enhancing alteration of the crust and the mechanical response of the basaltic crust near the lavas–dykes boundary. The high concentration of permeable structures including cataclasites in the lavas–dykes transition in the two holes favour interpreting this crustal interval as a (large-scale?) mechanically weak zone, possibly a fault zone at least for the intermediate-spreading ridge of Hole 504B. The lack of clear kinematic indicators prevents a conclusive interpretation. Other processes that may have led to fracturing include fluid overpressure and dyking. These processes are not mutually exclusive: intense fluid circulation concentrated in the transition zone is responsible for alteration inducing a dramatic weakening of the rock; conversely, rock fissuring due to thermal cracks, dyking and eventually faulting developed permeable structures that may channel the circulation of fluids. Cataclasites occurring at a shallower depth in Hole 1256D and at deeper depth in Hole 504B with respect to those described so far suggest that the extension of the lavas–dykes boundary should be revised by taking into consideration not only the change in lithology but also the structural features. In addition, our finding of chlorite and amphibole at shallower depths with respect to those described so far suggests that the alteration boundary across the lavas–dykes transition is thicker than previously thought, allowing the upflow of high-temperature hydrothermal fluids to shallower levels.",
    url = "https://doi.org/10.1144/jgs2024-117",
    doi = "10.1144/jgs2024-117",
    openalex = "W4412037157",
    references = "doi101016jepsl2024119116, doi101016jtecto201807007, doi1010292008gc002332, doi101038nature03358, doi101038s4158602407247w, doi101111j1365246x1974tb05468x, doi1011300016760619881001181piujot23co2, doi101144gsjgs13330191, doi101180claymin1988023413, doi101680geot1965153287, doi102138am199891004, doi102138am20103371"
}

SUMMARY Oceanic transform faults (OTFs) have long been viewed exclusively as vertical, strike-slip structures offsetting mid-ocean ridges, yet their deep geometry and structural complexity remain poorly constrained. Thus, key questions persist, including whether OTFs are single-stranded and continuous, whether they maintain vertical dip angles, if they accommodate mixed-mode slip, and what factors control their geometry. This study addresses these questions through a global statistical analysis of teleseismic earthquake focal mechanisms from 150 OTFs across diverse tectonic settings. We introduce stack maps, a novel method that quantifies fault dip and rake, providing a graphical representation of average focal mechanisms. Our findings reveal that while OTFs tend to conform to the standard vertical, strike-slip model, nearly half exhibit deviations, either in dip or motion, challenging the classical view of these plate boundaries. We identify four distinct OTF categories: (1) those adhering to the standard model, (2) non-vertical faults with transtensive/transpressive components, (3) non-vertical faults accommodating strike-slip motion and (4) vertical faults with a vertical component of motion. Tectonic regime shifts emerge as a primary driver of structural changes, with non-vertical geometries persisting even after the regime reverts to pure strike-slip motion. This structural memory suggests that fault geometry, once established, remains stable over geological timescales of several tens of Myr. By reconciling previously ‘unusual’ focal mechanisms with fault structure and dynamics, this work demonstrates that global seismic catalogues, when analysed statistically, offer robust insights into OTF geometry and tectonic regimes.

@article{doi101093gjiggag078,
    author = "Janin, Alexandre and Behn, M. D. and Tian, Xiaochuan",
    title = "Geometry, structure and tectonic regime of oceanic transform faults revealed by teleseismic earthquake focal mechanisms",
    year = "2026",
    journal = "Geophysical Journal International",
    abstract = "SUMMARY Oceanic transform faults (OTFs) have long been viewed exclusively as vertical, strike-slip structures offsetting mid-ocean ridges, yet their deep geometry and structural complexity remain poorly constrained. Thus, key questions persist, including whether OTFs are single-stranded and continuous, whether they maintain vertical dip angles, if they accommodate mixed-mode slip, and what factors control their geometry. This study addresses these questions through a global statistical analysis of teleseismic earthquake focal mechanisms from 150 OTFs across diverse tectonic settings. We introduce stack maps, a novel method that quantifies fault dip and rake, providing a graphical representation of average focal mechanisms. Our findings reveal that while OTFs tend to conform to the standard vertical, strike-slip model, nearly half exhibit deviations, either in dip or motion, challenging the classical view of these plate boundaries. We identify four distinct OTF categories: (1) those adhering to the standard model, (2) non-vertical faults with transtensive/transpressive components, (3) non-vertical faults accommodating strike-slip motion and (4) vertical faults with a vertical component of motion. Tectonic regime shifts emerge as a primary driver of structural changes, with non-vertical geometries persisting even after the regime reverts to pure strike-slip motion. This structural memory suggests that fault geometry, once established, remains stable over geological timescales of several tens of Myr. By reconciling previously ‘unusual’ focal mechanisms with fault structure and dynamics, this work demonstrates that global seismic catalogues, when analysed statistically, offer robust insights into OTF geometry and tectonic regimes.",
    url = "https://doi.org/10.1093/gji/ggag078",
    doi = "10.1093/gji/ggag078",
    openalex = "W7131092593",
    references = "doi101093gjiggae446"
}