Seismicity and tectonics of the subducted Cocos Plate

We have examined teleseismic earthquake locations reported by the International Seismological Centre (ISC) for the Middle America region and selected 220 as the most reliable. These hypocenters and other data are used to delineate the deep structure of the subducted Cocos Plate. The results indicate that the subducted plate consists of three major segments: Segment I extends from the Panama Fracture Zone to the Nicoya Peninsula. The structure of this segment is poorly defined. Segment II is the largest and best-defined segment. This segment consists of two parts, IIA and IIB. Part IIA extends from the Nicoya Peninsula to western Guatemala and is very well defined and continuous in structure. Its strike follows the curvature of the trench and dips at about 60°. Part IIB extends from western Guatemala to Orizaba, Mexico. The dip of this part of the segment decreases slightly toward the northwest, and its strike is more northward than that of the trench. Segment III extends from Orizaba to the Rivera Fracture Zone, and is not well defined due to a lack of earthquake activity beneath about 100 km. Its orientation differs markedly from segment II and strikes somewhat more westward than the trench. Between parts IIA and IIB of segment II the subducted plate seems to be continuous, bending smoothly to accommodate the change in geometry. Local network data from Costa Rica suggest there may be a tear between segments I and II. Between segments II and III there is a gap in the hypocenters which makes it difficult to define the boundary. The change in geometry between these two segments indicates that there may be a tear, and two strike-slip focal mechanisms in the region support this conclusion. We find no convincing evidence supporting the existence of segments smaller than the three described above. If there is smaller-scale segmentation in the shallow part of the subducting plate the plate must still maintain enough continuity to appear continuous at greater depths. There is no evidence for any major tear in the subducted plate associated directly with either the Tehuantepec Ridge or the Orozco Fracture zone. The shallow subduction at the northwestern end of segment II may be related to the bouyancy of the Tehuantepec Ridge. The Cocos Ridge is probably directly responsible for the change in geometry between segments I and II and may even be slowing or stopping subduction in segment I. The structure of the subducted plate in segment II and the changes in the character of volcanism along the arc can be related to the relative motion of the North American and Caribbean Plates. The present geometry of part IIB of segment II is more consistent with the probable configuration of the trench about 7 Ma ago than with the present configuration, indicating that the North American plate is overriding the subduction zone. Appendices 2, 3, and 4 are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B84-009; $2.50.

[1]  M. Bevis,et al.  Hypocentral trend surface analysis: Probing the geometry of Benioff Zones , 1984 .

[2]  W. D. Pennington.  The effect of oceanic crustal structure on phase changes and subduction , 1984 .

[3]  B. Burkart.  Neogene North American-Caribbean plate boundary across northern Central America: offset along the Polochic fault , 1983 .

[4]  A. Kafka,et al.  Motion of Caribbean Plate during last 7 million years and implications for earlier Cenozoic movements , 1982 .

[5]  E. R. Engdahl,et al.  Detection and location of earthquakes in the Central Aleutian Subduction Zone using island and ocean bottom seismograph stations , 1982 .

[6]  K. Klitgord,et al.  Northern East Pacific Rise' Magnetic Anomaly and Bathymetric Framework , 1982 .

[7]  J. Stephan,et al.  The middle america trench as an example of a subduction zone , 1982 .

[8]  R. Pilger,et al.  Controls of subduction geometry, location of magmatic arcs, and tectonics of arc and back-arc regions , 1982 .

[9]  G. Nixon.  The relationship between Quaternary volcanism in central Mexico and the seismicity and structure of subducted ocean lithosphere , 1982 .

[10]  Seiya Uyeda,et al.  Subduction zones: An introduction to comparative subductology , 1982 .

[11]  E. Chael,et al.  Recent large earthquakes along the Middle American Trench and their implications for the subduction process , 1982 .

[12]  W. D. Pennington.  Subduction of the Eastern Panama Basin and seismotectonics of northwestern South America , 1981 .

[13]  J. Curiale,et al.  Leg 67: The Deep Sea Drilling Project Mid-America Trench transect off Guatemala , 1980 .

[14]  J. L. Worzel,et al.  Continental margin and lower slope structures of the Middle America Trench near Acapulco (Mexico) , 1980 .

[15]  B. Isacks,et al.  A comparison of the spatial distribution of mantle earthquakes determined from data produced by local and by teleseismic networks for the Japan and Aleutian arcs , 1979 .

[16]  D. Forsyth,et al.  Earthquakes and bending of plates at trenches , 1979 .

[17]  L. House,et al.  Aleutian subduction zone seismicity, volcano‐trench separation, and their relation to great thrust‐type earthquakes , 1979 .

[18]  B. Isacks,et al.  Subduction of the Nazca plate beneath Peru: evidence from spatial distribution of earthquakes , 1979 .

[19]  W. Rose,et al.  Potassium content of lavas and depth to the seismic zone in central america , 1979 .

[20]  H. Kanamori,et al.  Back-arc opening and the mode of subduction , 1979 .

[21]  C. Frohlich,et al.  An efficient method for joint hypocenter determination for large groups of earthquakes , 1979 .

[22]  C. L. Drake,et al.  Focal Mechanism Solutions and Tectonics of the Middle America ARC , 1978, The Journal of Geology.

[23]  Thomas H. Jordan,et al.  Present‐day plate motions , 1977 .

[24]  G. L. Johnson,et al.  Recent plate motions in the Galapagos area , 1977 .

[25]  E. Engdahl,et al.  Plate effects in North Pacific subduction zones , 1977 .

[26]  B. Isacks,et al.  Spatial distribution of earthquakes and subduction of the Nazca plate beneath South America , 1976 .

[27]  J. Kelleher,et al.  Buoyant zones, great earthquakes, and unstable boundaries of subduction , 1976 .

[28]  M. J. Carr.  Underthrusting and Quaternary faulting in northern Central America , 1976 .

[29]  P. Vogt,et al.  Subduction of Aseismic Oceanic Ridges: Effects on Shape, Seismicity, and Other Characteristics of Consuming Plate Boundaries , 1976 .

[30]  T. Jordan.  The present‐day motions of the Caribbean Plate , 1975 .

[31]  S. T. Algermissen,et al.  Seismicity of the Middle America arc-trench system near Managua, Nicaragua , 1974, Bulletin of the Seismological Society of America.

[32]  M. J. Carr,et al.  Discontinuities in the Deep Seismic Zones Under the Japanese Arcs , 1973 .

[33]  M. J. Carr,et al.  Quaternary volcanic and tectonic segmentation of Central America , 1973 .

[34]  P. Molnar.  Fault Plane Solutions of Earthquakes and Direction of Motion in the Gulf of California and on the Rivera Fracture Zone , 1973 .

[35]  James W. Dewey,et al.  Seismicity and tectonics of western Venezuela , 1972, Bulletin of the Seismological Society of America.

[36]  T. Fitch.  Plate convergence, transcurrent faults, and internal deformation adjacent to Southeast Asia and the western Pacific , 1972 .

[37]  R. Hyndman.  Plate Motions relative to the Deep Mantle and the Development of Subduction Zones , 1972, Nature.

[38]  W. Stauder.  Fault motion and spatially bounded character of earthquakes in Amchitka Pass and the Delarof Islands , 1972 .

[39]  R. Moberly.  Origin of Lithosphere behind Island Arcs, with Reference to the Western Pacific , 1972 .

[40]  T. H. Andel,et al.  Tectonics of the Panama Basin, Eastern Equatorial Pacific , 1971 .

[41]  T. Atwater.  Implications of Plate Tectonics for the Cenozoic Tectonic Evolution of Western North America , 1970 .

[42]  D. McKenzie,et al.  Evolution of Triple Junctions , 1969, Nature.

[43]  P. Molnar,et al.  Tectonics of the Caribbean and Middle America Regions from Focal Mechanisms and Seismicity , 1969 .

[44]  F. Ender,et al.  Iron Resorption in Fish-induced Anaemia in Mink , 1967, Nature.

[45]  A. DOUGLAS,et al.  Joint Epicentre Determination , 1967, Nature.