HR: 14:15h
AN: U53A-02    [Abstracts]
TI: Estimates of Cenozoic Forearc Subduction off Southwestern Mexico: Constraints on Eocene- Miocene Reconstructions
AU: * Keppie, J D
EM: duncan@servidor.unam.mx
AF: Instituto de Geologia, Universidad Nacional Autonoma de Mexico, Mexico, D.F 04510, Mexico
AU: Moran-Zenteno, D J
EM: dantez@servidor.unam.mx
AF: Instituto de Geologia, Universidad Nacional Autonoma de Mexico, Mexico, D.F 04510, Mexico
AU: Martiny, B M
EM: martiny@servidor.unam.mx
AF: Instituto de Geologia, Universidad Nacional Autonoma de Mexico, Mexico, D.F 04510, Mexico
AU: Gonzalez-Torres, E
EM: eagtgaia@geologia.unam.mx
AF: Instituto de Geologia, Universidad Nacional Autonoma de Mexico, Mexico, D.F 04510, Mexico
AB: Traditional Cenozoic reconstructions of southern Mexico that place the Chortis block (Honduras and northern Nicaragua) adjacent southwestern Mexico using the Motagua Fault Zone violate plate tectonic principles, and are inconsistent with the presence of undeformed upper Cretaceous - Recent sediments in the Gulf of Tehuantepec that straddle the westward projection of the Motagua Fault Zone, the inferred northern boundary of the Chortis block. Rotating the Chortis block about a rotation pole near Santiago (Chile) places it to the southwest of its present position at 45 Ma with the Farallon Plate lying off southwestern Mexico. Examination of the geological record of southwestern Mexico (102-96°W) records several synchronous events in the upper Oligocene- lower Miocene (29-19 Ma): (1) an hiatus in arc magmatism; (2) removal of a wide (~210 km) upper Eocene-lower Oligocene forearc; (3) exhumation of 13-20 km of upper Eocene-lower Oligocene arc along the present day coast; and (4) breakup of the Farallon Plate. We suggest that the magmatic hiatus was caused by subduction of the forearc, which replaced the mantle wedge by relatively cool crust and has since been deeply subducted. Potential causes for subduction of the forearc include collision of an oceanic plateau with the trench, and a change in plate kinematics synchronous with breakup of the Farallon Plate and initiation of the Guadalupe-Nazca spreading ridge. Following removal of the forearc, arc magmatism resumed in two segments offset by the Veracruz-Oaxaca Line: (i) the 19 Ma to Present E-W Trans-Mexican Volcanic Belt; and (ii) the 22-10 Ma WNW part of the Central American Volcanic Arc. Estimates of the dip of the Benioff zone based on Miocene arc/forearc widths indicate that it changed from 11 to 39 degrees (west to east) across the Veracruz-Oaxaca Line, which is interpreted as a transform fault with a ~100 km dextral offset.
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 8102 Continental contractional orogenic belts and inversion tectonics
DE: 8110 Continental tectonics: general (0905)
DE: 8157 Plate motions: past (3040)
DE: 8178 Tectonics and magmatism
SC: Union [U]
MN: 2007 Joint Assembly