HR: 11:45h
AN: T42A-04    [Abstracts]
TI: Missing Forearcs: A Model for Oligocene Antithetic Thrusting and Subduction Erosion in the Xolapa Complex, Southern Mexico
AU: * Keppie, D F
EM: fkeppie@eps.mcgill.ca
AF: Earth and Planetary Science, McGill University, 3450 University St., Montreal, QC H3A 2A7, Canada
AB: Two distinct E-striking S-dipping zones of ductile mylonization were identified on four transects from Mexico's Pacific coastline roughly 25km north into the Sierra Madre del Sur centered on the town of Puerto Angel, Oaxaca. One of these shear zones appears to be coincident with the previously reported sinistral Chacalapa Fault. However, stretching lineations with significant down dip components predominate along three of these transects, together with smaller populations of horizontal stretching lineations on discrete planes within the shear zones. This two-group population of lineations suggests either two distinct episodes of shearing or, more likely, one episode of obliquely driven shearing with strain partitioning. Regardless, the down dip lineations coupled with an analysis of rotated porphyroblasts, strain shadow zones and mica fish all indicate a major component of thrusting antithetic to subduction on the Acapulco Trench. Since the mylonitization affects otherwise undeformed, probably Oligocene, granitoids, it is assumed here that the previously published upper (29 Ma) and lower (23 Ma) bounds for Chacalapa shearing constrain the time of thrusting. Previous studies suggest that a large sliver of fore-arc (>150 km wide perpendicular to the trench) was removed from southern Mexico during the Oligocene. Attempts to locate the suspected missing forearc at Earth's surface suffer from a lack of tectonic and geological support for the required motion. I propose that tectonic motion of the Farallon(-Cocos) plate(s) relative to North America brought the Farallon images of the Molokai Fracture Zone and the Clarion Fracture Zone into play as significant topographic elements capable of generating subduction erosion, especially when combined with increased convergence rates at the Middle America Trench associated with fragmentation of the Farallon plate into the Cocos and Nazca plates. I argue that the antithetic thrusting observed in the field is directly related to subduction erosion. Geochronological data tied to the structures observed in the field are required to compare the geometry and kinematics of arc deformation with times of arrival of oceanic fracture zones in Farallon(-Cocos)-North America reconstructions. Confirmation of this model will justify the creation of a predictive space-time map for similar cases of subduction erosion in the geological record of North America's Pacific margin.
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Joint Assembly