UPLIFT AND SUBDUCTION EROSION IN SOUTHWESTERN MEXICO SINCE THE OLIGOCENE - PLUTON GEOBAROMETRY CONSTRAINTS

Details of the late Oligocene to Middle Miocene uplift and tectonic er osion episodes of the southwestern continental margin of Mexico can be inferred using Al-tot geobarometry of igneous hornblendes, geochronol ogy, and field relations. On the basis of such analyses carried out be tween Acapulco and Huatulco we find the following: (1) Calc-alkaline b atholiths exposed along the coast from Acapulco to Huatulco, mostly in the 35-25 Ma age range, were emplaced at depths between 13 and 20 km. (2) The contact relationships between these plutons and their host ro cks, and the exposure of volcanic counterparts, 70 km from the coastli ne, indicate a landward decrease in the amount of uplift. (3) A compar ison of the time differences between intrusion and cooling ages of bat holiths along the coast suggest that cooling rates were, in general, h igher between Acapulco and Huatulco than those along the margin betwee n Puerto Vallarta and Manzanillo, 700 km northwest of Acapulco. (4) Th e uplift of this coastal belt occurred during the late stages of magma tism and after its cessation, triggering intensive subaerial erosion o f supracrustal rocks and the exposure of midcrustal rocks such as gran itic batholiths and amphibolite facies metamorphic assemblages of the Xolapa Complex. These findings, in conjunction with the geometry of th e present continental margin, as well as the offshore tectonic and str atigraphic features, support previous interpretations of very active l ate Oligocene to Middle Miocene subduction erosion after the onset of strike-slip tectonics related to the detachment and subsequent eastwar d displacement of the Chortis block. Subduction erosion involved both trench sediments and crystalline (continental framework) rocks. Differ ent rates of continental framework erosion are assessed on the basis o f the bathymetric fluctuations of the upper slope trench sediments and the age of the accretionary prism. Subsidence of the offshore contine ntal basement suggests intense episodes of basal erosion of lower cont inental crust, whereas the construction of the present day accretionar y prism and the uplift of the upper slope indicate a decline in the fr ontal and basal erosion of the continental framework. Comparing the ca lculated depths of pluton crystallization with the present depth of th e continental crust-subducted slab boundary, interpreted using previou sly published seismic refraction and gravity models, we conclude that onshore basal erosion played a subordinate role during Miocene episode s of subduction erosion. Major removal of lower crustal sections was p robably restricted to offshore regions.