GEOCHRONOLOGY AND PETROGENESIS OF THE PUERTO ESCONDIDO-HUATULCO INTRUSIONS IN SOUTHERN MEXICO AND STRUCTURAL ANALYSIS OF THE WESTERN CHACALAPA FAULT SYSTEM

A transect along the continental margin of Oaxaca between Puerto Escondido and Huatulco in southern Mexico displays a conspicuous record of shear zones parallel to the coast. Cenozoic batholiths and the Cretaceous-Paleogene metamorphic Xolapa Complex are affected by these regional features, which are related to the Chacalapa fault that has been designated as the limit between the Oaxaca and Xolapa terranes. The timing and kinematics of the shear zones seems to be related to continental margin truncation coeval with arc magmatism extinction, possibly linked to the displacement of the Chortis block from southwestern Mexico.

We carried out U-Pb geochronology, geochemistry and structural analyses in the transect. Data reveal two principal intrusive domains: a granitic-leucocratic unit ranging in age from 21-20 Ma crops out in the north part of the study area; in contrast, in the south part there are granodiorites and tonalites ranging from 32 to 31 Ma in age. Their geochemical patterns display that of a typical subduction tectonic setting. Additionally, Al-in-hornblende barometry from granodiorites indicate an emplacement depth from 10 to 12 km.

At least three discontinuous WNW-ESE trending shear zones were recognized, the main one about 3 km wide. Kinematic indicators and mylonitic foliation are consistent with left-lateral displacement. Deformational mechanisms show a crystal-plastic regime overprinted by fragile deformation. North of the main shear zone, metamorphic assemblages such as migmatites, schists and amphibolites have been attributed to the Xolapa Complex. Nevertheless, the recognition for the first time of deformed Permian plutons within the shear zones suggests that these metamorphic rocks do not belong to the Xolapa Complex. Instead, plutons seem to intrude a Paleozoic or older metamorphic assemblage. These findings implicate that the Chacalapa fault system might represent the limit of at least three different tectonostratigraphic terranes.