Abstract

TOLSON, Gustavo. The Chacalapa fault in southern Oaxaca. Bol. Soc. Geol. Mex [online]. 2005, vol.57, n.1, pp.111-122. ISSN 1405-3322.  https://doi.org/10.18268/bsgm2005v57n1a6.

Along the coast of Oaxaca between Puerto Angel and Santiago Astata a suite of rocks is exposed corresponding to the Xolapa metamorphic complex which is intruded by granitoid igneous rocks of Paleogene to Miocene age. Both of these rock units are in fault-contact along coast-parallel shear zones with granulite facies metamorphic rocks of the proterozoic age Oaxaca Complex and with cover rocks which unconformably overlie this crystalline basement. The largest of these shear zones is the Chacalapa shear zone exposed to the north of the town of Pochutla and formed by ultramylonites, mylonites, protomylonites, pseudotachylites, phyllonites and cataclasites. The kinematics of this shear zone are dominantly left-lateral.

The mylonitic rocks of the Chacalapa shear zone indícate temperatures ~500°C (as evidenced by crystal-plastic deformation of plagioclase feldspar) which decrease systematically to a strictly brittle deformation regime. Quartz crystallographic textures associated with crystal-plastic deformation processes are locally asymmetric, indicating left-lateral shear. Brittle structures also exhibit left-lateral shear-sense indicators, as do active deformation structures.

The age of the shear zone is constrained between 29±0.2 and 23.7±1.2 Ma. The upper limit is the age of the Huatulco granodiorite (U-Pb from zircon, Herrmann et al. 1994), whose northern contact is sheared, and the lower bound is fixed by the K-Ar age of hornblende separatedfrom granodioritic dikes which cut the mylonitic lineation.

Keywords : Fault; Mexico; Oaxaca; mylonite; pseudotachylite; Xolapa.