Abstract

ELIAS-HERRERA, Mariano et al. The Caltepec fault: exposed roots of a long-lived tectonic boundary between two continental terranes of southern Mexico. Bol. Soc. Geol. Mex [online]. 2005, vol.57, n.1, pp.83-109. ISSN 1405-3322.  https://doi.org/10.18268/bsgm2005v57n1a5.

This work presents the main geologic characteristics of the long-lived Caltepec fault zone, a N-S trending, righttranspressional shear zone that separates the Zapoteco terrane on the east from the Mixteco terrane on the west at their basement level (Oaxaqueño and Acatlan complexes). The high quality of the outcrops (Caltepec area), entirely exposing a 2-6 km wide tectonic contact, reveals protracted and polyphase tectonic activity on the fault from at least early Permian time to the present. Based on detailed geological mapping, petrography, geochronology, and isotopic studies, it is shown that the tectonothermal event in the Caltepec fault zone is related to the oblique collision of the metamorphic complexes during the amalgamation of Pangea. An anatectic leucosome and the resulting syntectonic granite (Cozahuico granite) in the fault zone yielded U-Pb zircon ages of 275.6 and 270.4 Ma. It is also established a half-flower structure for the fault zone, with the top lying almost vertical and its base thrusted westward with unknown displacements. The initial 87Sr/86Sr ratios (0.70435-0.70686) and Sm-Nd model ages (TDM) (1.0-1.6 Ga) for the Cozahuico granite and leucosome indicate a magmatic mixture that originated from melted Proterozoic crust and a component of depleted mantle. The leonardian age for the cover (Matzitzi Fm.) and a 40Ar/39Ar cooling age (muscovite) of 268.6 Ma for a mylonitic mica schist at the sole of the thrust, implies high ratios of cooling (~180°C/Ma) and uplift during the Permian. On the other hand, the adjacent sedimentological record indicates intense tectonic reactivation during Early Cretaceous, Paleogene, and Neogene times along the long-lived Caltepec fault zone, alternating with periods of relative tectonic quiescence during Triassic, Jurassic, and mid Cretaceous times. The trend of the Caltepec fault zone, parallel to the Oaxaca fault, distant only a few kilometers to the east, is interpreted as part of a synchronous and dynamically coupled tectonic system. This coupled system has liberated tectonic stresses associated with the rupture of Pangea and the evolution of the Pacific margin of southern Mexico from Jurassic to Holocene times.

Keywords : Major fault; terrane boundary; Acatlán Complex; Oaxacan Complex; southern Mexico.