Successive reactivation of older structures under variable heat flow conditions evidenced by K-Ar fault gouge dating in Sierra de Ambato, northern Argentine broken foreland

Abstract

The Argentine broken foreland has been the subject of continuous research to determine the uplift and exhumation history of the region. High-elevation mountains are the result of N-S reverse faults that disrupted a W-E Miocene Andean foreland basin. In the Sierra de Ambato (northern Argentine broken foreland) the reverse faults offset Neogene sedimentary rocks (Aconquija Fm., similar to 9 Ma) and affect the basement comprising Paleozoic metamorphic rocks that have been dated at similar to 477-470 Ma. In order to establish a chronology of these faults affecting the previous continuous basin we date the formation age of clay minerals associated with fault gouge using the K-Ar dating technique. Clay mineral formation is a fundamental process in the evolution of faults under the brittle regime (<<300 degrees C). K-Ar ages (9 fractions from 3 samples collected along a transect in the Sierra de Ambato) vary from Late Devonian to Late Triassic (similar to 360-220 Ma). This age distribution can be explained by a long lasting brittle deformation history with a minimum age of similar to 360 Ma and a last clay minerals forming event at similar to 220 Ma. Moreover, given the progression of apparent ages decreasing from coarse to fine size fractions (similar to 360-311 Ma for 2-1 mu m grain size fraction, similar to 326-286 Ma for 1-0.2 mu m and similar to 291-219 Ma of <0.2 mu m), we modeled discrete deformation events at similar to 417 Ma (ending of the Famatinian cycle), similar to 317-326 Ma (end of Gondwanic orogeny), and similar to 194-279 Ma (Early Permian - Jurassic deformation). According to our data, the Neogene reactivation would not have affected the K-Ar system neither generated a significant clay minerals crystallization in the fault gouge, although an exhumation of more than 2 Km is recorded in this period from stratigraphic data. (C) 2015 Elsevier Ltd. All rights reserved.