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., ~9 Ma) and affect the basement comprising Paleozoic metamorphic rocks that have been dated at ~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 °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 (~360-220 Ma). This age distribution can be explained by a long lasting brittle deformation history with a minimum age of ~360 Ma and a last clay minerals forming event at ~220 Ma. Moreover, given the progression of apparent ages decreasing from coarse to fine size fractions (~360-311 Ma for 2-1 μm grain size fraction, ~326-286 Ma for 1-0.2 μm and ~291-219 Ma of <0.2 μm), we modeled discrete deformation events at ~417 Ma (ending of the Famatinian cycle), ~317-326 Ma (end of Gondwanic orogeny), and ~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.