Deformation mechanisms accommodating progressive simple shear thrusting of quartzite and metacarbonate in the southwestern Espinhaço Range, Brazil

Abstract

The accommodation of low-temperature ductile deformation in foreland fold-and-thrust belts is often described in terms of outcrop-based geometric analysis, but microtextural observations are also important, as they relate stress, strain, fluids and temperature and thus the rheology in the peripheral part of the orogen. Such microtextural observations are lacking from the foreland part of the Araçuaí orogen in Brazil, and are here investigated through EBSD-based textural analysis of quartzites and metacarbonate samples from the southwestern Espinhaço Range. The quartzites overthrust the metacarbonates, and deform solely by the activation of dissolution-precipitation creep, whereas the metacarbonates show a much greater variety of deformation mechanisms that closely relate to grain-size, composition and strain, in the long (low-angle) and short (steep) limbs of shear-related folds. The metacarbonates show a centimeter-scale alternation of coarse-grained domains of calcite and quartz, and fine-grained domains of a mix of phases (calcite, dolomite, phyllosilicates, quartz, and apatite). In the strongly sheared long limbs, coarse calcite was deformed by dislocation creep with slip on (c)<a>, whereas fine calcite displays evidence of oriented dissolution-precipitation creep, with both cases displaying <c> axis orientations consistent with the top-to-the-west thrusting. Coarse calcite shows evidence of recovery by subgrain rotation and grain boundary migration. Coarse-grained calcite in the short limbs was deformed by dislocation creep, assisted by twin boundary migration recrystallization with twinning on {e}, indicating simple shear-dominated deformation. In contrast, fine-grained calcite was deformed by fluid-assisted grain boundary sliding. Dissolution-precipitation in both short and long limbs produced random textures, with the exception of coarse quartz in the short limbs, where dissolution-precipitation creep led to <c> axis orientation parallel to X. This study demonstrates that the foreland thrust zone of the Araçuaí orogen underwent consistent W-directed progressive thrusting, and that this deformation produced a wide variety of texture types in the metacarbonates while a simpler structural and textural pattern developed in adjacent quartzitic metasediments.