Structural overprinting and style of deformation at sierra Beauvoir and sierra de Apen: A geometric and kinematic model for the evolution of the internal thrust-fold belt, Fuegian Andes, Argentina

Abstract

A detailed study of the structural fabrics and overprinting relationships exposed at sierra Beauvoir and sierra de Apen, which include folds, foliations and thrust faults, allowed us to settle a deformation sequence of the internal thrust-fold belt of the Fuegian Andes. Our results, together with previous geochronology results, indicate that ductile structures developed during a Late Cretaceous simple shear deformation that formed asymmetric folds and related foliations under very-low grade metamorphism. This deformation affected rocks not younger than the Campanian, and is here related to the front of the orogenic wedge formed during arc-continent collision associated with the closure of a prior back-arc basin. Thrust systems overprint this ductile deformation, and even though they were considered in previous theoretical models, this is the first time their detailed geometry and time constraints are reported from field data. We identified an older, brittle-ductile thrust system formed during the Danian, with the slight penetrative deformation and very gentle folding of the Maastrichtian-Danian Policarpo Formation representing the frontal part of this thrust wedge. This thrust system formed at intermediate structural levels and was linked with basement-involved thrusting in the hinterland, which caused the shortening accommodated in the internal thrust-fold belt. The Eocene Apen-Malvinera thrust system, formed by brittle faults, was responsible for later exhumation of the Cretaceous rocks deformed by prior brittle-ductile structures. This deformation sequence implies that a significant supply of detritus to the foreland basin, originated from the erosion of the Cretaceous cover of the brittle-ductile structures of sierra Beauvoir, should be not older than the Eocene.