STYLES OF LAYER-PARALLEL SHORTENING AND VERTICAL-AXIS ROTATIONS IN THE PRECORDILLERA AND WESTERN SIERRAS PAMPEANAS, ARGENTINA

Initial studies of Triassic and Miocene to Quaternary red beds across parts of the Precordillera to western Sierras Pampeanas above the Pampean flat-slab segment have revealed widely developed, systematic minor fault sets, including conjugate wedge faults at acute angles to bedding and conjugate tear faults at high angles to bedding, as well as anisotropy of magnetic susceptibility (AMS) fabrics, with Kmax lineations parallel to the zone axis of kinked micas and to the intersection of layer-parallel shortening (LPS) and bedding fabrics. Analysis of both minor fault and AMS data from sites in structurally simple settings yields estimated LPS directions that range from 070 to 130 degrees and are typically at high angles to local structural trend. This pattern records overall E-W shortening in the Precordillera and adjacent foreland, with stress/strain refraction along the curved topographic front, likely reflecting primary sedimentary prism curvature combined with secondary vertical-axis rotation during curved thrust slip and differential shortening. Sites in some steep fold limbs display younger faults that developed during fold tightening with different shortening directions, recording temporal changes in the stress field. Limited data from limbs of basement-cored uplifts of the Sierras Pampeanas suggest local stress/strain refractions related to basement anisotropies. Red beds carry near primary remnant magnetization, with evidence for local vertical-axis rotations. Preliminary results suggest similarities and some differences with the Sevier and Laramide belts of the North American Cordillera, where thin- and thick-skin belts were active simultaneously during flat-slab subduction, with style partly controlled by sedimentary thickness. However, basement anisotropies appear to be more distinct and at higher angles to regional shortening above the Pampean flat-slab. Additional studies are underway to quantify regional rotation patterns and fully restore paleostress/ strain fields over time, which will provide data to test models of crustal deformation during flat-slab subduction