PRELIMINARY REPORT ON AN INTEGRATED STRUCTURAL, ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS), AND PALEOMAGNETIC STUDY OF A CLASSIC TRIANGLE ZONE BETWEEN THE PRECORDILLERA AND SIERRA PAMPEANAS OF ARGENTINA BETWEEN 28OS AND 33OS

As part of a larger effort to quantify spatial-temporal stress/strain fields and vertical-axis rotation patterns across the thick-skin Sierras Pampeanas, thin-skin Precordillera, and mixed mode Principal Cordillera in a region spanning flat-slab to normal subduction in northwest Argentina, we have heavily sampled the classic triangle zone that separates the overall east-verging thrust system of the western and central Precordillera with the overall west-verging eastern Precordillera and thick-skinned Sierra Pampeanas ranges. Nearly 200 sites have been collected within the triangle zone to understand the recent stress state inferred from fault-slip vector inversion of minor faults, AMS analysis used to estimate paleo-stress directions, and paleomagnetic data to correct all stress/strain data for any localized or regional vertical-axis rotation. Most collected data and samples are from late Miocene synorogenic strata of the broken foreland Bermejo basin, with additional sites collected from Mesozoic red beds associated with pre-Andean extensional basins. Sites were distributed as best as possible across the tightly imbricated thrust sheets of the western and central Precordillera, as well as the well-exposed folds associated with the slightly younger west-verging thrusts of the eastern Precordillera. Where exposures allowed, samples were also taken from the Sierras Pampeanas foreland, which represents the youngest deformation of the system. Previous work has established a well-constrained temporal evolution of the entire triangle zone, which allows us to put our sites into both a spatial and temporal framework. These data can be further compared to modern velocity-fields provided by GPS based studies as well as previously published inversions of modern earthquake moment tensors. Our data adds to existing studies to constrain the stress regime during Andean mountain building. Overall our preliminary result suggests a slight clockwise deflection of the maximum stress direction inferred from modern plate velocity models between the Nazca and South American plates and the shortening direction in the Andean foreland, suggesting that there is an important component of coupling between regional flat slab subduction and the transfer of plate boundary forces into the orogenic foreland.