A new constraint on the central Andean rotation pattern from paleomagnetic studies in the southern Subandes of Bolivia

摘要

New paleomagnetic and anisotropy of magnetic susceptibility (AMS) studies were performed in the southern Subandes of Bolivia in order to assess vertical axis rotations in a poorly studied area. Due to the presence of polarity reversals, it was possible to perform a reversal test that provided evidence of a reliable paleomagnetic record interpreted to be primary. In an area where vertical axis rotations are a principal component of deformation, we have developed a nonparametric method for the determination of tectonic rotations. We calculated a paleomagnetic pole for the Miocene Tariquia Formation (Lat. = 78.4 degrees S, Long. = 113.1 degrees E, A95 = 3.5 degrees, N = 72, K = 23.73, Slat = 22.08 degrees, SLon = 64.06 degrees) and from our results, rotations around vertical axes, commonly sought around the Arica-Santa Cruz bend were ruled out. From the integration of paleomagnetic and AMS results, it was seen that the magnetic lineation lies within the bedding plane, consistent with the early stages of layer parallel shortening. However, it is not parallel to the structural trend, implying material displacement parallel to it. In concomitance with paleomagnetic results, this allows us to argue that a local change in the azimuth of the structures at this latitude could be the consequence of an asymmetric or heterogeneous basement and/or exogenous agents like differences in erosion along the course of the structures during the evolution of the Bolivian fold and thrust belt.