Variable structural controls through time in the Southern Central Andes (~36ºS)

Abstract

The Southern Central Andes at 36ºS have been recognized as an orogenic belt where contraction, ac com-modated mainly by basement structures, is associated with the inversion of a Late Triassic-Early Jurassic extens ional detachment. Based on a structural cross-section, constrained by field data, 2D seismic and bore-hole information, and the processing of Bouger anomalies, we propose a polyphasic tectonic evolution. In the westernmost sector, along the axis of the Cordillera, NW to NNW basement structures were inverted, being a first order control in the generation of frontal narrow N-trending thin-skinned belts. This slip transfer is controlled by the Late Jurassic main detachment. These structures have low gravity anomalies that cross obliquely the main Andean trend. East of this inversion domain, beneath the frontal thin-skinned belts, seismic information reveals that Late Triassic wedge-like depocenters did not experience substantial inversion. To the east double-vergent basement blocks define the Andean emergent orogenic front at these latitudes. These contractional structures truncate gravity anomalies defined by basement discontinuities, indicating that they are not related to tectonic inversion, in contraposition to the westernmost domain. Two contractional phases were distinguished. The oldest is Late Cretaceous in age, as inferred from onlap relations in Upper Cretaceous strata identi-fied in seismic lines. These successions have a maximum age of 97 Ma as inferred by U-Pb in detrital zircons published in previous studies in the area. Contrastingly, the easternmost sector was mainly deformed in Late Miocene times as inferred from less than 18 Ma old synorogenic deposits. Moreover, contractional mechanisms varied through time for each specific sector. While Late Cretaceous contractional tectonics was generated by tectonic inversion and subordinate thin-skinned deformation, it is proposed that Late Miocene deformation was controll ed mainly by brittle-ductile transi-tions at the upper crust with no major influence of previous structures. This fact can be explained by a higher thermal flux achieved in the retroarc area in the last 17 Ma due to the eastward arc expansion during a shallow subduction regime