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, accommodatedmainly by basement structures, is associated with the inversion of a Late Triassic-Early Jurassic extensionaldetachment. Based on a structural cross-section, constrained by field data, 2D seismic and bore-hole information, andthe processing of Bouger anomalies, we propose a polyphasic tectonic evolution. In the westernmost sector, along theaxis of the Cordillera, NW to NNW basement structures were inverted, being a first order control in the generation offrontal narrow N-trending thin-skinned belts. This slip transfer is controlled by the Late Jurassic main detachment. Thesestructures have low gravity anomalies that cross obliquely the main Andean trend. East of this inversion domain, beneaththe frontal thin-skinned belts, seismic information reveals that Late Triassic wedge-like depocenters did not experiencesubstantial inversion. To the east double-vergent basement blocks define the Andean emergent orogenic front at theselatitudes. These contractional structures truncate gravity anomalies defined by basement discontinuities, indicating thatthey are not related to tectonic inversion, in contraposition to the westernmost domain. Two contractional phases weredistinguished. The oldest is Late Cretaceous in age, as inferred from onlap relations in Upper Cretaceous strata identifiedin seismic lines. These successions have a maximum age of 97 Ma as inferred by U-Pb in detrital zircons publishedin previous studies in the area. Contrastingly, the easternmost sector was mainly deformed in Late Miocene times asinferred from less than 18 Ma old synorogenic deposits. Moreover, contractional mechanisms varied through time foreach specific sector. While Late Cretaceous contractional tectonics was generated by tectonic inversion and subordinatethin-skinned deformation, it is proposed that Late Miocene deformation was controlled mainly by brittle-ductile transitionsat the upper crust with no major influence of previous structures. This fact can be explained by a higher thermalflux achieved in the retroarc area in the last 17 Ma due to the eastward arc expansion during a shallow subduction regime.