Tectonic evolution from Late Cretaceous to Miocene times and structural controls at the easternmost southern Central Andes (Sierra Azul; 36ºS).

ORTS, D., FOLGUERA A., GIMÉNEZ M., RAMOS, V.A.

ANDEAN GEOLOGY

SERVICIO NACIONAL GEOLOGIA MINERVA

Año: 2011 p. 1 - 20

0718-7092

The Southern Central Andes at 36ºS have been recognized as an orogenic belt where contraction, accommodated mainly by basement structures, is associated with the inversion of a Late Triassic – Early Jurassic extensional 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 being detached from shallower thin-skinned structures. 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 Late Cretaceous strata identified in seismic lines. These sequences have a maximum age of 97 Ma as inferred by U/Pb in detrital zircons published in previous works in the area. Contrastingly, the easternmost sector was mainly deformed in Late Miocene times as inferred from less than 18 Ma old synorogenic sequences. 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 controlled mainly by brittle-ductile transitions 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.