Mid-Cretaceous compressive deformation in Central Chile: The beginning of the Andean building

Abstract:

The Andean building has been traditionally considered as a consequence of successive orogenic phases mostly occurring during the Cenozoic. However evidence from Peru-northern Chile and southern Argentina have shown a first contractional pulse during the mid to late Cretaceous. Between these regions, however, there are no reports evidencing this event, hence which open the questions about the real impact of this orogeny along the western boundary of South America. Likewise, the occurrence of this event in central Chile-Argentina could be evidencing that this first contractional pulse in the Andes respond to a major synchronous orogeny along the entire former western margin of Gondwana after a long period of extension since the Triassic. In this context, our study, located approximately at the latitude of the Aconcagua mount (32º50'S), reveals the existence of contractional structures, developed east of the Early Cretaceous magmatic arc, deforming Aptian rocks deposited in an extensional setting. Particularly, the geometry exhibited by a regional east-vergent anticline evidences inversion tectonics in which a thick Albian molasse unit is developed in the frontal limb and beyond with growth strata. Later this synorogenic deposits were thrusted and then covered unconformably by Upper Cretaceous volcanic unit, sealing the deformation of the Aptian-Albian rocks. Finally all these deposits were gently deformed and unconformably covered by a Maastrichtian volcanic unit. Studies of provenance indicate a transitional arc origin for the sandstones contained in the synorogenic deposits, which is in agree with the reported exhumation of previous Andean magmatic arc. Our data evidence the beginning of the contractional deformation, marking the onset of the orogeny in central Chile, during the mid-Cretaceous. Therefore we can propose that the Peruvian orogenic phase extended along most of the Southamerican margin. The location farther west, close to the concurrent arc, and it slightly older age respect to that evidenced north and south of the study region would not indicate a diachronism in the orogenic onset, but that deformation progressed from the arc to the eastern foreland. Considering that this orogeny lasted since the Albian to the Upper Cretaceous.

This study has been supported by the FONDECYT project N° 1120272.