Structure of the North Patagonian Andes in the Piltriquitrón and Serrucho ranges and in the El Bolsón valley (41º30´-42º00´S), Río Negro province

Abstract

The Cenozoic structure of the North Patagonian Andes of Argentina (41º- 42º S) is characterised by an E-verging imbricated fold and thrust belt with some important back-thrusts. The western sector (west of 71º 15´ W) exhibits back-thrust systems that interact with the E-verging thrusts conforming triangular areas. There are two important back-thrust systems that have deformed both the Piltriquitrón (2,284 m) and Serrucho ranges (2,120 m). The first is composed of three W-verging thrusts with belts of intense deformation; the second system consists of at least two N-trending faults that form the great western scarp of the mountain range. As a consequence of the interaction with the E-verging thrusts, the Paleogene deposits of the El Bolsón valley have been deformed into a large N-trending drag-syncline. From a stratigraphical point of view, the back-thrust systems expose igneous and metamorphic rocks of the pre-Mesozoic basement. Their cartographic pattern suggests that both are branches of the Ventana - Catedral thrust, a main structure that is the boundary between the westernand the eastern sectors of the fold and thrust belt. It is proposed that the surface structure might represent a subsurface thick-skinned triangular zone, whose location is related to the Cenozoic tectonic inversion of normal faults formed during Mesozoic extensional regime, which controlled the palaeogeographic distribution of the Jurassic volcano-sedimentary deposits.