Structure of the North Patagonian thick-skinned fold-and-thrust belt, southern central Andes, Argentina (41°–42°S)

The structure of the southern sector of the central Andes in the North Patagonian Andes of the Argentinean slope (41°–42°S) is characterized by a Tertiary fold-and-thrust belt formed by an E-vergent imbricate thrust structure and a retrovergent thrust system. The paleogeographic distribution of Jurassic rocks suggests that some of the W-vergent thrusts may have been part of a Mesozoic extensional fault system inverted during Andean compression. On the basis of the structures and rocks involved in the deformation, we distinguish a western and an eastern sector. The western sector developed on pre-Tertiary basement rocks with E-vergent thrusts and an associated backthrust system that forms a triangle zone that exposes Mesozoic and pre-Mesozoic rocks. In the eastern sector, thrusting that formed the Ñirihuau foldbelt involved Cenozoic volcanic and sedimentary rocks and affected a subsurface synorogenic wedge. The boundary between the two sectors is probably a normal fault that was active during the middle Mesozoic. The regional cartography and microtectonic observations suggest predominant dip-slip movements of the thrust sheets. However, there is no evidence of major N–S strike-slip movements as has been proposed for the forearc region (Chilean Andes) and northwestern Patagonia on the basis of fault slip data analysis. A Tertiary sedimentary basin was developed in relation to the eastward migration of the orogenic front toward the foreland.