CONICET | Buscador de Institutos y Recursos Humanos

The Andes are an active subduction orogen formed by the Mesozoic-Cenozoic subduction of different oceanic (Pacific) plates beneath South America that is segmented along its strike due to the deformational, magmatic, and basin formation processes that have acted in a non-uniform way across the continental margin The North Patagonian Andes (NPA), located between ~ 39 ? 47º S, represent a considerably less known orogenic segment in terms of their deformational timing and lithospheric structure in comparison the Central Andes, even though their geological record extend from the earliest Andean evolutionary stages, covering vast regions from the forearc to the retroarc zones. Locally, the upper plate of the Patagonian Nazca-South American subduction system (41 - 45ºS), can be separated into a series of morphostructural units, characterized by a particular geology and associated structure. From the forearc to the retroarc regions, a Coastal Cordillera, a submerged Central Valley, the North Patagonian Andes, a wide Precordillera-Broken foreland system and the North Patagonian Massif can be recognized (Figure 1). In the forearc, the remnants of the Coastal Cordillera at the Chiloé Island and the Central Valley are cored by Paleozoic basement complexes that reflect different timings of forearc accretion in the SW margin of Gondwana with an early Mesozoic metamorphism. The NPA to the east is a narrow belt (~ 100 km) composed of intrusive suites of the North Patagonian Batholith mainly constructed in Cretaceous and Miocene times, and minor presence of Cenozoic sedimentary sequences. From the NPA to the southern margin of the North Patagonian Massif there is a series of N-trending mountain belts that interact with a NW-trending structure (Taquetrén thrust front, Figure 1) in the western margin of the Cañadón Asfalto Basin. This Precordillera-Broken Foreland System is a structural and morphological anomaly (Folguera and Ramos, 2011; Bilmes, 2013) that has suffered contractional and extensional collapse events in the Meso-Cenozoic, leading to an inhomogeneous lithospheric structure. In this study we integrate synorogenic structures recognized through a detailed field work in the Argentinean retroarc of the NPA, with identified deformational and exhumation stages, mostly from fission track data (Duhart and Adrisola, 2008; Thomson, 2010), in the Chilean NPA, attempting to unify the tectonic processes in this sector. We constructed a balanced cross section (Figure 1) that is deeply constrained by a thermomechanical model that predicts the lithospheric fragile-ductile transitions, based on a seismically constrained gravity model and a heat-transfer thermal model (Tassara, 2012; Tassara and Echaurren, 2012), and whose upper sector is constrained by field data integrated with regional previous information. Our results indicate that in the Late Early Cretaceous ? Upper Cretaceous the deformation extended episodically, at least, up to ~ 500 km from the trench, propagating the growth of the fold and thrust belt to a distal foreland area. This expansion was accompanied by progressive crustal thickening and shortening, interrupted by an extensional collapse in the Paleocene in the Chubut River Middle Valley area (CHRMV, Figure 1) possibly related to a crustal foundering event. This created an intra-crustal discontinuity, possibly enhanced by an extension-dominated regime in the Oligocene (Muñoz et al., 2000). This system was reactivated in the Miocene as a shallower decollement level, producing a short wavelength contractional deformation. The analysis of the distinctive parts of the orogenic system and its comparison with the constructional events emphasizes the multi-stage complex tectonic evolution of the North Patagonian Andes.