The lagos La Plata and Fontana fold-and-thrust belt: long-lived orogenesis at the edge of western Patagonia

Extensive fieldwork carried out in the inner sectors of the Cordillera Patagónica has revealed evidence of alternate of periods of crustal extension and orogen formation since the Jurassic. The Patagonian lithosphere was extended during the Mesozoic and fractured and collapsed into numerous depocenters within the arc and retroarc. The last pulse of extension during this period occurred in the Early Cretaceous, related to a series of Pacific transgressions that formed the embayment of the Rı́o Mayo basin, north of the Austral basin. The Rı́o Mayo basin is part of a widespread Early Cretaceous synrift episode found throughout the Patagonian region between 155 and 140 Ma. The last part of this rifting episode, between 140 and 125 Ma, was spatially more limited, restricted to the arc and inner retroarc. ea of extension can be related to an early Paleozoic suture between the Somuncura Massif and the hanging wall of the Deseado Massif. The correct identification of this suture zone helps delimit distinctive volcanic provinces, as well as metallogenic ore districts. l subsidence ceased as a consequence of a sudden tectonic inversion related to a major lithospheric reorganization at 120 Ma, during the breakup of southern Gondwana. The closure of the small Rı́o Mayo intra-arc-retroarc basin occurred as a result of a fold-and-thrust belt, mainly by tectonic inversion of previous normal faults, and was partially related to thin-skinned deformation. The emplacement of the Eastern Patagonian batholith between 100 and 80 Ma destroyed the inner sectors of the fold-and-thrust belt. During this time, the collapse of the inner sectors allowed for the development of a second, more limited intra-arc basin. The orogenic wedge grew through the inversion of progressively deeper decollements and out-of-sequence thrusts and lasted until the Late Paleogene, with a period of probable quiescence during the highly oblique subduction of the Aluk (Phoenix) Plate in the early Paleogene.