Timing and origin of deformation along the Patagonian fold and thrust belt

The Andean orogeny in the Patagonian Cordillera of southern South America r eflects the consequences of the Mesozoic and Cenozoic subduction of an ocea nic plate beneath the South American continental margin. The geological evo lution of the region has been influenced by the Eocene collision and subduc tion of the Farallon-Aluk Ridge and the Miocene-Recent subduction of the Ch ile Ridge. Another aspect of plate interaction during this period was two i ntervals of rapid plate convergence, one at 50-42 Ma, and the other at 25-1 0 Ma, between the South American and the oceanic plates. It has been propos ed that the collision of the Chile Ridge with the trench was responsible fo r the development, at least in part, of the Patagonian fold and thrust belt . This belt extends for more than 1000 km along the eastern foothills of th e southern Andes between 46 degrees and 54 degrees S along the southwestern rim of the Austral Basin. The interpretation of a link between subduction of the ridge and formation of the fold and thrust belt is based on assumed time coincidences between contractional tectonism and the collision of ridg e segments during Middle and Late Miocene times. The main Tertiary contract ional events in the Patagonian fold and thrust belt took place during lates t Cretaceous-Palaeocene-Eocene and during Miocene times. Although the timin g of deformation is still poorly constrained, the evidence currently availa ble suggests that there is little or no relationship between the timing of the fold and thrust belt and the collision of ridge segments. Most if not a ll of the contractional tectonism pre-dated the latest episodes of ridge co llision. Collision of a ridge crest with the continental margin has been ac tive for the past 14 to 15 million years. Contrary to the suggestion of a r elationship between ridge subduction and compression, the main result of th is collision has been fast uplift and extensional tectonism. The initiation of the Patagonian fold and thrust belt in latest Cretaceous or early Terti ary times coincided with a fundamental change in the tectonic evolution of the Austral Basin. Throughout the Cretaceous most of this basin subsided as a broad backarc continental shelf. Only in latest Cretaceous times, and co inciding with the initiation of the fold and thrust belt, the basin underwe nt a transition to a retro-arc foreland basin. This change to an asymmetric ally subsiding foreland basin, with an associated foreland fold and thrust belt, was related to uplift of the Andean orogenic belt in the west.