Southern Patagonia: opening of the weddell sea, magmatic arcs and terrane accretion,

RAMOS V.A; FOLGUERA, A.; GHIGLIONE, M.C.

Santiago de Chile

Congreso; GEOSUR; 2007

In recent years several new hypotheses have been advanced on the tectonic evolution of southern Patagonia during the Mesozoic. These new ideas can be summarized in three large themes. (1) The opening of the Weddell Sea as a result of the Peninsula Antarctica detachment; (2) the migration of the Jurassic magmatic arcs towards the trench, and (3) the collision of the Fitz Roy terrane against the continental margin of western Gondwana. The geological consequences of these tectonic processes are evaluated through  new geologic data obtained on the evolution of the Early Jurassic intra-arc and heavily attenuated basin of central Chubut and the related calc-alkaline magmatism; the subsequent opening of the Cañadón Asfalto basin during Middle to Late Jurassic times associated with poorly evolved alkaline lavas in a within-plate setting; and the final opening of the Río Mayo and Guenguel extensional basins in the Río Mayo embayment in the Neocomian, during generalized time of negative trench roll-back velocity along the Pacific continental margin. The sedimentary evolution and volcanic history are connected to the magmatic record of the different batholithic belts where arc affinities have been proposed: The Central Patagonian batholith (220-200Ma), the Precordilleran batholith (at approximately 180 Ma), and the Patagonian batholith (Jurassic to Neogene). As a result of that it is concluded that not all those three belts are arc-related, and the differences between extensional driven magmatism and subduction generated batholiths are discussed. On these bases is analyzed the need for collision against the Pacific margin of a major continental terrane, like the previously proposed Fitz Roy terrane. This terrane encompasses the eastern metamorphic complex of the Patagonian Cordillera. Although important metamorphism and deformation were recorded along the Patagonian Cordillera during Middle Jurassic times, some alternative mechanisms are proposed in order to understand the tectonic evolution of the Patagonian Andes. However, the regional tectonic evolution clearly indicates a local compressive regime in southern Patagonia, different from the dominant extensional tectonic regime observed in the southern and central Andes. The intense compressive deformation recorded during Middle Jurassic times, requires some ad-hoc explanation for the peculiar evolution of the southernmost Andes.