The Chubut Basin (new name for the Liassic Western Chubut Basin of other authors), developed during the Early Jurassic in the western part of central extra-Andean Patagonia in Argentina (42 30 S and 44 30 S), accumulated shallow marine and continental sedimentary beds, with pyroclastic input, of the Osta Arena Formation and equivalent units. To the west it was bounded by a coeval subduction-related magmatic arc (Subcordilleran Plutonic Belt) and a subduction complex of Late Triassic-Early Jurassic age (Chonos Metamorphic Complex). In turn, the eastern basin floor developed over the remnants of a previous Late Triassic-earliest Jurassic magmatic arc (Batholith of Central Patagonia) and over neopaleozoic sedimentary rocks of an accretionary prism. To the east, the basin was limited by a Toarcian volcanic system associated to the Karroo plume. The western magmatic arc continued to the north, where it is represented by the Icalma Member of the Nacientes del Biob□Formation, bounding this time the Neuqu•Basin developed to the east. The closure of the Toarcian basin in the Middle Jurassic coincides in time with a migration to the southwest of the Toarcian magmatic arc, from a position in central extra-Andean Patagonia and a NNW orientation, to a location in the Patagonian Cordillera and a north-south orientation. Synchronously, rift-related anatectic silicic volcanism developed east of the basin. Hence, a major change in tectonic setting took place during the Middle Jurassic. It is here proposed that the migration of the arc was related to the growth of a plume under north-eastern Patagonia.
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