STRATIGRAPHIC RECORD ACROSS AN EXTENSIONAL-CONTRACTILE BASIN TRANSITION, SOUTHERN PATAGONIA, CHILE

The Magallanes Basin of southern Chile is today a retro-arc foreland basin. The sedimentary succession preserved in the Andean fold and thrust belt reflects an inversion: from the early extensional phase of basin evolution (Rocas Verdes basin) to the subsequent contractile phase associated with Andean orogenesis. Similar inverted retro-arc basins are inferred for a number of shortened, arc-related belts (i.e.; Antler orogenic belt). Because of intense deformation, the transition from extensional to contractile tectonism is usually obscured. The Rocas Verdes-Magallanes basin sedimentary fill shows a well-preserved tensile-contractile transition, offering a unique opportunity to study inverted basins throughout their history. In the latest Jurassic, extension associated with the initial break-up of southern Gondwana culminated in the development of a quasi-oceanic extensional basin referred to as the Rocas Verdes Basin. Plate configuration in the Late Jurassic is speculative but a reported slowing for the Pacific spreading center (and re-arrangement of plates) perhaps enhanced retro-arc extension. Volcanic rocks and volcaniclastic strata of the Upper Jurassic Tobìfera Formation and fine-grained clastic strata of the Lower Cretaceous Zapata Formation filled the Rocas Verdes Basin. Compression associated with the onset of the Andean orogeny resulted in uplift along the western basin margin and concurrent foreland subsidence. A deep-water stage, caused by flexural loading of obducted ophiolitic blocks over the attenuated crust, is marked by turbidites of the Punta Barrosa Formation. The Punta Barrosa was derived from mixed sources, including the juvenile volcanic arc and metamorphic basement complexes exposed in the Andean belt. The shale-rich Upper Cretaceous Cerro Toro Formation represents the apex of deep-water sedimentation. Petrographic and geochemical evidence indicates that the northern portion of the basin was progressively isolated from the Andean arc by uplifted thrust-sheets and that much of the sediment was derived from Paleozoic metamorphic terranes along the Andean front. The overlying Tres Pasos Formation represents shoaling upward and filling of the Magallanes Basin, eventually capped by shallow marine and deltaic sequences of the latest Cretaceous.