Abstract

Newly collected seismic reflection data offshore Southernmost Chile, between 54 degrees S and 57 degrees S, reveal the complex tectonic setting of this active continental margin. This region is still poorly known, because frequent bad weather conditions among these latitudes have prevented the acquisition of geological and geophysical data. Three main tectonic domains are clearly imaged south of the strait of Magellan, (1) The oceanic area of the Antarctic plate, where a 2 km thick and largely undeformed sedimentary section, rests on the oceanic basalts; (2) The subduction complex, formed by a relatively narrow accretionary wedge of highly deformed sediments and a thick forearc basin; (3) The seaward dipping continental backstop formed partly by the Patagonian Batholith and partly by the Paleo-Mesozoic subduction complex. The accretionary complex is related to the subduction of the Antarctic plate beneath Scotia plate which resumed after the collision between the Chile Trench and the Chile Ridge (10-14 Ma). The structural style of the subduction complex, such as structural vergence, width of the accretionary wedge, taper angle and deformation in the forearc basin, varies along the margin. Large taper values are related to narrow wedges and seaward vergent structures. Low tapers occur where deformation at the toe of the accretionary complex is spread over wide areas and is related both to landward and seaward vergent thrust faults. The parameters which control these structural variations are: thickness of the offscraped sedimentary section (variations in the depth of the decollement level); presence of overpressured fluids (the BSR is extraordinarily continuous where the landward vergence structures are well developed); configuration of the continental margin after the consumption of the Chilean mid oceanic ridge (14 Ma) and its related phase of tectonic erosion. presence of strike slip faults belonging to the South America-Scotia plate boundary system which could bound crustal blocks with different mechanical behaviour (i, e. Strait of Magellan-Lago Fagnano and Beagle channel faults). (C) 1999 Elsevier Science Ltd. All rights reserved.