Structural variability at the active continental margin off southernmost Chile

Newly collected seismic reflection data offshore Southernmost Chile, betwee n 54 degrees S and 57 degrees S, reveal the complex tectonic setting of thi s active continental margin. This region is still poorly known, because fre quent bad weather conditions among these latitudes have prevented the acqui sition of geological and geophysical data. Three main tectonic domains are clearly imaged south of the strait of Magel lan, (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 wedg e of highly deformed sediments and a thick forearc basin; (3) The seaward dipping continental backstop formed partly by the Patagonia n Batholith and partly by the Paleo-Mesozoic subduction complex. The accretionary complex is related to the subduction of the Antarctic plat e beneath Scotia plate which resumed after the collision between the Chile Trench and the Chile Ridge (10-14 Ma). The structural style of the subducti on complex, such as structural vergence, width of the accretionary wedge, t aper angle and deformation in the forearc basin, varies along the margin. L arge taper values are related to narrow wedges and seaward vergent structur es. Low tapers occur where deformation at the toe of the accretionary compl ex is spread over wide areas and is related both to landward and seaward ve rgent thrust faults. The parameters which control these structural variatio ns are: thickness of the offscraped sedimentary section (variations in the depth of the decollement level); presence of overpressured fluids (the BSR is extraordinarily continuous whe re the landward vergence structures are well developed); configuration of the continental margin after the consumption of the Chilea n 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.