GEOMETRY AND EVOLUTION OF THE FRONTAL PART OF THE MAGALLANES FORELANDTHRUST AND FOLD BELT (VICUNA-AREA), TIERRA-DEL-FUEGO, SOUTHERN CHILE

The Magallanes foreland thrust and fold belt is a thin-skinned forelan d thrust and fold belt of Paleocene to Oligocene age that deforms Uppe r Jurassic through Tertiary volcanic, volcaniclastic, and siliciclasti c strata of the Magallanes basin, southern Andean Cordillera, Chile. T his paper is a detailed description and analysis of the geology and st ructural evolution of the thrust front (Vicuna area of southern Tierra del Fuego). Reflection seismic and well data, together with 1:50,000 scale geological mapping, have been used in the analysis. In the south ern part of the Vicuna area, two different thrust systems have been fo und: an upper imbricate fan that deforms Upper Jurassic and Cretaceous strata, and a younger, lower duplex composed of Cretaceous and probab ly Upper Jurassic rocks. The imbricate fan is characterized by fault-p ropagation folding in which listric thrust faults merge downward into a sole thrust that probably is located within the Upper Jurassic strat igraphy. This fan system generates an intercutaneous wedge with a well -developed triangle zone at its leading edge. The sole thrust of the u pper imbricates forms the roof thrust of the underlying duplex. In the northern part of the Vicuna area, the syntectonic sedimentary wedge o f the foredeep consists of late Cretaceous through Tertiary siliciclas tics that have been deformed and uplifted by passive back thrusting at the triangle zone. The structural style in the foreland region shows three main subhorizontal detachment levels located within the sediment ary wedge as a result of the progressive transfer of slip from die thr ust belt to the foreland. Minor blind thrusts produce stacked ''pop up '' and triangle structures that result in complex geometries in the co res of anticlines. A forward-breaking sequence of thrusting is interpr eted. During deformation, the active foredeep wedge migrated at least 10 km northward. Balanced geological cross sections indicate approxima tely 60% (-30 km) shortening for this part of the Magallanes thrust be lt.