TECTONICS IN THE ANDES OF THE TIERRA DEL FUEGO ISLAND – SOUTHERMOST SOUTH AMERICA

The Fuegian orogen is the southernmost part of the Andean Cordillera, that with a N-S-trending for 3800 km, bends progressively towards E-W in correspondence of the Tierra del Fuego linking to the eastern northern Scotia Ridge. The progressive rotation is marked by left lateral wrench faults of the Magallanes-Fagnano system. The geological and geophysical surveys in Tierra del Fuego Island carried out in the last decade in the frame of the Italian PNRA project give a new insight of the kinematic evolution of the region. The Mesozoic-Cenozoic Andean orogenic cycle started with crustal stretching a silicic volcanism and the development of the ophiolitic complex of back-arc basin. In the Late Cretaceous the intercontinental polyphase compression occurred with horizontal shortening and crustal thickening and with a widespread magmatic emplacements. The first compressive deformation phase in the Middle-Upper Cretaceous, produced shortening with isoclinal folds and widely distributed slate cleavage structures. The high grade metamorphic rocks of the Upper Paleozoic to Lower Tertiary form the major stacks of the internal thrusts. Thrusts sheets, in- or out-of-sequence, produced also rapid exhumation of the basement rocks and the shortening of the siliciclastic sedimentary cover of the Magellan foredeep basin, with the formation of a NE verging fold-and-thrust belt. Larger folds and crenulation cleavage affect the whole orogen while the slaty cleavage is well developed in the inner and deeper part of the chain. Shear zones are characterized by penetrative structural fabrics with ductile kinematic indicators with S-C mylonites, fractured and displaced feldspar grains, rotated porphyroclasts and mica fish. From the Oligocene to present an E-W sinistral wrench tectonics affected the region as a component of the relative motion between South America and the Antarctic Peninsula related to the development of the oceanic floor in the western Scotia Sea. The principal tectonic features are transtensional wrench faults arrays arranged en-echelon with a E-W trend, connected with restraining and releasing side-steps structures along which are distributed several restricted asymmetric pull-apart basins and ridges.