Mesozoic extensional tectonics in the Southernmost Andes.

MENICHETTI, MARCO.; LODOLO, EMANUELE; TASSONE, ALEJANDRO

Mendoza. Argentina.

Simposio; Gondwana 12.; 2005

Godwana

The Patagonian and Fuegian Andean region and the Antarctic Peninsula were affected from the Middle Jurassic to Lower Cretaceous by extensive back-arc crustal stretching and widespread silicic volcanism associated with the break-up of Gondwanaland (Dalziel, 1981; Vaughan and Storey, 2000). The crustal thinning involved the Palaeozoic metamorphic basement, with large extensional structures associated with the development of the Rocas Verdes back-arc basin (Dalziel et al., 1974; Mukasa & Dalziel, 1996). The geometries and the structural features of the basin were controlled mainly by the extensional stress field, with an important component of wrench tectonics. At least three corridors of ocean crust are connected by transform faults, more than 800 km in length, from the actual ophiolitic outcrops in the north of the Sarmiento Cordillera to the Tortuga ophiolitic complex in the Cabos de Hornos area, in the south. In the Tierra del Fuego region, many of these extensional structures are represented in the field by mafic dyke swarms hosted in the basement rocks and in the Upper Jurassic Tobífera and Lower Cretaceous Yahgán formations (Winslow, 1982; Wilson, 1991). These normal faults are well displayed in the seismic reflection profiles both on-shore and in the Atlantic off-shore of the Magallanes and Malvinas basins. The extensional faults show a NW–SE general trend in the Atlantic off-shore, while close to the Fuegian cordillera they are rotated anticlockwise by Cenozoic compressional and strike slip faults (Biddle et al., 1986; Robbiano et al., 1996; Galeazzi, 1998). A system of asymmetric tilted fault blocks with a NW–SE general trend was deformed on a listric normal sole fault, dipping NE and SW. The geometrical pattern is constituted by alternating and partially overlapping graben and half-graben with basins a few kilometres wide and tens of kilometres long, bounded by secondary fault planes. In several areas the basins show roll-over structures with grow-fault systems, a few of them possibly reactivated in further tectonic events. The extensional structures are arranged in a right-stepped geometry with WNW–ESE orientated transfer zones that possibly constitute local depressions, where the extensional crustal thinning was renewed by magmatic accretion. In the extensional structures, more than 2000 metres of the filling of the volcaniclastic complex of the Tobífera Formation was deposited. A Palaeozoic succession constitutes the basement, which is separated from the Tobífera Formation by a regional unconformity in the Magallanes basin, dated at 150.5 Ma by Biddle et al. (1986). In the Rocas Verdes basin there was important diachroneity, both in the fault evolution and in the basin filling with Tobifera Fm dated Middle Jurassic (164 Ma, Mukasa and Dalziel, 1996) in Argentine Tierra del Fuego. In the seismic lines of the Malvinas and Magallanes basins, the Tobífera Formation displays two sedimentary sequences of the same thickness separated by an unconformity (Galeazzi, 1998). These probably represent Upper Jurassic rift sequences, which reflect the mechanical and thermal subsidence of the basin. The normal fault systems include sub-vertical structures with cumulative variable offsets from many hundreds to more than one thousand metres. In this sector of the Rocas Verdes marginal basin, the geometry of the structures allows estimation of the extension, ranging 15–20 %. A depth of 10 kilometres for the detachment of the listric sole normal fault can be inferred from geometrical reconstructions, compatible with a thinning extensional crust. Crustal stretching increases towards west and southwest, where the progressive thinning locally permitted the formation of the ophiolitic floor.