CONICET | Buscador de Institutos y Recursos Humanos

The southwest boundary of Gondwana in South America is the counter-part of the Cape fold belt of South Africa. This fold and thrust belt, known as Gondwanides, was subject to deformation during the Paleozoic, however, the geological evolution of this region is still a subject of debate. Rocks of different lithologies and ages ranging from Cambrian to Permian-Triassic were studied with AMS (anisotropy of magnetic susceptibility) in different localities along this belt. The AMS is an effective tool to measure the rock petrofabric and it allow to analyze the tectonic stress in samples of zones with deformation. Each locality have its own magnetic signature according to their lithological and ages intrinsic characteristics. Nevertheless, in almost all samples there is a gradual tectonic development overprinted that differences the AMS patterns into and between the studied localities. In the Permian rocks, as Pillahuincó Group, Carapacha Basin and Sierra Chica there is a clear regional magnetic imprint that indicate NW-SE elongation directions (Kmax) and therefore a NW-SW shortening (Kmin). There is also a movement of the Kmin from the horizontal, in the western sites at the bases of the respective sequences (olders), to the vertical in the eastern sites situated at the tops (youngers), and a change of the respective shape parameter. Thus, indicate a transition from a magnetic fabric with a clear tectonic imprint to sedimentary fabrics, similar to the conceptual models proposed by several authors in thrust and fold belts. The younger localities as Gonzales Chavez (Claromecó Basin) and Lopez Lecube, practically have no signature of deformation. Nevertheless, in the patterns of the old rocks (Cerro de los Viejos, Puelches, Cerro Colorado, Lolén Formation and Agua Escondida) the signature is no constant and the main stresses could be placed from the southwest, from the west or from the south. They have complex patterns that are related to the overlapping of different orogenic phases and/or to local lithological control of each locality. The AMS, in addition with other geological data, indicate a tectonically active and changing environment during the Permian of Gondwana. The deformation in the area came from the southwest and it attenuate during the Middle Permian, evidencing an orogenic front migration to the foreland basin to the northeast. The Permian deformation is linked to the San Rafael orogenic phase, that coincide with an abrupt curvature in the apparent polar wander of Gondwana in the Upper Paleozoic. In the southwest of Gondwana, small continental plates were accreted to the main center of the continent during the Medium Devonian. This devonian deformation known as Cháñica orogenic phase in Argentina have been related with the Chilenia and Patagonia collision with Gondwana from the west or southwest respectively. Based on that have been proposed the possibility that Chilenia and Patagonia where the same alloctonous plate. The Permian deformation was interpreted as a global process as the consequence of a paleogeographic re-organization of Gondwana and Laurentia that moves to lowest latitudes to makes the Pangea continent during the Triassic.