New evidence of the importance of faulting in the structure of the Sierras Australes de la Provincia de Buenos Aires

The Sierras Australes de la Provincia de Buenos Aires or Sierra de la Ventana are situated in the south part of Buenos Aires province, Argentina, between 37°-39°S and 61°-63°W, and constitute a curved fold and thrust belt with a general northwest strike. They are formed of Precambrian to Permian rocks. The oldest and more intensely deformed rocks out in the western ranges, whereas the younger and less deformed are located in the east. Traditionally, these ranges were considered as an example of pure folding without any associated faulting. In this paper, mesoscale structural sections that prove the importance of faulting in the ranges development are described. On a regional scale, the presence of tectonic contacts between the Lolen Formation (upper Devonian) and Providencia Formation (lower Devonian), and between the latter and the Neposta Formation (lower Devonian) are suggested in Parque Tornquist, Estancia Las Vertientes and at Cerro Tres Picos. Field evidence, suggests that the Pillahunico Ranges are structurally higher than the Ventana Ranges, because of a north-east dipping underthrust.Fil:Tomezzoli, R.N. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Preliminary paleomagnetic results from the Carboniferous-Permian El Imperial Formation sedimentary rocks in the San Rafael Block, Mendoza province

During the Early Permian an extensive deformation belt well-known as Gondwánides developed in the southwest Gondwana margin. A foreland basin has been settled during the Late Carboniferous - Early Permian in the western end of this strip, and is represented by the El Imperial Formation, which crops out in the San Rafael Block. The upper section of this unit records an inversion of the ancient slopes, which has been interpreted as the first evidence of the migration of the orogenic front towards the foreland. Therefore, a paleomagnetic study has been carried out on this unit, searching for elements that allow to constraint the deformation age to evaluate the possible diachronism of it. First paleomagnetic results obtained in EI Imperial Formation, from the lower member in the Represa Agua del Toro and from the upper member in the Puesto La Josefa, are presented and discussed. These rocks have reverse magnetizations carried by hematite, of possible Permian age. Pole positions are discordant with the apparent polar wander path from South America. These differences could be explained because: secular variation was not properly averaged; rotations about vertical axes of the sampling localities; or the combination of these factors. © 2006 Asociación Geológica Argentina.Fil:Tomezzoli, R.N. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Japas, M.S. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina