Magnetotelluric Study in the Western Border of the Río de la Plata Craton (Chacopampeana Plain and Eastern Sierras Pampeanas)

FAVETTO, ALICIA; PERI, VERÓNICA GISEL; POMPOSIELLO, M. CRISTINA; ROCHA FASOLA, V.; GARCÍA RODOLFO

Foz de Iguassu

Encuentro; Meeting of Americas; 2010

American Geophysical Union (AGU)

Two dimensional inversions of MT data acquired along two transects in the Chacopampeana plain provided resistivity models of the western border of the Rio de la Plata Craton (2.1? 2.3 Ga). One at around 24SL, across the Formosa Province, in a NW-SE direction (A) and other at approximately 27SL in the Santiago del Estero and Chaco provinces in a W-E direction (B). Also, a positive comparison with previous results from the profile in Córdoba province at the Eastern Sierras Pampeanas around 31.5SL in the W-E direction (C) is shown. In the Chacopampeana plain, many authors inferred that the western border of the Río de la Plata craton is the southern continuation of the Trans-Brazilian Araguaia lineament in Brazil. All transects cross the described zone and their integration permitted to characterize, regionally, a very high resistive sector at the eastern part with typical values of resistivity (>5.000 Ohm-m) for the ancient structures corresponding to the Río de la Plata craton. Through dimensional analysis of the data, it was demonstrated that regional-scale electrical structures are mainly two-dimensional with a strike direction oriented parallel to the surface geological strike. Models generated from long period responses from all the profiles show signifiCant lateral variations in the lithosphere. The models corresponding to profile A and B show that the highly resistive sectors (greater than 5.000 Ohm-m) are found approximately at depths between 5 and 100 km. In profile B the resistive zone is observed at both ends of the profile separated by a more conductive (50-100 Ohm-m) zone extended along 100km. Profiles A and C show a lateral discontinuity being two Orders of magnitude more conductive at the west than at the east. In profile C the resistivity model shows a subvertical limit approximately along the eastern border of the Sierra Chica de Córdoba. To the east below the shallower Chaco-paranense basin the Río de la Plata craton shows resistivities in a range of 1.000?10.000 Ohm-m. Based on both, the geotectonic context and the magnetotelluric results, the sharp lateral discontinuity observed in the resistivity model of the profile A y C and the discontinuity more conductive in the profile B are conjectured to represent the boundary between the Río de la Plata craton and the Pampean terrane which could correspond to the Early Cambrian suture. A tentative limit between approximately 23.5SL and 31.5SL for the western border of the Río de la Plata craton was provided from the joint interpretation of the models. The relevance of these studies is enhanced by the fact that no outcrops and deep geophysical studies exist.