Regional scale electrical structures delimiting the Rio de la Plata craton at 31.5S, Argentina.

A. FAVETTO C. POMPOSIELLO, J.BOOKER, A. BURD

El Vendrell España

Workshop; 18th International Workshop on Electromagnetic Induction; 2006

Magnetotelluric (MT) data were obtained along a 550 km almost west-east profile at around 31.5o S, extended from La Rioja to Entre Rios provinces in Argentina. New MT data, corresponding to the twelve most eastern sites, allowed to complete the determination of the crystalline basement depth below the Chaco-Paranense basin and characterize different geological structures up to upper mantle depths. From dimensionality analysis it was obtained that electrical structures are almost one dimensional (1D) for periods less than 30 s (representing an average depth of about 7 km in the model) while for longer periods they are mainly two-dimensional (2D). In the last case a strike direction oriented parallel to the surface geological strike (around N 6◦E) was found. Previously, a resistivity model for the western part of the profile permitted to delimitate the Rio de la Plata craton border which was identified with the suture between it and the Pampean terrain. The model obtained including the new data showed that the bottom of the sedimentary basin extends to a depth of about 5 - 6 km in the central part and tends to be shallower to the eastern end. The craton is the geological structure with the highest resistivity values (¿1000 ohm-m) and a sharp decrease of resistivity appears at a depth of about 200 km, which can be interpreted as the lower limit of the Rio de la Plata craton. The possibility of anisotropy effects in the upper mantle is discussed.