Geophysical constraints on tectonic models of the Taubaté Basin, southeastern Brazil

Abstract

Abstract Magnetotelluric, gravity and geothermal flux data are analysed to constrain the tectonic evolution of the Taubate Basin, a SW-NE 20 km wide and 170 km long rift-like Tertiary basin in southeastern Brazil. The basin is characterized by half-grabens and contains up to 1000 m of sediments cut by normal faults. The magnetotelluric measurements show the presence of two layers: an upper highly conductive one (less than 10 Ωm from 1-D inversions), associated with Tertiary sediments, and an extremely resistive (more than 10000 Ωm) lower layer, related to the crystalline basement. Gravity anomalies in the region are ascribed to both near-surface sources, sediments and granites, and a deep-seated source associated with crustal thinning towards the continental margin. Available geothermal flux values within the basin are anomalously high; however, they have probably been affected by convective transport because they were determined in water boreholes. These results suggest a stable thermal regime at present and, viewed together with the absence of magmatism and thermal subsidence, indicate that the mechanism related to the basin formation did not originate a deep-seated thermal anomaly in the region. The event can be seen as a shallow one, involving a small area in the upper part of the crust. The region can be characterized typically as a strike-slip mobile zone, with horizontal displacements between juxtaposed blocks. A transtensional model is suggested for the Taubate Basin to explain some of its principal geological features.