MULTISCALE INVERSION OF ONSHORE-OFFSHORE ELECTROMAGNETIC DATA FROM SOUTHEAST BRAZIL AND SANTOS BASIN

Abstract

In this work, magnetotelluric (MT) and Controlled Source Electromagnetic (CSEM) methods have been used to assess the deep and shallow geoelectrical nature of the Earth underneath Southeastern Brazilian onshore margin and the adjoining offshore Santos basin (SB). It was investigated the Southern Brasilia and Ribeira Orogenic Belts (BROB) and SB underlying lithosphere by three-dimensional MT imaging using 174 amphibious MT dataset. 3D conductivity models reveal the presence of a highly heterogeneous crust in BROB, whereas it is uniformly resistive below the SB continental shelf region. A resistive segmented layer (of about 60-80 km), and another moderately resistive lower layer (of 80-100 km) are key features of the lithosphere. Excepting the above, a steeply dipping sub-lithospheric conductor is associated with a confined asthenosphere upwelling and might be related with the surface deformation hills of BROB below the coast-parallel São Paulo-Rio de Janeiro dyke swarms. Deep lithospheric roots of ~200 km were found beneath part of BROB, which is thinning out to be ~75 km below the SB continental shelf and probably becomes thinner towards the deeper ocean coincident with the lithosphere-asthenosphere boundary (LAB) found in seismic tomography, speculating that the opening of the South-Atlantic Ocean probably uplifted the lithosphere underneath it. It is also investigated the relationship between deep crustal structure and the deformation in the overlying sedimentary wedge in SB and evolution of the salt-related ‘Albian Gap’. It was found beneath this wedge, the resistive continental crust is ~35 km thick across the Cretaceous hinge line and thereafter thins seaward to ~21 km over a lateral distance of ~80 km defining a domain of highly extended and faulted crust. The models show a mantle-associated basement high and evidence of significant dislocation of the overlying sedimentary wedge which spatially coincides with the Albian Gap and a previously proposed Moho high. This implies a coupled deformation of the basement and the sedimentary wedge. It is proposed that magmatism or lower crustal flow may have played a significant role in the inferred displacement at the Albian Gap. Finally, it is presented a CSEM processing workflow from a data set recently public available in SB intesecting the main MT profile. A methodology involving joint inversion of MT and CSEM data incorporating a practical weighting scheme and constraints is also employed resulting in better data fit and enhanced electromagnetic images that are validated through geological considerations.