Lithospheric structure of central Brazil revealed by combined three-dimensional electromagnetic and seismic tomography: Relationship to the distribution of magmatism and intra-plate seismicity

The origin of intraplate seismicity and Cretaceous-Eocene alkaline magmatism in central Brazil has been the subject of long-standing debate. Here, we investigate the structure of lithosphere using a combined three-dimensional (3D) electrical resistivity and seismic P-wave velocity (Vp) tomography in order to understand the deformation patterns and relationship to the distribution of magmatism and seismicity in the region. Magnetotelluric and geomagnetic depth sounding data and P-wave travel times from seismological arrays were inverted for 3D resistivity and Vp variations in the ~1500×1000 km2 area of study. The results reveal a strong correlation between electrical resistivity and seismic velocity anomalies in the lithosphere and upper asthenosphere. The basic structure consists of an electrically resistive and seismically fast upper crust, a low-resistivity and low-wavespeed lower crust, a resistive and fast uppermost mantle and a basal low-resistivity and low-wavespeed asthenosphere. This layering is dissected by through-going steep NE-SW and NW-SE shear-zones (or alteration haloes) of low resistivity and slow wavespeeds yielding a completely fragmented lithosphere. There is a strong correlation between these steep shear-zones and zones of gravity lows, past magmatism and present-day seismicity in the region. These anomalous steep zones likely tapped sub-lithospheric sources of magma and assisted the movement of lithospheric blocks in the ensuing compressional deformation (and basin inversion). Given the occurrence of mafic and alkaline rocks, basin inversion and through-going shear zones (to facilitate deep fluid migration), we suggest that this region may be important for future exploration for geothermal and natural hydrogen resources needed for a low-carbon future.