Lithospheric structure of central Brazil revealed by combined 3-D electromagnetic and seismic tomography: relationship to intraplate magmatism and seismicity

SUMMARY The origins of the diverse Mesozoic magmatism and present-day intraplate seismicity in central Brazil, part of the South American plate currently undergoing compression/shortening, have been the subjects of long-standing debate. Here, we investigate the structure of the lithosphere using a combined 3-D electrical resistivity and seismic P-wave velocity (Vp) tomography to understand the deformation patterns and the relationship to the distribution of magmatism and seismicity in the region. Magnetotelluric and geomagnetic depth sounding data and P-wave traveltimes from seismological arrays were inverted for the 3-D resistivity and Vp variations in the ∼1500 × 1000 km2 area of study. The results reveal a basic structure consisting of an electrically resistive and seismically fast upper crust, a low-resistivity and low wave speed lower crust, a resistive and fast uppermost mantle and a basal low-resistivity and low wave speed asthenosphere. This layering is thinned in places and dissected by steep cross-cutting NE-SW and NW-SE shear zones of low resistivity and slow wave speeds yielding a fragmented lithosphere. Thick blocks of high resistivity and fast wave speed bounded by the steep shear zones correlate spatially with zones of gravity lows across the Neoproterozoic fold belts. The steep shear zones correlate with the Middle Proterozoic and Mesozoic mafic dyke swarms (likely emplaced during the opening of Neoproterozoic and younger proto-Atlantic oceans) and Cretaceous-Eocene alkaline and alkaline-carbonatite magmatism, indicating that inherited structures controlled the lithospheric deformation and magmatism. They also correlate spatially with zones of high seismicity, suggesting that fluid migration plays a role in generating the ongoing seismicity.