Paulo Araujo de Azevedo

Lithospheric Features of the São Francisco Craton

Studying the thick lithosphere of cratons is important to help understand their formation and the mechanisms for their preservation. We present a synthesis of the information available for the deep structure in Eastern Brazil, from seismological and gravity data, to characterize the Sao Francisco Craton (SFC) and help better define its lateral boundaries at depth. Crustal thicknesses of the SFC, known mainly from receiver function studies, range from 38 to 42 km, except for a localized thickening (up to 44 km) in the northern part, and crustal thinning towards the Atlantic continental margin in Bahia state. Overall, the crust is slightly thicker near the geologically-defined surface boundaries (40–42 km) and slightly thinner in the center (38–40 km), which is consistent with generally low Bouguer anomalies and high topography to the East and to the West of the craton probably defining the suture zones during the Gondwana amalgamation. Modeling of gravity anomalies with some seismic constraints indicates a relatively low-density lithospheric mantle for the SFC, despite higher Pn velocity, which is consistent with a Fe-depleted, buoyant lithosphere, which helps preserve the cratons’s root. Surface-wave continental-scale tomography suggested the thickest lithosphere, around 200 km, to be in the Archean southern part of the SFC, consistent with regional P- and S-wave tomography. Both the surface-wave and the body-wave tomographies show high upper mantle velocities beneath the Brasilia fold belt, next to the SFC’s surface limits, which is interpreted as a continuation at depth of the craton’s lithosphere, beneath the low-grade external metamorphic domain of the Brasilia fold belt. Analysis of the SFC seismicity shows that most earthquakes now occur on shallow (<2 km) normal faults formed during the formation of the Brasiliano continental margin, now reactivated under the present E–W compressional stresses.