Seismic Studies of Continental Lithosphere Beneath SE Brazil

The Brazilian Lithosphere Seismic Project is part of a multinational geological and geophysical program to investigate the structure, tectonics, and geologic evolution of the continental lithosphere beneath SE Brazil. The project, which will run for a period of at least 18 months, is a collaborative effort by the Carnegie Institution of Washington and the Institute of Astronomy and Geophysics of the University of Sao Paulo. The broad band seismic studies are aimed at mapping the fine structure of the continental crust and lithospheric mantle beneath Brazil and interpreting it in terms of the origin, evolution, and composition of the Brazilian shield. The first nine stations of a planned 14 station portable array of digital REFTEK dataloggers and the broad band threecomponent Streckeisen STS-2 seismometers have been installed, or are in the process of being installed, in southeast Brazil (see Figure 1). The seismic array traverses the four major tectonic provinces of the region --- the Archean Sao Francisco craton, the surrounding late Proterozoic BrasilianolPan-African mobile belts, the intracratonic Parana Basin, and the coastal Ribeira belt, a complex transcurrent shear system. Stations are configured about 50-100 km apart in an approximately east-west saw-tooth array along 200 S latitude, with an E-W aperture of approximately 800 km. The station configuration is designed to capitalize on the abundance of high-quality seismic sources, particularly Andean, that surround and illuminate the Brazilian shield. Digital data are recorded continuously at 10 slsec and in triggered mode at 50 slsec. Timing is via GPS clocks and is accurate to better than 1 msec absolute. The matched STS-2 three component recording covers the frequency band 0.0083 to 20 Hz. Preliminary results from receiver function analysis indicate that crustal thicknesses beneath the Sao Francisco craton and surrounding Archean crust are about 40 to 42 km. Crustal velocities in these regions increase approximately linearly with depth and there is no evidence of major midcrustal discontinuities. Results from receiver functions and from two station surface wave phase velocity inversion indicate that mantle shear wave velocities are high, about 4.8 to 4.9 krn/sec as in other cratonic areas, in the uppermost mantle beneath the Sao Francisco craton and surrounding area and to at least 130 km depth beneath the Parana Basin.