HR: 1340h
AN: S13B-1055    [Abstracts]
TI: New Interpretation of Deep Seismic Refraction data from the Tocantins Province, Brazil
AU: * Perosi, F
EM: fabio@iag.usp.br
AF: US Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025 United States
AU: * Perosi, F
EM: fabio@iag.usp.br
AF: University of Sao Paulo, Rua do matao, Sao Paulo, 05508 Brazil
AU: Mooney, W D
EM: mooney@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025 United States
AU: Berrocal, J
EM: berrocal@iag.usp.br
AF: University of Sao Paulo, Rua do matao, Sao Paulo, 05508 Brazil
AB: We interpret seismic refraction data in the central sector of the Tocantins Province, central Brazil, producing a seismic crustal model with a well-defined upper, middle, and lower crust with smooth velocity gradients in each layer. The depths to Moho vary from 32 to 43 km, and we find mean crustal P-wave velocity to be 6.3 - 6.4 km/s. The behavior of the lower crustal layer allows an improved understanding of regional gravimetric features of central and northern sectors of the Tocantins Province, and suggests that there was subduction of the Amazon plate beneath central Brazil. In the southeastern sector, the refraction experiment resulted in a detection of a thinner crust (38 km) below the Brasilia fold belt, and a thicker crust (41 km) below the Parana basin and Sao Francisco craton (42 km). The upper crust beneath the Parana basin is around 20 km thick, and thins to less than 10 km below the craton. Gravimetric measurements in the central sector of the Tocantins Province delineate a high and a low anomaly separated by a steep gradient with a NE direction. The axis of the gradient seems to bend still further to NE in the northern sector of that province, whereas the gravimetric high continuous northwards, defining a separation between them. This suggests those features belong to different tectonic processes. The gravimetric model, which incorporates seismically resolved structure beneath the Tocantins Province, matches the observed gravimetric data quite well. Although tectonic movements have been monitored with high precision GPS for only a short time interval (1999- 2001), the results suggest observable deformations. The main seismicity of central Brazil, the Goias-Tocantins seismic belt, seems to be spatially associated with a large gravimetric high and with observed tectonic deformation. These results bring new insights into the geological history of the central and southeastern sectors of the Tocantins Province.
DE: 1212 Earth's interior--composition and state (8105)
DE: 1219 Local gravity anomalies and crustal structure
DE: 1242 Seismic deformations (7205)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting