HR: 1400h
AN: S33A-01    [Abstracts]
TI: Upper Mantle Anisotropy in Brazil With SKS Splitting: Predominance of Asthenospheric Flow From Absolute Plate Motion
AU: * Guarido, M
EM: mguarido@iag.usp.br
AF: Universidade de Sao Paulo, IAG-USP Rua do Matao, 1226, Cidade Universitaria, Sao Paulo, SP 05508-090, Brazil
AU: Assumpcao, M
EM: marcelo@iag.usp.br
AF: Universidade de Sao Paulo, IAG-USP Rua do Matao, 1226, Cidade Universitaria, Sao Paulo, SP 05508-090, Brazil
AU: van der Lee, S
EM: suzan@earth.northwestern.edu
AF: Northwestern University, Dept. of Geological Sciences 1850 Campus Drive , Evanston, IL 60208-2150, United States
AB: Upper mantle anisotropy, due to lattice preferred orientation of minerals, like olivine, can be caused by shear deformation from the last major orogeny (usually preserved in the lithosphere) or from the lithosphere/asthenoshere strain related to the present absolute plate motion. Anisotropy causes shear wave splitting, with the fast polarization direction usually sub-parallel to the flow direction of the upper mantle rocks. Upper mantle anisotropy in Brazil was studied with SKS and SKKS shear wave splitting, where the delay time and the fast polarization directions were measured with the method of Silver and Chan (1991). Measurements of splitting parameters have been made at 10 stations of the BLSP02 project, mainly in northern and NE Brazil, complementing the previous study of Assumpcao et al.(2006) in SE Brazil. Detectable anisotropy was found at all stations with delay times ranging from 0.5s to 1.6s (average about 1s). Fast polarization directions tends to be roughly EW oriented, on average, consistent with the absolute plate motion direction given by the HS3-NUVEL1A model. The present data set confirms the deviation from the absolute plate motion caused by upper mantle flow around the keel of the the Sao Francisco craton in SE Brazil. In other parts of Brazil, no significant large deviation from the absolute plate motion could be observed, except at stations PDCB (BLSP02) and RCBR (GT), near the northeastern coast, where a clearly NNE to NE fast direction was determined. This fast direction can be correlated with local geological structures near each station. Alternatively, the NE to NNE fast direction is consistent with the predictions of the upper mantle flow model of Conrad et al.(2007). However, while this flow model is roughly consistent with the splitting observations in SE and NE Brazil, the deviations in the Amazon craton are very large. The fact that most fast directions in Brazil tend to be roughly parallel to the absolute plate motion implies that the contribution to the shear-wave splitting from the frozen lithospheric anisotropy is relatively small, compared to other continental plates.
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 9360 South America
SC: Seismology [S]
MN: 2007 Joint Assembly