HR: 0800h
AN: T31A-0275    [Abstracts]
TI: Upper Mantle Anisotropy in South America With SKS Splitting: Predominance of Asthenospheric Flow From Absolute Plate Motion and Test of the Frequency-Dependent Polarization Filter in the SKS Analysis
AU: Guarido, M
EM: mguarido@iag.usp.br
AF: IAG - USP, Rua do Matao, 1226, Cidade Universitária., Sao Paulo, SP 05508-090, Brazil
AU: * Assumpcao, M
EM: marcelo@iag.usp.br
AF: IAG - USP, Rua do Matao, 1226, Cidade Universitária., Sao Paulo, SP 05508-090, Brazil
AU: Schimmel, M
EM: schimmel@ija.csic.es
AF: IJA - CSIC, Lluis Sole i Sabaris s/n, Barcelona, 08028, Spain
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 has been 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). Our data, together with published data for the Andean region, were compared with the absolute plate motion as well as with upper mantle flow direction. 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 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. Comparison with the upper mantle flow model developed by Conrad et al. (2007) showed some good results. While this flow model is roughly consistent with the splitting observations in SE and NE Brazil, the deviations in the Amazon craton and in the Andean region are very large. In the Amazon, the upper mantle flow model has low resolution (the San Francisco and the Amazon cratons are not separate units). In the Andes region the shear-wave splitting may be due to frozen lithospheric anisotropy, or the upper mantle flow model is not accurate. A frequency-dependent polarization filter was tested to improve estimates of SKS splitting. With synthetic data good results were obtained for larger windows and higher powers. Tests with real data will be presented.
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7218 Lithosphere (1236)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting