Tectonic implications of S ‐wave anisotropy beneath SE Brazil

SUMMARY SKS shear‐wave splitting has been analysed for fast‐polarization direction and delay time at 14 broad‐band portable seismic stations arrayed in SE Brazil across the São Francisco craton, the Brasília mobile belt, the intracratonic Paraná Basin, the Rio Apa Block to its west, and the coastal transpressional Ribeira Belt. Splitting parameters are consistent with fossil mantle anisotropy resulting from major geological events and exhibit only coincidental correlation with absolute plate motion. For the coastal Ribeira Belt, the anisotropy, while not unusually large (1.05 s). parallels the local and regional fault trends at the surface, consistent with anisotropy formed by regional transpressional tectonics. In the region comprised of the São Francisco craton, the Brasília Mobile Belt, and the eastern margin of the Paraná Basin, fast‐polarization directions trend monotonously WNW‐ESE, parallel to the dominant regional faulting and consistent with their having formed during a NE‐SW Brasiliano ( ca. 600 Ma) collisional event between the São Francisco craton and the Paraná Basin. The delay time is smallest (0.6 s) for station CDCB in the craton interior and largest (1.6 s) for station RIFB located near the north‐eastern edge of the Paraná Basin and on the prominent Bouguer gravity anomaly gradient inferred by Lesquer et al. (1981) to mark the cryptic (i.e. basement) suture between craton and Basin. Within the Paraná Basin, shear‐wave splitting is uniformly small (< 1.0 s), indicating relatively weak mantle foliation in the horizontal plane. Fast‐polarization directions within the cratonic nucleus of the Basin (near the Basin axis) average approximately E‐W. Within the marginal regions of the Basin, fast‐polarization directions tend to be subparallel to the trend of the bordering Brasiliano basement structures, indicating the pervasive influence of the Brasiliano accretionary events. In the eastern and northern Paraná Basin, fast‐polarization directions are dominanlly WNW‐ESE, whereas in the western Basin the fast‐polarization direction is NE‐SW, parallel to observed anisotropy in the Rio Apa Block to the west. Minimal delay times (<0.5s) are observed in the eastern Paraná Basin above the axis of the seismic low‐velocity anomaly interpreted to be the fossil conduit of the Paraná plume‐head (VanDecar, James & Assumpção 1995). While the splitting results are not definitive, the regional anisotropy of the eastern Paraná Basin may also reflect plume‐head spreading beneath the continental lithosphere and partial destruction of pre‐existing mantle fabric. Large shear‐wave splitting (1.55 s) is observed for the station in the Rio Apa Block adjoining the western margin of the Paraná Basin. The fast‐polarization direction (NE‐SW) is oriented parallel to that of the station in the western Paraná Basin and almost normal to the fast‐polarization direction of the stations on the eastern margin of the Paraná Basin. The orientation of anisotropy in the Rio Apa Block and western Paraná Basin appears to be consistent with the inferred regional trends of Brasiliano structures, suggesting that the anisotropy may have been produced by NW‐SE convergence and collision between the cratonic basement of the Paraná Basin and the Rio Apa Block.