Tectonic implications of S-wave anisotropy beneath SE Brazil

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 Sao Francisco craton, the Brasilia mobile belt, the intracratonic Parana 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 Sao Francisco craton, the Brasilia Mobile Belt, and the eastern margin of the Parana 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 Sao Francisco craton and the Parana Basin. The delay time is smallest (0.6 s) for station CDCB in the craton interior and largest (1.6 s) for station RIFE located near the north-eastern edge of the Parana 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 Parana 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 Parana Basin, fast-polarization directions are dominantly 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 (<0.5 s) are observed in the eastern Parana Basin above the axis of the seismic low-velocity anomaly interpreted to be the fossil conduit of the Parana plume-head (VanDecar, James & Assumpcao 1995). While the splitting results are not definitive, the regional anisotropy of the eastern Parana 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 Parana Basin. The fast-polarization direction (NE-SW) is oriented parallel to that of the station in the western Parana Basin and almost normal to the fast-polarization direction of the stations on the eastern margin of the Parana Basin. The orientation of anisotropy in the Rio Apa Block and western Parana 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 Parana Basin and the Rio Apa Block.