UPPER MANTLE ANISOTROPY IN SE AND CENTRAL BRAZIL FROM SKS SPLITTING: EVIDENCE OF ASTHENOSPHERIC FLOW AROUND A CRATONIC KEEL

Abstract

We present results of upper mantle anisotropy derived from measurements of core refracted shear wave splitting (mainly SKS phases) recorded at 48 stations covering the major tectonic provinces in Central and SE Brazil, such as the Tocantins Province (Paraguay-Araguaia and Brasília belts between the Amazon and São Francisco cratons), the Paraná intracratonic basin, the southern part of the São Francisco craton, and the Mantiqueira province (with the coastal Ribeira belt). Although the fast polarization directions vary across the region, consistent orientations are observed over hundreds of kilometers. The fast polarization directions tend to be close to the absolute plate motion given by the hot-spot reference model HS3-NUVEL1A. However, correlations with geological structures are also observed in the southern Brasilia belt and in the Ribeira belt, respectively located SW and S of the São Francisco craton. On the other hand, in the northern Tocantins province, the fast shear-wave direction (~ N60°S) is oblique to the SW-NE trend of the geological units and faults, and no anisotropy contribution from lithospheric sources can be clearly identified. Overall, the fast polarization directions show a fan-shaped pattern strongly suggesting asthenosperic flow around a thick and stiff keel in the southern part of the São Francisco craton, consistent with the high-velocity anomaly revealed by recent surface-wave tomography. The observed NW-SE directions in the southern part of the Brasília belt may also be interpreted as resulting from asthenospheric flow channeled between the São Francisco craton and a cratonic block beneath the Paraná basin. The largest splitting delays observed in the southern Brasília belt and in the Ribeira belt (up to 2.4 s) suggest contributions from both lithospheric and asthenospheric sources in those two areas. Our preferred model for the anisotropy causing the observed pattern of SKS splitting consists of flow-induced deformation in the asthenosphere caused by the absolute plate motion (roughly towards W or WSW) with flow channeled around a thick cratonic keel. In the southern Brasília and in the Ribeira fold belts, the large delays and the parallelism with geological structures indicate additional anisotropy contribution from lithospheric sources. © 2006 Elsevier B.V. All rights reserved.