Lithospheric and sublithospheric anisotropy beneath central-southeastern Brazil constrained by long period magnetotelluric data

Abstract

Electric anisotropy calculated from geoelectric strikes of magnetotelluric (MT) data and seismic anisotropy derived from shear-wave splitting parameters are jointly analyzed to estimate the degree and orientation of strain in the subcontinental mantle of central-southeastern Brazil. High-quality long-period MT soundings are available at 90 sites concentrated along four profiles at the southwestern and southern borders of the Paleoproterozoic-Archean São Francisco craton. This data set is complemented by a previous study of SKS and SKKS splitting measurements available at more than 40 sites covering a slightly wider region. For this study, the MT data were processed with modern techniques, including recovery of the undistorted EM field polarizations through correction of static shift and phase mixing due to galvanic distortions. Three-dimensional forward modelling of MT and GDS (geomagnetic depth soundings) transfer functions supports interpretation of deep electrical anisotropy in the region. Magnetotelluric phase responses of orthogonal propagation modes present slight splitting at long periods for most of the sites, indicative of overall electrically low mantle anisotropy to depths greater than 250 km. The electrical strike azimuths are parallel to the fast NW directions of shear-wave splitting along the southern borders of the São Francisco craton, suggesting that the seismic anisotropy also resides within the same depth range. Since this direction is very different from that of present-day South American westward absolute plate motion, it is inferred that no significant lateral mantle flow or deformation related to the plate motion is observed under the study area, either because it is absent or the rigid lithosphere is much thicker than expected. To explain the prevailing electric and seismic common direction it is suggested that a mechanical coupling between lithospheric and sublithospheric mantle exists. A relic strain, possibly resulting from ancient continental collision processes, is interpreted to have induced a general alignment of olivine down to mantle depths beneath the continental rigid plate. The observed slightly enhanced conductive texture could be associated with hydrogen diffusion along the aligned olivine a-axis, especially within mantle patches subjected to metasomatic processes. The coincidence of mantle strikes with the trend of surface deformation pattern also suggests that crust, lithospheric and sublithospheric upper-mantle have deformed and translated coherently, preserving the regional NW direction since the tectono-thermal event responsible for the deformation. Distinct electric azimuths from the general NW trend are observed in regions probably perturbed by Cretaceous rift-related intraplate magmatism and in zones of complex deep structures produced by superposed nearly simultaneous oblique Neoproterozoic collisions in the southeast of the São Francisco craton.