Deep structure of a stretched lithosphere: Magnetotelluric imaging of the southeastern Borborema province, NE Brazil

Abstract

Abstract Magnetotelluric data were collected at 25 stations along a NNW–SSE linear profile deployed perpendicularly to the main structures in the southeastern portion of the Borborema province, northeast Brazil. The geotectonic scenario is characterized by a broad range of tectomagmatic events including periods of compressive tectonism associated with amalgamation of lithospheric blocks during the Brasiliano orogeny in the Late Neoproterozoic, superimposed by extensional events related to the continental break-up and plate drift that led to the formation of the southern Atlantic Ocean in the Early Cretaceous. Strike analysis and distortion decomposition show that most of the data are two-dimensional (2D) but sensitive to distant off-profile structures. Data fit an ENE regional geoelectric strike direction, consistent with the geological and tectonic setup. A joint 2D inversion of the TE and TM modes defined by this strike direction was undertaken and the obtained model exhibits signatures of the different tectonic events in the area. The model suggests that the SE Borborema province is electrically separated into two parts. The first, comprising the southern domain of the province (Sergipano belt and Pernambuco–Alagoas block), has a highly heterogeneous crust and an anomalously conductive upper mantle. The second, comprising the transversal domain of the province (Alto Moxoto terrain), displays crust and upper mantle predominantly homogeneous and resistive. A deep Neoproterozoic lithospheric shear zone (Pernambuco lineament) marks the limit between the two contrasting domains. Also, the region where the Jatoba basin is located in the southern domain presents a striking conductor at upper- to mid-crustal depths beneath the basin. These results correlate well with seismic refraction data that indicate thicker crust for the transversal domain than for the southern domain, with the thinnest crust coinciding with the location of the Jatoba basin. The data support the conclusion that the lithosphere of the southern domain of the Borborema province was significantly stretched in the Cretaceous during the opening of the South Atlantic Ocean, allowing refertilization of depleted upper mantle with incompatible elements from enriched deeper mantle material and reactivation of older crustal fabrics. The conductivity increase below the Jatoba basin is associated with ionic conduction in a currently reactivated fracture zone filled with high salinity fluid. Apparently, the Alto Moxoto terrain worked as a region of higher resistance to stretching, causing the crustal thinning and geoelectric effects to be concentrated to the south of the Pernambuco lineament.