Magnetotelluric evidence for a Rhyacian suture zone hidden underneath the Seridó belt, Borborema Province, Northeastern Brazil

Abstract

The Borborema Province is a complex orogenic system in northeast Brazil, whose current structural configuration is associated with the late Neoproterozoic-early Phanerozoic Brasiliano/Pan-African Orogeny. Magnetotelluric (MT) data were collected along four linear profiles across the northeastern corner of the province to assess the regional deep electrical resistivity structure. The site distribution is non-ideal for 3-D modeling, but dimensionality analysis showed that the data are dominated by 3-D effects. The model resulting from the 3-D inversion was critically evaluated for robustness and was found to provide a coherent view of the regional structure. A major crustal conductivity anomaly was modeled beneath Neoproterozoic supracrustal rocks of the Seridó belt and this anomaly deepens to upper mantle depths in the northwest direction. Considering independent geophysical information and the geometry and location of the conductor, it likely reflects remnants of a northwest-directed subduction zone with enhanced conductivity associated with interconnected magnetite along the suture/shear zone. Constrained by geochemical data, it is proposed that subduction occurred during the Rhyacian orogenic event and since then became a key pre-existing intra-basement mechanical weakness, prone to concentrate tectonomagmatic events. Its reactivation during several episodes of extension and compression in the Borborema Province was linked to the nucleation of the original Seridó rift basin (pre-Brasiliano), emplacement of mineralized bodies and crustal deformation in the Seridó belt (Brasiliano), and tectonic control for volcanics erupted during the Cenozoic (post-Brasiliano). All these processes after the Rhyacian collision in this sector of the province took place in an intracontinental setting.