Deep magnetotelluric signatures of the early Neoproterozoic Cariris Velhos tectonic event within the Transversal sub-province of the Borborema Province, NE Brazil

摘要

The Borborema structural province, in northeast Brazil, consists of a complex set of crustal blocks of different ages, origin and evolution, with final amalgamation during the late Neoproterozoic Brasiliano Orogeny. In the central part of the province, its Transversal sub-province is characterized by elongate NE-SW terranes largely affected by the Brasiliano metamorphic overprinting and by a suite of early Neoproterozoic metavolcanic and metaplutonic rocks, reflecting a distinct tectonic event (Cariris Velhos). Magnetotelluric (MT) data were collected at 64 stations along four profiles across this central region of the province to assess its deep electrical resistivity structure. Dimensionality analysis shows that the subsurface is electrically dominated by 3D structure, and 3D inversions of the full impedance tensor and vertical magnetic transfer function (tipper) were carried out. Data fit is relatively good with an acceptable distribution of the misfit between the sites and the dominant features in the final model are north-dipping crustal conductors, which can be tentatively correlated between the different MT profiles and cross contrasting terranes at the surface of the province. Considering the geometry and distribution of the conductors, it is proposed that they might reflect remnants of a north-directed subduction zone with enhanced conductivity associated with graphitized biogenic material in metasedimentary rocks originally deposited in a restricted oceanic environment and subsequently metamorphosed and introduced into the deep crust by the subduction of the oceanic lithosphere. Constrained by isolated outcrops of early Neoproterozoic rocks with oceanic affinity, the proposed subduction event may have taken place during the Cariris Velhos Orogeny with the crustal remobilization by the Brasiliano Orogeny being restricted to the brittle upper crust. This interpretation gives support for an accretionary model to the Neoproterozoic geodynamic evolution of the Borborema Province. (C) 2015 Elsevier B.V. All rights reserved.