2011-INCTET Progress Report on Magnetotelluric and Geomagnetic Depth Soundings in the Borborema Province and Adjacent Terrains, NE Brazil

In order to enhance our knowledge about the tectonic processes responsible for the geological evolution of the Borborema Province and adjacent regions, the National Institute of Space Research (INPE) and the National Observatory (ON) are conducting Electromagnetic Induction studies in northeastern Brazil, which are being supported by the National Institute of Science & Technology for Tectonic Studies (INCTET) project during the past three years. Such studies contemplate the deep geophysical probing of this region with Magnetotelluric surveys (MT) and Transient Electromagnetic soundings (TEM) along several geoelectric transects and an array of Geomagnetic Depth Soundings (GDS), which complement other geophysical methods. The focus of this study is directed to the identification of geoelectric variations in electric conductivity at distinct lithospheric depths and associations of geoelectric strikes and anisotropy with structural grain and stress patterns, particularly in the long ubiquitous structural features that cross the province and that constitute major geoelectric heterogeneities, at the near surface crust and also in the lower crust and upper mantle. The preliminary results indicate a lithosphere with a very variable electrical resistivity. The upper-mid crust shows up as a very resistive layer, overlying a less resistive lower crust and upper mantle. In some areas the mantle is very resistive, characteristic of a cratonic origin. Exceptions to a resistive crust are found under the Serido belt, Jatoba rift basin and the Araripe intracratonic basin, marked by an underlying conductive crust. Laterally, the deeper portion of the crust and upper mantle is highly segmented in blocks with alternating juxtaposed higher and lower resistivities. This pattern is suggestive of highly deformed regions by transform dominated tectonic regimes, with the presence of contrasting mechanically strong cratonic-like keels against weaker zones likely subjected to delamination processes that might have produced magmatic related lithospheric re-fertilization by metasomatism. Introduction The National Institute of Science & Technology for Tectonic Studies congregates many geoscientists from several Brazilian research institutions with the purpose of studying the deep crust and upper mantle structures. It is multidisciplinary in its approach, merging different techniques and methodologies, which allow the study of the Earth’s interior based on different properties of its constituting rocks. The focus of this research is the study of the crust and upper mantle of the Borborema Province and adjoining Sao Francisco Craton and Parnaiba Basin. The Borborema Province is a complex association of crustal blocks with differing ages, origin and geologic evolution, and which were amalgamated during the last orogenic event that took place in what is now the Brazilian territory, namely the late Neoproterozoic-early Phanerozoic Brasiliano orogeny, composing the Gondwana supercontinent. Therefore, Electromagnetic Induction studies are being developed with the goal of making a major contribution to a better comprehension of the actual articulation of the amalgamated crustal blocks and the role of large transcurrent lineaments in bringing these blocks together, through an integrated multidisciplinary geophysical study.