Evidence for Early Tonian (Ca. 1000-940 Ma) continental rifting in southern Borborema Province (NE Brazil): tectonic inheritance and shear zone nucleation during assembly of West Gondwana

Abstract

ABSTRACT The factors controlling the inception of continental-scale transcurrent shear zones are still under debate. In order to a better understanding of the origin and evolution of the 400 km long West Pernambuco shear zone (WPSZ), we undertook a structural, geochronological and geochemical study in an area around its eastern termination. The WPSZ is one of the most expressive strike-slip shear zones in the Borborema Province of NE Brazil, which resulted from orogenic processes associated with the assembly of western Gondwana in the Late Neoproterozoic. Previous studies have shown that 1000–940 Ma-old granitic orthogneisses are located immediately to the south of the western and central portions of the WPSZ. Based on LA-ICP-MS U-Pb zircon dating, we show that rocks with similar ages (995–956 Ma) also occur to the East. The geochemical characteristics of this magmatism and the alignment of the bodies along a narrow, less than 40 km-wide belt point to emplacement in a continental rift-related setting. The WPSZ thus follows the trend of a pre-existing rheological discontinuity, which induced strain localization during the post-collisional stage of the Brasiliano-Pan-African orogeny.