Structural evolution of the Cruzeiro do Nordeste shear zone (NE Brazil): Brasiliano-Pan-African- ductile-to-brittle transition and Cretaceous brittle reactivation

Abstract

The Borborema Province (NE Brazil) is characterized by the development of continental-scale transcurrent shear zones related to the Neoproterozoic Brasiliano-Pan-African Orogeny. These shear zones commonly border Cretaceous intraplate sedimentary basins. This work presents a structural and microstructural study of the Cruzeiro do Nordeste shear zone (CNSZ), which limits the northern border of the Jatobá Basin. The ductile deformation of the CNSZ is marked by high-angle, ENE-trending foliation bearing subhorizontal stretching lineation, with numerous kinematic indicators showing dextral shearing. We documented a continuous transition from high-temperature (high-T) to low-temperature (low-T) (c. 650 °C to c. 300 °C) ductile fabrics characterized, at the high-T end, by quartz recrystallization by grain boundary migration and feldspar recrystallization by subgrain rotation, and, at the low-T end, by bulging recrystallization of quartz and extensive fracturing of feldspars. The cooler semi-brittle to brittle deformation superimposed on the mylonites is characterized by conjugate pairs of strike-slip mesoscopic faults. The orientation of these faults (WNW-ESE, dextral, and N–S, sinistral) suggests they were formed under the same stress field than the ductile fabrics and thus evidence a continuum deformational from the ductile to the brittle field associated with exhumation during transcurrent tectonics. Brittle reactivation of the CNSZ is characterized by normal faults overprinting the mylonitic foliation. We report a U–Pb age from fault-hosted calcite slickenfibres of 135 ± 4.7 Ma, which provides constraints on the timing of brittle reactivation that can be associated with opening of the South Atlantic Ocean.