Evolution of brittle-ductile deformation to brittle fault zones and the role of fluid migration in Cruzeiro do Nordeste shear zone, Borborema Province, NE Brazil

Abstract

The Borborema Province in northeastern Brazil is an ideal location for investigating the tectonic evolution of crustal-scale strike-slip shear zones. These structures exhibit an anastomosing network with numerous well-exposed mylonitic belts linked to the Neoproterozoic Brasiliano-Pan-African Orogeny. However, there is a gap in information on fluid-rock interaction related to both brittle-ductile and brittle deformation. This work aims to describe the control of brittle-ductile structures on the development of brittle fault zones with significant fluid interaction associated with the Cruzeiro do Nordeste shear zone, which limits the northern border of the Jatobá Basin. Our study is based on multiscale structural analyses, integrating aeromagnetic data, UAV images, outcrop-based measurements and microstructural characterization. We documented that the mylonitic foliation is represented by ENE-WSW magnetic positive anomalies (~14 nT/m) and is characterized by S-C fabrics indicating dextral kinematics. Brittle-ductile deformation is marked by dextral C'-type shear bands (WNW-ESE) and mesoscopic strike-slip faults (NW-SE and N-S). These structures exhibit bulges defined by fine-grained, peripheric quartz grains. C’ shear bands evolve into brittle fault zones composed of mosaic and chaotic breccias, veins filled by epidote, epidote+calcite, and calcite, which are associated with hydraulic brecciation. Cathodoluminescence analysis revealed variations in luminescence along the calcite-filled veins, suggesting at least two phases of fluid interaction. The older phase exhibits higher luminescence and is brecciated by the younger calcite fluid, which displays lower luminescence activation. This fluid-rock interaction can modify the permoporous system of analog reservoirs, which can be observed through the variation in cementation intensity in the Tacaratu Formation, sandstones of the Paleozoic sequence of the Jatobá Basin. Our results indicate that the Cruzeiro do Nordeste shear zone is an excellent example that preserves the record of ductile, brittle-ductile and brittle deformation due to exhumation. Furthermore, enhancing our knowledge of brittle deformation associated with the late stages of the Brasiliano-Pan-African cycle (Cambrian?) at shallow crustal levels may be the key to understanding the tectonic evolution of Paleozoic and Cretaceous sedimentary basins in northeastern Brazil.