TRACKING PALEOSEISMIC CYCLES TROUGH U-Pb CARBONATE DATING: EVIDENCE FROM THE BRITTLE DEFORMATION OF THE CRUZEIRO DO NORDESTE SHEAR ZONE, NE BRAZIL

The Borborema Province (NE Brazil) is a key region to investigate fluid-assisted brittle deformation related to the reactivation of continental-scale shear zones during South Atlantic rifting. The Cruzeiro do Nordeste shear zone (CNSZ), defines the northern boundary of the Jatobá Basin and displays brittle structures such as faults, veins, breccias and pseudotachylytes, which contain evidence of paleoseismic cycles. These structures formed during the brittle reactivation of pre-existing ductile and brittle-ductile fabrics. They are filled by multiple stages of mineralization, first epidote, followed by two phases of calcite. Calcitic carbonate veins appear as cement in tectonic breccias, or as late veins associated with quartz. Fault breccias are characterized by angular clasts with crackle textures, while hydrothermal-type breccias are distinguished by matrix-supported subangular to subrounded clasts, indicative of multiple fluid injection events. To determine the timing of these deformation events, five carbonate samples were dated using U-Pb geochronology. The samples include spatic calcite in hydrothermal breccias, late sub-horizontal veins, and spatic calcite that crosscuts epidote-filled veins. The textures of these carbonates range from coarse tabular crystals with mechanical twins to fine-grained, granular shapes. Cathodoluminescence imaging reveals two distinct calcite mineralization phases reflecting episodic fluid flow during deformation. The samples yielded ages from 140 to 114 Ma, spanning the Lower Cretaceous (Berriasian) to Early Cretaceous (Aptian), a timeframe associated with South Atlantic rifting and the development of the intraplate Brazilian sedimentary basins. The structural and chronological data suggest that reactivation of brittle-ductile structures played a crucial role in channelling carbonate-rich fluids during successive seismic cycles. The variations in orientations and ages of brittle structures indicates that deformation occurred episodically, driven by regional stress variations during rifting. These findings highlight the importance of brittle-ductile deformation in accommodating tectonic stresses and facilitating fluid circulation throughout rift evolution. This study enhances our understanding of the tectonic and fluid-flow processes associated with shear zone reactivation during South Atlantic opening. By integrating structural analysis with U-Pb dating of carbonate minerals, we provide a framework for reconstructing paleoseismic cycles and their role in shaping the geological record of intraplate deformation. These insights contribute to broader discussions on rifting dynamics and the evolution of continental margins.