Deciphering the brittle deformation events in the Potiguar Basin-NE Brazil: Insights from U-Pb dating of calcite

The dating of carbonates using LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) presents a remarkable leap forward in our geological understanding. It holds promise for directly dating sedimentary sequences, hydrothermal alterations, whether linked to mineralization or not, and the timing of deformation events. Furthermore, it offers a broad scope for examining the temporal progression of brittle deformation within shear zones and fault planes.

In this context, a study was conducted on brittle structural analysis and the evolving stress systems over time in the Potiguar Basin and its underlying geological formations. The Precambrian rocks of the Borborema Province exhibit a complex network of shear zones with a multi-stage evolution, shaped during the Neoproterozoic with the formation of the Western Gondwana Orogen. These shear zones exert structural control over Paleozoic-Mesozoic basins, experiencing reactivation phases in both ductile-brittle and purely brittle regimes. These reactivations give rise to fault slickensides, carbonate and quartz-feldspar veins, intricately linked to paleostresses associated with the rifting of the Pangea Supercontinent and the subsequent opening of the South Atlantic Ocean.

Given this geological backdrop, understanding the temporal sequence of these reactivations becomes paramount for elucidating the role of ancient structures in the formation of basins in northeastern Brazil. To this end, fieldwork was conducted in the Potiguar Basin (Mesozoic), focusing particularly on intrabasinal deformation. Extensive data collection was undertaken, including structural observations and oriented sampling of fault planes containing slickensides, veins, and carbonate-filled stylolites. These samples underwent U-Pb in situ dating on carbonates using LA-ICP-MS at the Laboratory of Isotopic Geology of the Institute of Geosciences-UNICAMP.

The mapped structures delineate two distinct compressional stress regimes: firstly, a N-NE orientation characterized by reverse faults displaying down-dip slickensides; and secondly, an E-W orientation marked by E-W veins featuring N-S growth fibers alongside strike-slip faults. Structural elements associated with these stress regimes were dated accordingly. To ensure the reliability of the obtained ages, samples representing syn-deformational filling were carefully selected, bolstering the robustness of the data. The resulting ages reveal that these two stress fields operated during discrete time intervals, delineating two deformation episodes spanning from the Paleogene to the Neogene. Notably, these ages also temporally align with magmatic events in the region.