Petrographic, structural, and deformational analysis in the Curitiba Shear Zone, Ribeira Belt: a case study at São Jorge quarry, Eastern Paraná State, Brazil

Abstract The Ribeira Belt in southern Brazil records the Neoproterozoic tectonic evolution associated with the amalgamation of Western Gondwana, and its shear zones provide critical constraints on transcurrent–transpressive deformation regimes. Within this framework, the Atuba Complex in the Curitiba Terrane hosts the São Jorge quarry, where metatexites preserve evidence of progressive deformation. This study investigates feldspar intracrystalline plasticity and quartz–feldspar microstructures to better understand deformation mechanisms and their significance for strain partitioning during the evolution of the Curitiba Shear Zone. Petrography, structural analysis, finite strain quantification (Fry, Polar, Rf/ϕ), and electron backscatter diffraction (EBSD) were combined to characterize deformation fabrics. This research analyzes feldspar crystals to investigate intracrystalline plasticity, while finite strain quantification was primarily based on quartz grains, which serve as reliable markers for bulk strain estimates. Three deformation stages were identified: (i) ductile shear producing mylonitic metatexites with steep NE–SW foliation and sinistral kinematics, (ii) overprinting brittle–ductile deformation, and (iii) late brittle fracturing. Quartz microstructures, including lobate boundaries and grain boundary migration, indicate deformation temperatures exceeding 500°C, while feldspar exhibits undulatory extinction, subgrain formation, and moderate dynamic recrystallization. EBSD data reveal a crystallographic preferred orientation of albite consistent with 100 slip, also suggesting deformation at ~500°C. Finite strain ratios are low (R ≈ 1. 5-2. 1), reflecting the last local increment of deformation rather than the total bulk strain, and indicating limited strain accumulation despite localized intense shearing. Brittle–ductile overprints are marked by fractures, fault zones, grain-size reduction, and comminution. Taken together, we interpret the deformation at the São Jorge quarry to be progressive, transitioning from ductile to brittle regimes within a transcurrent–transpressive setting. This interpretation provides new constraints on the structural evolution of the Atuba Complex and Curitiba Terrane and contributes to broader models of strain accommodation in the southern Ribeira Belt during the Brasiliano Orogeny.