Boundary curvature as a first-order control on strain localization in arcuate shear zone: Insights from field observations, analogue and numerical modelling

Curved shear zones are common in orogenic systems, yet the role of boundary curvature in governing strain localization remains underexplored. Using the Singhbhum Shear Zone (SSZ) in eastern India as a natural analogue, we develop an integrated approach combining analytical Couette flow theory, physical analogue modelling with Newtonian viscous Polydimethylsiloxane (PDMS), and finite element simulations. The results consistently reveal a non-monotonic velocity profile with a neutral radius where flow reverses direction, creating distinct zones of localized shear. Strain markers in the experiment deform most intensely near the inner arc and progressively less outward, mirroring field evidence from the SSZ, where high-strain mylonites and strong non-cylindrical folds transition into overprinting crenulations and open folds. This convergence between model predictions and field observations demonstrates that the boundary curvature alone, absent of mechanical or lithological heterogeneity can explain strain partitioning, flow reversal, and fold overprinting in a ductile regime. The study provides a transferable kinematic framework for interpreting curved tectonic interfaces and underscores the often-underestimated role of geometry in shaping deformation.