Geometry of conjugate shears with conflicting shear-sense indicators in the western Palghat–Cauvery Shear Zone, Southern India

Shear zones are the areas of localised deformation that contain differential movement within the lithosphere. As such it plays a vital role in the tectonic evolution of continental margins and orogenic belts. The formation, geometry and kinematics of these zones are important for reconstructing the tectonic history. The Indian subcontinent exposes several crustal-scale shear zones, which are major zones of deformation that accommodate the movement of tectonic plates. Among these, the ~30 km wide east-west trending Palghat-Cauvery Shear Zone (PCSZ) forms one of the major transpressive dextral systems in the Southern Indian Granulite Terrain (SGT).

The PCSZ records D1–S1 fabrics that were overprinted by widespread dextral D2 mylonitisation (S2). This structural configuration is altered by brittle to brittle–ductile D3 structures, indicating significant structural heterogeneity in the area. On close examinations, the region is found to preserve evidence of conflicting nature of shears with brittle, brittle-ductile and ductile signatures. The structural complexity of the PCSZ is envisaged as a product of multiple deformation events, tectonic reworking and the overprinting of successive fabrics. The dextral and sinistral senses of shears include σ- and δ-type porphyroclasts, folds, minor faults, and fractures. The ductile dextral shears are characterised by well-developed S-C fabrics, σ- and δ-type porphyroclasts, and mica fish and folds that have oriented nearly in the E-W direction, while both the ductile and brittle sinistral shears are oriented mainly in the NNE to NNW direction. The younger brittle shears such as minor faults and fractures overlap the earlier ductile, which is oriented in the NW-SE direction. The structural analysis of the western PCSZ reveals that two principal stress regimes were in operation in this region. The early N-S compressive stress is associated with the collision of the Madurai and Madras blocks, producing E-W trending foliations, folds and σ- and δ-type porphyroclasts. The later E-W-oriented stress developed due to the transpressional movements leads to the development of conjugate brittle and brittle-ductile shear sets. Thus, the PCSZ form an ideal section to understand how alternating stress orientations and multiple deformations can form conjugate conflicting shear systems, exhibiting the interplay of ductile flow, strain partitioning, and brittle fracture in the deep crustal response to orogenic processes.