Ductile to brittle tectonic evolution of the Achankovil Shear Zone, Southern Granulite Terrane – Constraints from statistical analysis of fabric and paleostress inversion

The Achankovil Shear zone (AKSZ), juxtaposing the Trivandrum Granulite Block (TB) and the Madurai Granulite Block (MB) of the Southern Granulite Terrane (SGT), represents a paleo-suture zone related to the late Neoproterozoic to early Cambrian Gondwana assembly (Rajesh et al., 1998; Praharaj et al., 2021). The polyphase deformational history of the Achankovil Shear Zone (AKSZ) reveals progressive transition from a ductile to brittle deformation regime concomitant with high-grade granulite facies metamorphism and subsequent cooling–exhumation of the granulites. Progressive evolution of the state of stress and the variation of crustal dynamics during the ductile to brittle deformation regime transition, and the genetic link between these two contrasting episodes, if any, have been evaluated from statistical analysis of solid-state fabric and kinematic analysis of brittle fractures.

Three ductile deformation phases, D₁, D₂, and D₃, and associated solid-state fabrics, i.e., S₁, S₂, and S₃, are discernible at the mesoscopic scale. S₁ fabric is gneissic in character and is only preserved in strain shadow regions. Elsewhere, S₁ is transposed along the later S₂ fabric, which is axial planar to fold on S₁. Prevalence of high-temperature deformation conditions during D₂ and D₃ deformation stages is manifested by the presence of S₂-parallel stromatic leucosomes and diatexite leucosome along dilatant S₃ fabric, developed parallel to the axial planes of fold on S₂. Significant simple shear component during D₃ deformation is evidenced from the asymmetric nature of S₂ folds, asymmetric porphyroclast tail and other shear sense markers. Eigen vector analysis reveals a change of maximum eigen vector, i.e., pole to the mean foliation, from NW-SE (D₁: 311⁰/62⁰ NE) to N-S (D₃: 187⁰/80⁰ W). The maximum eigen vector of D₂ (125⁰/53⁰ SW), though similar in trend with D1, shows a reversal of dip direction. Fabric shape analysis reveals a progressive change from girdle to a strong clustered distribution of solid-state fabric from D₁ to D₃ deformation regime, suitably accounting for intense ductile shearing and transposition of earlier fabric during the D₃ deformation stage.

Minor conjugate faults are ubiquitous at different locations along the AKSZ. Dihedral angle of ~60 or less for these faults suggests a shear or hybrid fracture origin, diagnostic of a compressive stress regime. Also, the observed slip of striations on slickensides suggests a consistent oblique reverse kinematics. Fault kinematic and paleo-stress analysis further reveals two distinct stress regimes with NW-SE and NE-SW directed maximum compressive stress (s₁). Stress ratios for these faults imply a compressional to transpressional tectonic regime. Superposition of the slip tendency of NW–SE directed stress tensor over NE–SW directed stress tensor and vice-versa suggests that the NW-SE stress tensor precedes the NE-SW stress tensor during a progressive brittle deformation regime. Summarily, the cooling and exhumation and the switch over from ductile to brittle deformation regimes of the granulites took place under a compressive stress field developed during terrane accretion along the AKSZ. The brittle faults seemingly result from the relaxation of the orogenic far-field stress.