Crustal structure along Permanallur-Pallapatti deep seismic reflection/refraction profile and inferred evolutionary history of Palghat-Cauvery Shear System, Southern Granulite Terrain, India

Deep seismic studies over Permanallur-Dharapuram-Oddanchatram-Pallapatti profile were conducted for better understanding the structure and tectonic frame work of the northern part of Southern Granulite Terrain (SGT), India. The northwest-southeast trending, 140 km long profile over the SGT depicts the basic nature of crustalscale shear zones like Palghat-Cauvery Shear Zone (PCSZ), Karur-Kambam-Painavu-Trichur Shear Zone (KKPT) and Karur-Oddanchatram Shear Zone (KOSZ). The seismic evidences in the form of opposing reflectivity trends, horst and graben structure and abrupt south dipping mid-lower crustal reflections along transect indicate episodes of crustal extension and stress accumulation. These episodes might have influenced development of weak zones or faults. Subsequent compressional events might have lead to undulations/uplifting of crustal columns as indicated by five-layered 2D velocity-depth model (Vp 6.0, 6.6, 6.9, 7.3 and 7.5 km/s). A combined interpretation of long range refraction/wide angle reflection and the vertical reflection data confirms the basic deep seated nature of the crustal scale shear zones. Presence of diffused reflection Moho is noticed at a depth of similar to 45 km along the profile. The unusually high velocity of 7.5 km/s in the lower crust, presence of diffused Moho and reflections from upper mantle suggest the magmatic underplating in the lower crust during crust-mantle interactions.