Crustal tomographic imaging and geodynamic implications toward south of Southern Granulite Terrain (SGT), India

The crustal structure toward southern part of SGT is poorly defined leaving an opportunity to understand the tectonic and geodynamic evolution of this high-grade granulite terrain surrounded by major shear and tectonically disturbed zones like Achankovil Shear Zone (AKSZ) and Palghat Cauvery Shear Zone (PCSZ). To develop a geologically plausible crustal tectonic model depicting major structural elements, a comprehensive tomographic image was derived using deep-seismic-sounding data corroborated by Bouguer gravity modeling, coincident-reflection-seismic, heat-flow and available geological/geochronological informations along the N–S trending Vattalkundu–Kanyakumari geotransect. The final tectonic model represents large compositional changes of subsurface rocks accompanied by velocity heterogeneities with crustal thinning (44–36 km) and Moho upwarping from north to south. This study also reveals and successfully imaged anomalous zone of exhumation near AKSZ having transpression of exhumed rocks at mid-to-lower crustal level (20–30 km) with significant underplating and mantle upwelling forming a complex metamorphic province. The presence of shear zones with high-grade charnockite massifs in the upper-crust exposed in several places reveal large scale exhumation of granulites during the Pan-African rifting (~ 550 Ma) and provide important insights of plume-continental lithosphere interaction with reconstruction of the Gondwanaland.

Research highlights
► Crustal tomographic image in south of SGT reveals velocity anomalies at depth.
► Tomographic image corroborates with coincident seismic section and density image.
► Anomalous zone of exhumation is imaged at mid-to-lower crustal level below AKSZ.
► Underplating and Moho upwarping is due to mantle upwelling and intense magmatism.
► Pan-African tectonothermal activity played a major role for geodynamic evolution.