Abstract:
Understanding the mechanism of evolution of continents- comprising Archean Cratons flanked by younger mobile belts- through time is challenging. Cratons have a cold, thick, buoyant lithosphere, which remained stable for billions of years. They preserve Earth’s history and help to understand the continent's evolution. The lithosphere consists of the crust and the upper mantle, separated by Moho. The structure and composition of the crust provide constraints to help model its evolution as well as its interaction with the mantle. The continental crust is broadly divided into upper, middle, and lower based on the chemical composition and the seismic wave velocity. While the upper crust is better understood due to its surface exposure, the middle and lower crust remain poorly investigated and understood. There are significant variations in their composition interpreted using seismological and petrological data. Equally important is to understand the difference in thickness and composition of the lithosphere of Archean cratons and the surrounding terrains, the longevity of cratons, and their possible destruction due to magmatic processes.
Southern India provides an excellent location to investigate most of the above-discussed geological issues. I use the high-density seismological data acquired at every 15-20 km along a 630 km long profile from the west to the east coast of India through the western and eastern Dharwar craton, Proterozoic Cuddapah basin, and the eastern ghat. The western end of the Dharwar craton has been subjected to interaction with the Marion mantle plume and rifting from Madagascar, while the eastern end rifted from eastern Gondwanaland.
I significantly improved the velocity image of the crust and mantle lithosphere-asthenosphere to a depth of ~300 km using the joint inversion of receiver function and improved surface wave phase velocity dispersion (3-140 sec) with better resolution in space and time. Most significant findings include a homogenized mid and lower crust with Moho at ~40 km and a more felsic crust beneath the eastern Dharwar; No extensive underplating signatures in the western and eastern ends of the profile. We observe the unusual low velocity layer in the upper crust at the boundary of CB and EG representing the signature of Nallamalai Fold Belt (NFB). We observe a thick lithosphere beneath most of the Dharwar craton having 160-180 km thickness and weak/absence (highly viscous) of asthenosphere suggesting possible presence of tectosphere; the partial destruction of craton on the western and eastern ends of the profile due to Marion plume (~85 Ma) and the Eastern Gondwanaland (~130 Ma) respectively. The western and eastern ends of the profile have lithospheric thicknesses 100-120 km and 80-90 km respectively showing extensive lithospheric thinning and strong low velocity asthenosphere due to the intrusion of hydrous melts/supercritical fluids and re-fertilization. We observe a distinct signature of Mid Lithospheric Discontinuity (MLD) across the diamond-bearing kimberlite field between 100-120 km depth along the E-W profile possibly suggesting accumulation of seismically slow metasomatic minerals and the acquisition of fabric due to deformation during craton construction involving plume or collisional tectonics.
