Thinning of hot Indian lithosphere and apparent geodynamical consequences
|Category||Tectonic & Seismotectonic|
|Location||International Geological Congress,oslo 2008|
|Author||Pandey, Om Prakash|
|Holding Date||17 September 2008|
Our studies since last two decades, have now affirmed the extremely dynamic nature of the Indian lithosphere since at least late Archean times. During this period, it confronted several long sustained tectonothermal events, unheard elsewhere, which severely affected the underlying lithosphere and caused intense deformation, degeneration and shearing of its bottom. As a result, Indian shield has become quite warm and lost almost two-third of its mantle lithosphere. We estimate its current lithospheric thickness to be only about 105 km on an average, compared to at least twice (250-300 km) in comparable terrains elsewhere. Recent multimode surface waveform tomographic, shear-wave receiver function and other studies have confirmed our findings, which have been a subject of debate for a long time.
It is felt that due to anomalous nature of the lithosphere, the upper mantle beneath this shield has become less viscous, fertile and buoyant leading to crustal thickness shortening, exhumation of high velocity lower crustal layers, erosion of almost entire granitic-gneissic crust from several geotectonic segments, crustal densification and thick magmatic underplating at the base of the crust. These phenomena have visibly also altered the observed surface gravity field. Besides, the anomalous thermal nature has resulted into a much higher input of heat flow from the mantle (~32 mW/m. sq. ), thereby enhancing the insitu temperatures. This appears to be causing generation of additional thermal stresses, which act over and above to that generated by northward movement of Indian plate, thus creating seismic instability and in turn accelerating the process of earthquake nucleation. It could be one of the prime reasons why the entire Indian shield is in grip of moderate but damaging earthquakes since historical times.