CRUST AND UPPER MANTLE STRUCTURE IN NORTHEAST INDIA

Abstract

The North East Region (NER) of India is one of the world's most seismically active and tectonically complex regions, forming a continent-continent triple junction joining the Indian plate with the Asian plate and the Burmese plate. The NER of India and adjoining areas exhibit complex tectonics owing to India-Asia collision/convergence and rotational tectonics in the Eastern Himalayan Syntaxis. Such complex tectonics can alter and reshape the crustal composition and structure, playing a key role in deciphering the region's geodynamics. The complex tectonics of the region not only changed the crustal configuration but also caused several devastating earthquakes, such as the 1897 Great Shillong Plateau earthquake (revised Mw 8.1), the 1950 Great Assam earthquake (revised Mw 8.4), and about 25 large (Mw > 7.0) earthquakes in the region. The diverse subsurface structures of the region play a key role in generating destructive earthquakes. To understand the seismogenesis and geodynamic evolution of NER India, studying the crustal configuration and upper mantle structure is crucial. While several studies have addressed crustal thickness, fewer studies have explored crustal composition based on physical parameters like the Poisson’s ratio. Furthermore, a significant portion of the region remains unexplored due to the limited availability of geophysical data. This region possesses significantly different tectonic domains, such as Pop-up tectonics in the Shillong Plateau, collision tectonics in the Himalaya, subduction tectonics in the Indo-Burma Ranges, and rotation tectonics in the EHS. Although the surface geology of these tectonic units was well-studied, a complete understanding necessitates examining the spatial variations in the subsurface structure. Thus, one of the primary motivations of the present study is to investigate such changes in the crustal configuration and shallow upper mantle structure by application of receiver function analysis of teleseismic earthquakes. Moreover, this study attempted to study crustal deformation in the IBR and adjoining areas with the help of seismic anisotropy investigations. The results obtained in this study have been interpreted in association with similar results available to provide a more comprehensive perspective.