Lithospheric Structure and Deformation of the Eastern Himalayan System using Broadband Seismology

Abstract:

We investigate the lithospheric structure beneath northeast India and combine it with earthquake focal mechanisms and GPS derived velocity vectors to study the kinematics of the Eastern Himalayan System. Our experiment comprised an array of 13 broadband seismographs located on the Eastern Himalayan foreland, Shillong Plateau, Mikir Hills, and Bengal Basin. We use P-receiver functions and joint inversion with Rayleigh wave group velocity dispersion to model the crust and upper mantle structure. We observe that the crust beneath Shillong Plateau, Mikir Hills and the Eastern Himalayan foreland has similar Vs and Vp/Vs, and felsic in nature. However, the crustal thickness varies 3-dimensionally. The central Shillong Plateau and Mikir Hills have the thinnest crust (31±2 km). The crustal thickness increases abruptly by 10±2 km north of Mikir Hills, while it increases gradually north of the Shillong Plateau. From earthquake source mechanisms, we observe that the crustal thickness variation is associated with thrusting of the Plateau and Mikir Hills over the eastern Himalayan foreland. The Kopili fault zone between the Plateau and Mikir Hills is marked by dextral strike-slip earthquakes, whose slip vectors match the GPS derived relative velocity vectors. Across the southern edge of the Plateau, marked by topographic break and surface trace of Dwaki fault, the crustal thickness changes abruptly from 36±2 km (south Shillong Plateau) to 22±2 km (north Bengal Basin) and thins further southward to 16±2 km. This change is associated with a sharp transition from felsic to mafic crust. This intermediate thickness mafic crust resembles thinned passive continental margin crust, and underlies the entire eastern Bengal Basin. This crust is overlain by the thickest pile (18-20 km) of continental sedimentary strata. Moderate earthquakes south of the Shillong Plateau originate within the Bengal Basin crust and have strike-slip mechanism. These earthquakes obliquely accommodate a part of the GPS inferred convergence between the Plateau and the Bengal Basin. We finally conclude that the uplift of the Shillong Plateau and Mikir Hills is controlled by flexure and faulting of the Indian crust to the north and by abrupt change in crustal structure to the south. This in turn controls the distributed deformation in the Eastern Himalayan System.