Deep Structure of the Eastern Himalayan Collision Zone: Evidence for Underthrusting and Delamination in the Postcollisional Stage

摘要

Based on broadband seismic data from 102 stations in the eastern Himalayan collision zone, the crustal thickness and Poisson's ratio were determined by using the H-kappa stacking analysis of teleseismic P wave receiver functions. The Moho topography rapidly deepens northward in the northern Lhasa terrane, inferring the northern limit of the Indian plate underthrusting beneath Tibetan Plateau. Our seismic images show that steep subduction of the Indian plate occurs to the west of the Eastern Himalayan Syntaxis while gentle subduction to the east. The variation of subduction geometry of the Indian plate may be one of the causes for the formation of the eastern Himalayan Syntaxis. The 2-D crustal density modeling of the gravity measurements shows that the average crustal density in the eastern Himalayan collision zone is less dense than the global average continental crust, and our preferred model has 5- to 10-km-thick, high density layer (2,970-3,000 kg/m(3)) in the lower crust beneath the eastern Lhasa terrane, consistent with mafic underplating. The attributes of a thickened crust with northward deepening Moho and low-to-normal Poisson's ratio might be the geophysical signature of delamination beneath the Lhasa terrane and underthrusting of Indian plate. We hypothesize that the orogenic root of the Lhasa terrane was removed by convective-driven delamination, followed by northward subduction of the Indian plate. Thus, delamination and continental subduction are the dominant deep processes in the postcollisional stage in the eastern Himalayan collision zone.