Palaeoslab and plume signatures in the mantle transition zone beneath Eastern Himalaya and adjoining regions

A comprehensive data set of 73 876 high quality receiver functions computed using waveforms recorded by 327 broad-band seismic stations is used to investigate the mantle transition zone (MTZ) structure beneath the eastern Himalaya, southern Tibet, Assam valley and the previously unexplored Bunnese arc and Bengal basin regions. A highly variable and perturbed mantle transition zone, with depressed 410 and 660 km discontinuities, is observed beneath the Bengal basin and to the east of the eastern Himalayan syntaxis. The 410 is elevated by similar to 10 km along the Himalayan collision front, while it deviates in the range of +/- 5 km beneath most parts of Tibet and the Himalayan Foredeep. In northern Tibet and along the Red River Fault, delayed conversions from the 410 reveal a deepening of more than 10 km. The 410 and 660 km discontinuities arc uplifted by nearly 10 km beneath the Arunachal Himalaya, due to the presence of a subducting Indian lithosphere, as evident in the regional tomographic images. We observe a thick (>20 km) transition zone beneath the Burmese Arc and close to the Tengchong volcano. An uplifted 410 together with a depressed 660 km discontinuity requires presence of lithospheric slabs within the MTZ. Delayed P-to-s conversions from the 410 and 660 km discontinuities in the proximity of the Jinsha suture zone seem to be consistent with the earlier results that invoke flow of a hot Tibetan asthenosphere into the mantle transition zone, as an explanation. Interestingly, results from the Bengal basin reveal a deepening (similar to 10 km) of both the 410 and 660 km discontinuities. Similar results from other plume affected regions prompt us to interpret this as a signature of the Kergulean plume.