Alteration in the mantle transition zone structure beneath Sikkim and adjoining Himalaya in response to the Indian plate subduction

We map the depth variations of 410 km and 660 km discontinuities (d410 and d660) beneath the Sikkim Himalaya and adjoining regions using 18,816 P wave receiver functions. The average variation of d410 and d660 for the entire study region is found to be 395 +/- 4 km and 652 +/- 5 km, respectively, with the mantle transition zone (MTZ) thickness being similar to 255 km. A major upliftment in the upper mantle discontinuities is seen in the southeastern portion of the study region close to Shillong plateau, where the average depth of d410 and d660 is 373 +/- 4 km and 648 +/- 6 km, respectively. Overall, 90% of the grids have an uplifted transition zone. An uplift of similar to 20 km in d410 is observed beneath the central Nepal Himalaya and east of the Bhutan Himalaya. For the remaining grids, the d410 is elevated by similar to 10 km. For d660, similar to 15 km of elevated topography is observed in central Nepal Himalaya and the northern part of Arunachal Himalaya, while the remaining areas have a relatively flat d660. The upliftment in the upper mantle discontinuities reflects presence of a thick Indian lithosphere beneath the study region. Furthermore, clear positive and negative conversions are observed between 450 km and 550 km depths, reflecting a highly heterogeneous nature of the MTZ. In addition, strong negative amplitude conversions corresponding to similar to 350 km depth suggest presence of low velocity zones in the northernmost parts of Himalaya and southern Tibet. The low velocity zones may reflect compositional changes in response to the relict slab dynamics.