Crustal structure under the eastern Himalayan Syntaxis seismic array and its geodynamic implications derived from receiver functions

On the eastern margin of the Himalayan orogenic belt, the rapid uplift of the Namche Barwa metamorphic terrane and significant bending of the Yarlung Zangbo suture zone occur. The formation mechanism of the Eastern Himalaya Syntaxis (EHS) is still debated. We used the teleseismic data observed by 48 broadband seismic stations around the Namche Barwa Mountain to estimate P-wave receiver functions and study crustal thickness, V-P/V-S ratio and crustal structure based on improved H-kappa analysis and common conversion point (CCP) stacking method. The results demonstrate that the average crustal thickness is about 64.03 km with most stations ranging from 60.48 km to 66.55 km and the average V-P/V-S ratio is about 1.728 with most stations ranging from 1.696 to 1.742. The crust thickness exhibits drastic lateral variation beneath the EHS, which is thicker in the western and northern area and thinner in the middle area; thus the crust of the EHS presents an uplifting structural feature. The Moho boundary is uplifted about 6 similar to 7 km in the W-E direction, and more than 9 similar to 10 km in the N-S direction. The uplifted Moho boundary is formed because high-density and high-velocity lithospheric tearing slab could be delaminated and sank into asthenosphere in the upper mantle and result in unbalanced gravity. The anomaly of the average V-P/V-S ratio, which is more than 1.8, is distributed on both sides of the EHS, which can be interpreted as a crustal partial melting body around the EHS. The uplifting Moho beneath the EHS and the existence of partial melting in the crust might be associated with the upwelling of hot mantle material, and the upwelling of the asthenosphere may be caused by a tear of the India plate.