The Mantle Transition Zone Structure Beneath the Pamir Plateau and Western Tian Shan and Adjacent Regions.

Systematical investigation of deep mantle structure beneath the Pamir Plateau, western Tian Shan and their surroundings is of great significance to understand dynamics of continental collision, intracontinental orogenesis and deformation in response to the Indo‐Eurasian collision. In this research, we imaged the mantle transition zone (MTZ) structure beneath these regions using 42,560 P‐wave receiver functions obtained from 352 seismic stations and 6,173 teleseismic events. Our results reveal significant 15–20 km depression of the 410‐km discontinuity (d410) mainly beneath the southern Kazakh Shield, which is consistent with the low‐velocity anomaly in tomographic models and thus attributed to the mantle upwelling from the MTZ, providing evidence for the fossil Tian Shan plume responsible for the Late Cretaceous‐Paleocene basaltic magmatism (74–52 Ma) at the western Tian Shan. Considering that the d410 is slightly depressed by ∼8 km beneath the western Tian Shan, deep subduction of the Tarim lithosphere is likely excluded and its subhorizontal indentation into the Tian Shan is preferred. As a result, segments of thickened Tian Shan lithosphere delaminated and accumulated near the 660‐km discontinuity (d660), which induce small‐scale upwelling across the d410 there. The d410 is depressed by ∼10–15 km beneath Tarim, which is interpreted to be caused by the mantle upwelling originating from beneath the d410. The d660 below the central Hindu Kush is extremely depressed by 25–30 km, providing direct evidence for the deep subduction of Indian lithosphere into the bottom of the MTZ and suggesting different mechanisms for continental collision between the Hindu Kush and Pamir Plateau. Plain Language Summary: Intensive tectonic activities are ongoing in the Pamir Plateau, western Tian Shan and their surroundings, whereas the deep mantle structure and related dynamical processes are poorly constrained by independent seismological observations. Here we mapped the detailed mantle transition zone (MTZ) structure utilizing large data sets of seismic data by imaging the 410‐km (d410) and 660‐km (d660) discontinuities. Our results show prominent depression of d410 (15–20 km) below the southern Kazakh Shield in agreement with the tomography‐derived low‐velocity anomalies, which we interpret as mantle upwelling from the MTZ likely associated with the fossil Tian Shan plume and the eruption of basaltic magmatism (74–52 Ma) in the western Tian Shan. Furthermore, the subhorizontal indentation of Tarim lithosphere into western Tian Shan is supported by slightly depressed d410 (∼8 km) there, followed by the delamination of thickened Tian Shan lithosphere into the bottom of the MTZ. Furthermore, mantle upwelling from the MTZ beneath the Tarim Basin is indicated by the depressed d410 there (∼10–15 km). While the Indian slab underthrusts northward beneath the Pamir Plateau, it subducts deep into the bottom of the MTZ beneath the central Hindu Kush. Key Points: The mantle transition zone structure beneath the Pamir Plateau and western Tian Shan and adjacent regions are imaged with great detailThe 15–20 km depression of the 410‐km discontinuity beneath southern Kazakh Shield is likely associated with the fossil Tian Shan plumeSignificant depression of the 660‐km discontinuity (25–30 km) suggests deep subduction of the Indian slab beneath the central Hindu Kush [ABSTRACT FROM AUTHOR]