East‐West Differential Underthrusting of the Indian Lithospheric Plate Beneath Central Tibet Revealed by Imaging V_P/V_S

Abstract

Although numerous research studies have already provided basic information on deeper lithospheric structure under Tibet, the northernmost edge and the geometry of the northward underthrusting of the Indian mantle lithosphere are yet to be understood. The P wave to S wave velocity ratio (VP/VS) can provide more insight into the mantle physical state than either VP or VS alone, owing to their different sensitivities to partial melting, temperature, and composition. High VP/VS usually indicates that partial melting exists in the upper mantle, while anomalously low VP/VS in the upper mantle reflects the enrichment of orthopyroxene in the subduction zone mantle wedge. Using teleseismic records of 164 broadband stations deployed in central Tibet, we obtained a high‐resolution velocity ratio image of the upper mantle. A striking difference is seen between western and eastern Tibet. A distinct high VP/VS zone exists at relatively a shallow depth (<200 km) to the west of the Yadong‐Gulu Rift, while the eastern part exhibits a shallow region (south of the Bangong‐Nujiang suture in the depth range of 120–160 km) with abnormally low VP/VS. Moreover, there are high VP/VS areas existing below 270‐km depth, and it appears further east at depth of 340 km than that at depth of 270 km. We propose that this difference reflects different geometries of the underthrusting Indian mantle lithosphere from west to east. Slab tearing under eastern Tibet and lithospheric delamination under western Tibet are likely good candidates to explain the observation.

Ps‐P Tomography of a Midcrustal Magma Reservoir Beneath Cleveland Volcano, Alaska

“…Here we build on the work of Janiszewski et al (2020) by taking an inverse approach to assessing the virtual source Ps travel times. Similarly to prior studies of the upper mantle (Si et al, 2017(Si et al, , 2019, we invert the virtual source travel times, but at the crustal scale. Importantly, our technique mitigates the limitations of the forward modeling approach of Janiszewski et al (2020) by solving for the three-dimensional (3-D) crustal velocity structure surrounding Cleveland Volcano and iteratively re-tracing rays through the 3-D heterogeneous structure.…”