Deep Crustal Structure Beneath the Pamir–Tibetan Plateau: Insights From the Moho Depth and Vp/Vs Ratio Variation

Davlatkhudzha Murodov,Sherzod Abdulov,Wang Xin,Ilhomjon Oimuhmmadzoda,Amirkhamza Murodov,Wang Mi

doi:10.3389/feart.2022.821497

Abstract

The Cenozoic convergence between India and Asia has created Earth’s thickest crust in the Pamir–Tibetan plateau, leading to broadly distributed deformation and extensive crustal shortening; however, the crustal deformation of the high plateau is still poorly constrained. The variation of the Moho topography and crustal composition beneath the Pamir–Tibetan plateau has an important correlation with the major tectonic units. In this study, the results of the receiver functions have been reviewed and analyzed to observe variations in the Moho depth and crustal Vp/Vs ratio beneath the Pamir–Tibetan plateau. We found a notable SE–NW-oriented deep Moho interface that starts from the southeast of the Tibetan plateau and continues to the eastern Pamir with a northward dipping direction, which may indicate the northern frontier of the decoupled lower crust of northward underthrusting of the Indian plate. In contrast, the deepest Moho beneath the Pamir plateau has a southward dipping direction indicating the southward underthrusting Asian plate. In general, the average crustal Vp/Vs ratio is relatively low beneath the South-Central Pamir (∼1.70), while it is relatively higher (∼1.75) under the Himalaya–Lhasa terrane, suggesting more felsic to intermediate rock composition with locally high values indicating a low-velocity zone, possibly caused by partial melting. Elevated Vp/Vs ratios are observed beneath the northern Pamir (&gt;1.77) and Qiangtang and Songpan–Ganze terranes (&gt;1.80), which can be related to the high mafic rock content and upwelling hot materials from the upper mantle. The Vp/Vs ratio beneath the Pamir–Tibetan plateau presents complex north–south variations with a relatively low crustal Vp/Vs ratio in the south, while it gradually increases toward the north of the Pamir and central-northern Tibet, which is probably caused by the joint effects of the northward underthrusting Indian lower crust and southward subduction of the Asian plate, the low-velocity zones within the mid-upper crust, and substantial crustal shortening and thickening. The low to average crustal Vp/Vs ratio throughout the plateau (except the central Tibet) indicates a limited amount of hot materials to support the low crustal channel flow model, instead suggesting that crustal thickening and shortening is the main uplifting mechanism of the Pamir–Tibetan plateau.

Highlights

Lateral variation of Moho discontinuity, crustal thickness, and bulk composition preserve first-order information in plate tectonic evolution and provide important clues in understanding the structural evolution of the crust and upper mantle
We closely observed the deepest Moho imaged by N-S receiver function cross-sections along seismic arrays deployed within the Pamir–Tibetan plateau
The maximum Moho depth with a northward dipping interface from the southeast of Tibet to the southwest of the Tarim basin may indicate the northern limit of the Indian lower crust, suggesting sub-horizontal underthrusting of the Indian plate beneath the Tibetan plateau

Summary

INTRODUCTION

Lateral variation of Moho discontinuity, crustal thickness, and bulk composition preserve first-order information in plate tectonic evolution and provide important clues in understanding the structural evolution of the crust and upper mantle. During subduction of the dense Indian lithosphere beneath Tibet, its upper crust scrapped off and formed the highest mountain range in the world, the Himalayas, in the south of the Tibetan plateau (Capitanio et al, 2010) While it has been widely accepted by the research community that the Pamir–Tibetan plateau has uplifted in response to the Cenozoic India–Asia convergence, its uplifting mechanism and crustal structure remain poorly constrained. The most supported model for southern Tibet is the Argant-type subduction model (Argand, 1922), where the Indian subcontinent horizontally underthrusts beneath the Tibetan plateau along the Main Boundary Thrust and its upper-crust scrapes off, forming the high Himalayan mountains, and its northward lower crust terminates at the Moho doublet (Kind et al, 2002; Nábělek et al, 2009). The algorithm sums the weighted receiver function amplitudes at predicted arrival times for the Ps phase and its multiples for different values of possible crustal thicknesses (H) and Vp/Vs ratio (κ)

DISCUSSION

CONCLUSION

DATA AVAILABILITY STATEMENT