Crustal and mantle velocity models of southern Tibet from finite frequency tomography

Xiaofeng Liang,Yongshun John Chen,Yang Shen,Yong Ren

doi:10.1029/2009jb007159

Xiaofeng Liang, Yongshun John Chen + Show 2 more

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https://doi.org/10.1029/2009jb007159

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Abstract

[1] Using traveltimes of teleseismic body waves recorded by several temporary local seismic arrays, we carried out finite-frequency tomographic inversions to image the three-dimensional velocity structure beneath southern Tibet to examine the roles of the upper mantle in the formation of the Tibetan Plateau. The results reveal a region of relatively high P and S wave velocity anomalies extending from the uppermost mantle to at least 200 km depth beneath the Higher Himalaya. We interpret this high-velocity anomaly as the underthrusting Indian mantle lithosphere. There is a strong low P and S wave velocity anomaly that extends from the lower crust to at least 200 km depth beneath the Yadong-Gulu rift, suggesting that rifting in southern Tibet is probably a process that involves the entire lithosphere. Intermediate-depth earthquakes in southern Tibet are located at the top of an anomalous feature in the mantle with a low Vp, a high Vs, and a low Vp/Vs ratio. One possible explanation for this unusual velocity anomaly is the ongoing granulite-eclogite transformation. Together with the compressional stress from the collision, eclogitization and the associated negative buoyancy force offer a plausible mechanism that causes the subduction of the Indian mantle lithosphere beneath the Higher Himalaya. Our tomographic model and the observation of north-dipping lineations in the upper mantle suggest that the Indian mantle lithosphere has been broken laterally in the direction perpendicular to the convergence beneath the north-south trending rifts and subducted in a progressive, piecewise and subparallel fashion with the current one beneath the Higher Himalaya.

Highlights

[3] Since the work of Argand [1924], the nature and extent of the underthrusting of the Indian lithosphere under the Tibetan Plateau has been a topic of debate [e.g., Willett and Beaumont, 1994]
[2] The unique role of the Himalayan range and Tibetan Plateau in understanding continent‐continent collision has led to numerous geological and geophysical studies in the region, including several temporary seismic networks operated from the southern Himalaya in Nepal to the Lhasa Terrane of southern Tibet in China during the past two decades [e.g., Hirn et al, 1995; Nelson et al, 1996; de la Torre and Sheehan, 2005; Nabelek et al, 2005; Sol et al, 2007; Velasco et al, 2007]
Receiver function studies reveal a doublet phase in the Tibetan crust, which was interpreted as evidence for the underthrusting Indian lower crust beyond the Indus‐Yalu suture (IYS) [Kind et al, 2002; Nabelek et al, 2005; Schulte‐ Pelkum et al, 2005; Jin et al, 2006; Nabelek et al, 2009]

Summary

Introduction

[2] The unique role of the Himalayan range and Tibetan Plateau in understanding continent‐continent collision has led to numerous geological and geophysical studies in the region, including several temporary seismic networks operated from the southern Himalaya in Nepal to the Lhasa Terrane of southern Tibet in China during the past two decades [e.g., Hirn et al, 1995; Nelson et al, 1996; de la Torre and Sheehan, 2005; Nabelek et al, 2005; Sol et al, 2007; Velasco et al, 2007]. The “sandwich” rheology structure divides the continental lithosphere into two seismogenic layers: The upper crust and the uppermost mantle, which are separated by a ductile/weaker lower crust This concept has been used to construct numerical models of the Tibetan Plateau [e.g., Zhao and Morgan, 1985, 1987] and to interpret the geophysical observations from the INDEPTH projects (International Deep Profiling of Tibet and the Himalaya) in southern Tibet. [6] In this study we combine data from the Hi‐CLIMB [Nabelek et al, 2005], INDEPTH [Nelson et al, 1996], BHUTAN [Velasco et al, 2007], and HIMNT [de la Torre and Sheehan, 2005] projects and apply the finite frequency traveltime tomography method to image the velocity structure beneath southern Tibet (Figure 1b). The traveltimes used in the inversion are 11137 high‐frequency and 4121 low‐frequency P waves, and 1850 high‐frequency and 1233 low‐frequency S waves

Method

Null‐Space Shuttle Method

Results

Discussion

Findings

Conclusion