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Joint inversion of surface waves and teleseismic body waves across the Tibetan collision zone: The fate of subducted Indian lithosphere


Type

Article

Change log

Authors

Nunn, C 
Roecker, SW 
Priestley, KF 
Liang, X 
Gilligan, A 

Abstract

We carry out a joint inversion of surface wave dispersion curves and teleseismic shear wave arrival times across the Tibetan collision zone, from just south of the Himalaya to the Qaidam Basin at the northeastern margin of the plateau, and from the surface to 600 km depth. The surface wave data consist of Rayleigh-wave group dispersion curves, mainly in the period range from 10 to 70 s, with a maximum of 2877 source–receiver pairs. The body wave data consist of more than 8000 S-wave arrival times recorded from 356 telesesmic events. The tomographic images show a ‘wedge’ of fast seismic velocities beneath central Tibet that starts underneath the Himalaya and reaches as far as the Bangong–Nujiang Suture (BNS). In our preferred interpretation, in central Tibet the Indian lithosphere underthrusts the plateau to approximately the BNS, and then subducts steeply. Further east, Indian lithosphere appears to be subducting at an angle of ∼45°. We see fast seismic velocities under much of the plateau, as far as the BNS in central Tibet, and as far as the Xiangshuihe-Xiaojiang Fault in the east. At 150 km depth, the fast region is broken by an area ∼300 km wide that stretches from the northern edge of central Tibet southeastwards as far as the Himalaya. We suggest that this gap, which has been observed previously by other investigators, represents the northernmost edge of the Indian lithosphere, and is a consequence of the steepening of the subduction zone from central to eastern Tibet. This also implies that the fast velocities in the northeast have a different origin, and are likely to be caused by lithospheric thickening or small-scale subduction of Asian lithosphere. Slow velocities observed to the south of the Qaidam suggest that the basin is not subducting. Finally, we interpret fast velocities below 400 km as subducted material from an earlier stage of the collision that has stalled in the transition zone. Its position to the south of the present subduction is likely to be due to the relative motion of India to the northeast.

Description

Keywords

Tomography, Mantle processes, Body waves, Surface waves and free oscillations, Seismic tomography, Continental margins: convergent, Asia

Journal Title

Geophysical Journal International

Conference Name

Journal ISSN

0956-540X
1365-246X

Volume Title

198

Publisher

Oxford University Press (OUP)

Rights

DSpace@Cambridge license
Sponsorship
Our study has included data from GSN (including IC, IU and II), China Digital Seismograph Network, GEOSCOPE, IRIS-IDA, Pacific-21, Kyrgyz Digital Network, Kyrgyz Seismic Telemetry Network and IRIS-USGS permanent seismic networks and the MANAS, Tien Shan Continental Dynamics, Tibetan Plateau Broadband Experiment, INDEPTH II, INDEPTH III, INDEPTH IV/ASCENT, HIMNT, Bhutan, Nanga Parbat Pakistan and GHENGIS PASSCAL temporary seismic deployments. We thank IIEES and LGIT for seismic data from Iran and also SEISUK for provision and assistance with instruments operated in northeast India. CN was supported by a Natural Environment Research Council studentship (grant NE/H52449X/1), with CASE funding from AWE Blacknest. We thank Nick Rawlinson and an anonymous reviewer for their constructive and helpful reviews. Figures were prepared with Generic Mapping Tools (GMT) software (Wessel & Smith 1998).