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

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.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).This is the version of record, which can also be found on the publisher's website at: http://gji.oxfordjournals.org/content/198/3/1526.full © The Authors 2014. Published by Oxford University Press on behalf of The Royal Astronomical Societ

The crustal structure of the western Himalayas and Tibet

We present new, high-resolution, shear velocity models for the western Himalayas and West Tibet from the joint inversion of P receiver functions recorded using seismic stations from four arrays in this region and fundamental mode Rayleigh wave group velocity maps from 5–70 s covering Central and Southern Asia. The Tibetan Plateau is a key locality in understanding large-scale continental dynamics. A large number of investigations has examined the structure and processes in eastern Tibet; however, western Tibet remains relatively understudied. Previous studies in this region indicate that the western part of the Tibetan Plateau is not a simple extension of the eastern part. The areas covered by these arrays include the Karakoram and Altan-Tagh faults, and major terrane boundaries in West Tibet and the Himalayas. The arrays used include broadband data collected by the West Tibet Array, a U.S.-China deployment on the western side of the Tibetan Plateau between 2007 and 2011. We use the shear wave velocity models to obtain estimates of Moho depth. The Moho is deep (68–84 km) throughout West Tibet. We do not observe significant steps within the Moho beneath West Tibet. A large step in Moho depth is observed at the Altyn-Tagh fault, where Moho depths are 20–30 km shallower to the north of the fault compared to those to the south. Beneath the Lhasa Terrane and Tethyan Himalayas, we observe a low-velocity zone in the midcrust. This feature is not interrupted by the Karakoram Fault, suggesting that the Karakoram Fault does not cut through the entire crust.The collection and archiving of the data used in this study were supported by the IRIS PASSCAL and DMC programs and by NSF-Geophysics grants 0440062 and 0439976. Data from the Y2 and YT networks were downloaded from IRIS DMC. Amy Gilligan was supported by a NERC studentship, with CASE funding from Weston Geophysical. Figures were prepared using Generic Mapping Tools (GMT) software (Wessel and Smith, 1998). We would like to thank an anonymous reviewer for their constructive comments that have helped improve the manuscript.This is the final published version of the article. It first appeared at http://dx.doi.org/10.1002/2015JB01189

Imaging the lithosphere beneath NE Tibet: Teleseismic P and S body wave tomography incorporating surface wave starting models

The northeastern margin of the Tibetan Plateau, which includes the Qiangtang and Songpan-Ganzi terranes as well as the Kunlun Shan and the Qaidam Basin, continues to deform in response to the ongoing India–Eurasia collision. To test competing hypotheses concerning the mechanisms for this deformation, we assembled a high-quality data set of approximately 14 000 P- and 4000 S-wave arrival times from earthquakes at teleseismic distances from the International Deep Profiling of Tibet and the Himalaya, Phase IV broad-band seismometer deployments. We analyse these arrival times to determine tomographic images of P- and S-wave velocities in the upper mantle beneath this part of the plateau. To account for the effects of major heterogeneity in crustal and uppermost mantle wave velocities in Tibet, we use recent surface wave models to construct a starting model for our teleseismic body wave inversion. We compare the results from our model with those from simpler starting models, and find that while the reduction in residuals and results for deep structure are similar between models, the results for shallow structure are different. Checkerboard tests indicate that features of ~125km length scale are reliably imaged throughout the study region. Using synthetic tests, we show that the best recovery is below ~300km, and that broad variations in shallow structure can also be recovered. We also find that significant smearing can occur, especially at the edges of the model. We observe a shallow dipping seismically fast structure at depths of ~140–240km, which dies out gradually between 33°N and 35°N. Based on the lateral continuity of this structure (from the surface waves) we interpret it as Indian lithosphere. Alternatively, the entire area could be thickened by pure shear, or the northern part could be an underthrust Lhasa Terrane lithospheric slab with only the southern part from India. We see a deep fast wave velocity anomaly (below 300?km), that is consistent with receiver function observations of a thickened transition zone and could be a fragment of oceanic lithosphere. In NE Tibet, it appears to be disconnected from faster wave velocities above (i.e. it is not downwelling or subducting here). Our models corroborate results of previous work which imaged a relatively slow wave velocity region below the Kunlun Shan and northern Songpan-Ganzi Terrane, which is difficult to reconcile with the hypothesis of southward-directed continental subduction at the northern margin. Wave velocities in the shallow mantle beneath the Qaidam Basin are faster than normal, and more so in the east than the west.This work was supported by a Natural Environment Research Council studentship (grant NE/H52449X/1)This version of record of this article can be found in Geophysical Journal International (March, 2014) 196 (3): 1724-1741. doi: 10.1093/gji/ggt47