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

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.

Early‐Stage Lithospheric Foundering Beneath the Eastern Tibetan Plateau Revealed by Full‐Wave Pn Tomography

“…J. Zhao et al, 2011). Our model exhibits low velocities in the upper mantle under the AKMS, in general agreement with previous tomographic observations (Ceylan et al, 2012;Nunn et al, 2014;Obrebski et al, 2012), though some existing teleseismic body wave tomography models show a wider low-velocity area south of the AKMS (X. F. Liang et al, 2012;Wei et al, 2016). Our resolution test (supporting information Figure S9) and existing models all rule out a continuous high-velocity structure in the upper mantle extending from the Qaidam Basin to the plateau as expected from a subducted Qaidam lithosphere across the AKMS.…”

The magmatic plumbing system of the Askja central volcano, Iceland, as imaged by seismic tomography

“…We use the tomographic inversion method of Roecker et al [2006], previously used to image the 3-D velocity structure of the San Andreas fault near Parkfield [Roecker, 2004], Western Tibet [Nunn et al, 2014], and more recently, the Krafla central volcano north of Askja [Schuler et al, 2015]. The key feature of the Roecker et al [2006] method is that the traveltimes are generated using a finite difference (FD) solution to the eikonal equation [Vidale, 1988;Hole and Zelt, 1995].…”

Lateral variations in the crustal structure of the Indo–Eurasian collision zone

“…(2014), and data at 8 and 12 s are only from new paths, Gilligan et al (2014) and Nunn et al (2014). Figure S2.…”