The fate of the Indian lithosphere beneath western Tibet: Upper mantle elastic wave speed structure from a joint teleseismic and regional body wave tomographic study

We investigate the fate of the Indian lithosphere following its descent beneath western Tibet by means of tomographic imaging based on arrival times of body waves from regional and teleseismic sources recorded by a portable network deployed in the region from 2007 to 2011. We use a non-linear iterative algorithm that simultaneously models absolute, regional, and relative teleseismic arrival times to obtain a 3-D velocity structure in a spherical segment that extends from 26°N to 37°N, from 76°E to 89°E, and from the surface to 430 km depth. We find that variations in P and S wave speeds in the upper mantle are similar, and identify a number of prominent fast anomalies beneath western Tibet and the adjacent Himalayas. We associate these fast anomalies with the mantle lithosphere of India that is likely colder and hence faster than the ambient mantle. Resolution tests confirm the ability of our dataset to resolve their shapes in the upper 300 km, and the lack of significant downward smearing of these features. We interpret the presence of faster material below 300 km as being consistent with former Indian lithosphere having reached these depths. There are two main fast anomalies in our model. One resembles a ∼100 km wide sub-vertical column located directly beneath the India-Asia plate boundary. The other anomaly is thinner, and has the shape of a dipping slab that spans the north-south width of the Lhasa block. It dips towards the NE, starting near the Indus-Yarlung suture and ending north of the Bangong-Nujiang Suture at depths in excess of 300 km. Another finding of our study is the absence of major fast anomalies west of ∼80°E, which our resolution tests show to be significant. Our results do not support the notion of a continuous body of formerly Indian lithosphere being presently underthrust northward, and extending all the way to the northern boundary of the plateau. Rather, shapes of fast anomalies in western Tibet suggest colder material beneath the northernmost Himalaya and the southernmost part of the plateau. The presence of multiple relatively small-scale anomalies, rather than a single large fast body aligned with the India-Asia boundary, rules out subduction of the intact Indian lithosphere. Our preferred scenario for the process that currently removes the mantle lithosphere of India from the collision zone envisages viscous behavior, with gravity-driven instabilities leading to the formation of multiple drips of different size. To reconcile our findings with evidence of past underthrusting of Indian lithosphere beneath Tibet we postulate a geologically recent change in the mode of lithosphere removal. Our scenario includes initial underthrusting, subsequent gravitational destabilization, and removal of the mantle lithosphere of Indian plate. At present the mantle lithosphere of India that continues to underthrust southern Tibet develops a drip-like instability that is seen as a fast columnar feature in our model.