The crustal structure of the western Himalayas and Tibet

Abstract

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.

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

“…As Chun & Yoshii (1977) first observed, the fundamental mode Rayleigh wave crustal Airy phase has a minimum at ∼35 s, a significantly longer period than that observed for most regions of the continents. In western Tibet, the depth of the Moho defined from receiver function studies (Wittlinger et al 2004;Rai et al 2006;Zhang et al 2014;Gilligan et al 2015) agrees well with the depth of the steepest gradient in increasing shear velocity in our surface-wavederived model (Fig. 4).…”

“…We have an improved resolution over a wider area than previous studies and can confirm that fast material is present across the entire width of the Plateau in western Tibet. Given the interpretation (Gilligan et al 2015) that Indian crust is present beneath the entirety of west Tibet, it is likely that the fast uppermost mantle material that we observe extending across the entire western part of the Plateau is Indian mantle lithosphere. Furthermore, the uppermost mantle shear velocities we observe beneath western Tibet are similar (∼4.5-4.6 km s −1 ) to those beneath areas of the Indian Shield, providing further evidence that cratonic Indian material is likely to be present beneath west Tibet.…”

“…1). In addition to new dispersion measurements made for this study, we include the dispersion measurements made by Acton et al (2010), Nunn et al (2014a), Gilligan et al (2014) and Gilligan et al (2015) which were all made in an identical manner. From the stations shown in Fig.…”

“…Of these, 4424 are new dispersion curves from this study for earthquake data and 5698 are new measurements from ambient noise data. The remaining dispersion curves come from previously published studies (Acton et al 2010;Nunn et al 2014a;Gilligan et al 2014Gilligan et al , 2015. There is considerable redundancy in the dispersion data set and we use this redundancy to identify and remove outliers, and to estimate errors in the dispersion data.…”

“…4) suggesting that the Indian lithosphere underthrusts the western Plateau possibly as far as the Altyn Tagh Fault (e.g. Wittlinger et al 2004;Rai et al 2006;Gilligan et al 2015). Beneath western ( Fig.…”