Crustal structure beneath western Tibet: evidence from receiver functions and newly constrained P and S-wave velocity models

Abstract:

Ps converted waves recorded by 29 stations deployed from 2007 to 2011 are used to develop constraints on the crustal velocity discontinuities beneath western Tibet. We use the frequency-domain multiple-taper correlation (MTC) algorithm (Park and Levin, 2000) to develop receiver functions (RFs), and plot back-azimuth and epicentral gathers to identify clear converted waves beneath our study area.

To evaluate depths of major converting boundaries we use a S-wave velocity model of Gilligan et al., (2014), and the P-wave velocity model of Razi et al., (2014). From these models, we compute vertically averaged ratios of Vp and Vs for the crust beneath each site. We find Vp/Vs ratios vary between 1.71 and 1.8, with sites showing the high Vp/Vs ratio (1.75-1.8) concentrated along the plate boundary between India and Eurasia. The Vp/Vs ratio is lower in the Lhasa block (1.71-1.75), while south of the Indus-Yarlung suture (IYS), the highest Vp/Vs ratio (1.8) is observed.

We observe strong positive Ps converted phases attributable to the Moho with Ps-Ptimes ranging from 5.3 s south of the IYS to 10.1 s north of the Bangong-Nujiang suture (BNS), with corresponding range of crustal thicknesses of 41-82 km. Within the Lhasa block the Moho appears at 65-80 km depths. However, south of the IYS the crust is 43-60 km. We find the thickest crust north of the BNS at 82 km.

Additionally, at many locations our RFs contain another positive phase with Ps-Ptimes of 5-7 s, corresponding to an interface at 40-55 km. This phase is seen mostly beneath the sites along the Karakoram fault (KKF). The relative timing between this phase and the later one delimiting the extent of the crust is in 2-3 s range, implying a ~20 km thick layer in the lower part of the crust bounded by two sharp increases in seismic impedance.

Our results also show coherent negative phases at some of the sites, mostly along the KKF. Ps-P delays of 2-5 s suggest boundaries at mid-crustal depths, and negative polarity indicates low velocity beneath them, a finding broadly consistent with vertical profiles of Vs in the model of Gilligan et al. (2014). A lack of the corresponding zone of low Vp velocities in the model of Razi et al. (2014) suggests that the origin of low Vs is not thermal. A plausible alternate is systematic sub-horizontal texture of the rocks aligned in the course of the formation of the plateau.