Seismic Imaging of Crust Beneath the Western Tibet-Pamir and Western Himalaya Using Ambient Noise and Earthquake Data

We present a new high-resolution image of the crust beneath the western Himalaya-Asia convergence zone encompassing the geographical domain of western Himalaya-western Tibet-Ladakh-Karakoram-Pamir-Hindu Kush, using ambient noise cross-correlations from 530 seismological stations along with surface wave observations from 1,261 earthquakes recorded over the network. The 3-D shear wave velocity image is created using 22,726 inter-station Rayleigh wave dispersion measurements from 5 to 60 s period at a horizontal resolution of less than 0.5 degrees x 0.5 degrees following the Bayesian Trans-dimensional Tree tomography approach. The Moho beneath the Himalayas and south Tibet correlates with a velocity transition of 4.4-4.6 km/s and a reduced velocity transition of 4.0-4.2 km/s in northern Tibet and the Pamir. We used the Moho depth and the nature of high-velocity lower crust (Vs > 4.0 km/s) to map the northern limit of the Indian crust that extends beyond the Qiangtang block in western Tibet (77-82 degrees E) from its previously assumed boundary in the Lhasa block and till the central Pamir farther west. The velocity image reveals discontinuous low-velocity zones (LVZs; Vs < 3.4 km/s) in the mid-crust of western Tibet and the Pamir that do not support the existence of the channel flow model. The LVZs in the Pamir correlate with the surface distribution of gneiss domes. The lowest velocities (Vs < 3.2 km/s) are observed over the Ladakh-Karakoram batholith and the Nanga Parbat region. The study suggests a continuation of LVZs across the Karakoram Fault at a depth below 20 km, indicating the fault's upper crustal depth extent.