Crustal structure in the Pakistan Himalaya from teleseismic receiver functions

Subsurface structure in the Pakistan Himalaya is poorly known despite its geological importance in understanding the trend and growth of the Himalaya. In this paper, we compute receiver functions from a broadband seismic network in northern Pakistan and construct crustal structure beneath the stations. P to S converted phases at the Moho are clearly observed on stacked receiver functions. We model the conversions from the Moho and shallower interfaces using the trial and error method. Assuming a Vp/Vs ratio of 1.75, the modeled crustal thickness is 52-53 km in the Peshawar Basin, 55-62 km in the Lesser Himalaya, and 65-76 km in the Higher Himalaya. The variation of the Moho depth correlates well with surface topography, indicating an Airy type of isostasy in the Pakistan Himalaya. A large negative phase before the Moho conversion is consistently observed at all three stations to the north of the India-Asia suture, which is best modeled by a low velocity layer in the lower crust. The slow lower crust is likely caused by partial melt resulted from crust thickening. Volatile in the crust of the Kohistan island arc and the subduction and melting of the Indian upper crust may also contribute to the low velocity layer. Like the weak middle-lower crust in Tibet, this slow lower crust probably has an important role in the formation and growth of the western Himalaya.