Crustal Structure in the Pakistan Himalayas

The Pakistan Himalayas, located at the western end of the India-Asia collision boundary, is a unique place to study the development of mountain belts. Despite its geological importance in understanding the trend and growth of the Himalayas, the area is less well studied in geophysics.

In this study, we have imaged crustal thickness beneath the Pakistan Himalayas by analyzing receiver functions recorded at a PASSCAL seismic experiment (PAKH) in the region. The PAKH network consists of 9 broadband and 2 short-period stations and operated from September to December in 1992. We have analyzed 32 earthquakes with magnitude greater than 5.5 at a distance range of 44 to 90 degrees. At least 6 P receiver functions are obtained at 8 of the 9 broadband stations. To improve signal to noise ratio of the P to S conversion phase from the Moho, receiver functions are stacked by station after moveout correction for epicentral distance. We construct crust models with three to four layers using a constant Vp/Vs ratio of 1.75. The best crust model at each station is determined by matching synthetic receiver functions with the observed ones.

Our preliminary results show that the Moho depth varies from 44 km at the most southern station in the Peshawar Basin to 74 km in the Pakistan Himalayas. The increase of roughly 30 km in the crustal thickness occurs in a NS distance of 100 km. Such a sharp change in crustal thickness must be contributed from both crust shortening of the Eurasia plate and the subduction of the Indian crust.