The main collision between the Indian and Asian lithospheric plates occurred during late Eocene time (40 to 50 m.y. ago). Continued northward migration of the Indian plate since that time at the rate of 5 cm/yr has resulted in approximately 2,000 km of closure between the two plates. In northern India it has been suggested that, while perhaps 500 to 1,000 km of the closure can be accounted for by crustal shortening, underthrusting, and thickening, the balance (1,000 + km) may be accounted for by eastward motion along strike-slip faults in China. However, in northern Pakistan, the problem is complicated because there is no Tibetan plateau analog and no evidence of strike-slip structures that could have removed significant amounts of crustal material.

In order to place tighter constraints on tectonic models for the Indian-Asian collision in the western Himalaya, it is important to be able to estimate the amount of crustal shortening that has occurred. Current estimates of 500 to 700 km of crustal shortening in northwestern Pakistan are calculated from balanced cross-section methods.

An important step in estimating the amount of shortening that has occurred is to determine the volume of crust that remains in the orogen. The crustal models based on observed gravity profiles presented in this chapter suggest that there may be enough crustal volume in the western Himalaya to account for between 570 and 1,140 km of shortening. This is still significantly less than the 2,000 km of closure that has possibly occurred. The balance of the closure might be accounted for by erosion and/or diffuse deformation, or it might suggest that less than 2,000 km of closure has occurred in the northwestern Himalaya.

The crustal models also suggest that the style of underthrusting in the northwestern Himalaya may be significantly different than that proposed for the Himalaya of northern India. Here the underthrusting seems to be occurring at a very steep angle when compared to the shallow underthrusting proposed for northern India.