Crustal Shortening in the West Himalaya
Abstract
The main collision between the Indian and Asian lithospheric plates occurred during the late Eocene (40 million years ago) and continued closure at the rate of 5 cm/yr. has resulted in approximately 2000 km of crustal shortening between the two plates (Molnar and Tapponnier, 1975; Molnar, 1984). In northern India it has been suggested that while some of the shortening is by underthrusting (Molnar and Tapponnier, 1975) much may be the result of diffuse deformation in China (Tapponnier, 1982). 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 of 700 km of convergence in northwestern Pakistan (Butler, 1986), are calculated from balanced cross section methods. This is significantly less than the 2000 km required by closure models based on paleomagnetic data (Powell, 1979; Molnar and Tapponnier, 1975; Molnar, 1984).
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 upon observed gravity profiles presented in this paper suggest that there may be enough crustal volume to account for between 550-1100 km of shortening. This is still significantly less than the 2000 km of closure that has presumably occurred. The balance of the closure might be accounted for by erosion and/or diffuse deformation or it might suggest that less than 2000 km of closure has occurred in the northwestern Himalaya.
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