Gravimetric constraints on the rheology of the Indian and Tarim Plates in the Karakoram continent–continent collision zone

Abstract

Bouguer gravity anomalies in the region of the Western Himalayas, Karakoram and Tien Shan show large negative values, but classical isostatic models are insufficient to account for the detailed pattern of the observed anomalies. In the past years gravimetric surveys in the Karakoram (Marussi, Caputo and others in 1954) have been extended and densified. The full body of available gravimetric data, including the pendulum observations by De Filippi in 1913–14 and Hedin in 1929–33 have been re-analyzed. Terrain corrections have been computed systematically for all available data using an algorithm and Digital Terrain Model. The isostatic anomalies along a profile from the Indo-Gangetic foredeep, across the Karakoram Range and terminating in the Tarim Basin show the oscillating values already noticed by Marussi. This oscillatory pattern can be explained by a model in which the convergent boundaries of the Indian and Tarim plates deform by elastic flexure, besides isostasy. The gravity data are used to constrain the numerical values of the model parameters, particularly the flexural rigidity of the plates. For the Indian Plate it is found that the best fitting value of the flexural rigidity is D=5×10 24 N m, a value very similar to those reported in the Central Himalayas. The flexural rigidity of the Tarim Plate turns out to be considerably larger, D=7×10 25 N m, which makes the Tarim more rigid than the neighboring Central Tibet. Both plates are loaded by an estimated shear force of 7 10 12 N m −1 located in a region corresponding to the Nanga Parbat Haramosh syntaxis. It is concluded that the Indo–Asian continental collision in Western Himalaya and Karakoram results in the development of flexural basins on both sides, unlike in Central Himalaya where the collision produces a flexural basin, the Ganga Basin, to the South and, to the North, the indentation of an isostatically supported Tibetan Block with possible rheological layering and eastward lateral extrusion.