Gravity anomalies, flexure and the elastic thickness structure of the India–Eurasia collisional system

We have used Bouguer gravity anomaly and topography data to determine the equivalent elastic thickness of the lithosphere, Te in the region of the India–Eurasia collisional system. Comparison of observed and modelled gravity anomalies along 1-dimensional profiles suggest there are significant variations in Te along-strike of the Himalaya foreland. Estimates decrease from a high of 70 km in the central region to 30–50 km in the east and west. We have verified these inferences of spatial variations using a 2-dimensional, non-spectral, interative flexure and gravity anomaly modelling technique. The Himalaya foreland forms a high Te (40 < Te < 100 km) rigid block with a well defined edge, as shown by the localisation of faulting and deformation along its northern margin. Other high Te blocks occur to the north beneath the Qaidam and Sichuan basins. The Tibetan plateau forms a low Te (0 < Te < 20 km) weak region that extends from the central part of the plateau into south-western China. Tectonic styles in the India–Eurasia collisional system therefore involve both ‘rigid’ and ‘non-rigid’ blocks. Where high Te rigid blocks are present the styles dominated by underthrusting of the more rigid block. Where the collisional zone is not constrained by rigid blocks, however, the style appears to be dominated by lower crustal flow and a more continuous style of deformation.