CONTINENTAL LITHOSPHERE FOLDING IN CENTRAL-ASIA .2. CONSTRAINTS FROM GRAVITY AND TOPOGRAPHY

Periodical sub-parallel fold-like structures observed in the Western G obi have two characteristic dominant wavelengths of approximately 50 a nd 300-360 km. We explain these observations in terms of independent q uasi-viscous folding of the crustal and upper-mantle parts of the lith osphere due to horizontal transpressional stresses in Central Asia ind uced by the collision with the Indian plate. We derive a model for the mechanical response of the continental lithosphere to horizontal stre sses for a rheologically-layered plate with non-Newtonian power law rh eology overlying a low-viscosity asthenosphere. The differentiation in the effective viscosity and thickness of the strong upper-crust and u pper-mantle lithosphere, along with the presence of a low-viscosity lo wer crust between them, leads to their partial decoupling during compr essional deformation. These features result in the appearance of diffe rent wavelengths of the folds in the Western Gobi region. We derive si mple semi-analytical estimates for the dominant wavelengths and rates of growth of surface undulations, constrained by data from experimenta l rock mechanics, topography, and gravity.