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

Periodical sub-parallel fold-like structures observed in the Western Gobi have two characteristic dominant wavelengths of approximately 50 and 300-360 km. We explain these observations in terms of independent quasi-viscous folding of the crustal and upper-mantle parts of the lithosphere due to horizontal transpressional stresses in Central Asia induced by the collision with the Indian plate. We derive a model for the mechanical response of the continental lithosphere to horizontal stresses for a rheologically-layered plate with non-Newtonian power law rheology overlying a low-viscosity asthenosphere. The differentiation in the effective viscosity and thickness of the strong upper-crust and upper-mantle lithosphere, along with the presence of a low-viscosity lower crust between them, leads to their partial decoupling during compressional deformation. These features result in the appearance of different wavelengths of the folds in the Western Gobi region. We derive simple semi-analytical estimates for the dominant wavelengths and rates of growth of surface undulations, constrained by data from experimental rock mechanics, topography, and gravity.