Rheology of the lithosphere beneath the central and western Tien Shan

The distribution of crustal velocity over a 800 by 400km region of the Tien Shan shows strain rates with principal contractional axes aligned perpendicular to the mountain chain and with negligible velocity gradients in the along-strike direction. This configuration allows us to describe the dynamics by a force balance along profiles perpendicular to the chain. The velocities on each of these profiles fit, with root-mean-square misfits of approximate to 1mm/yr, a constant rate of contractional strain. Gravitational potential energy per unit area varies along these profiles by at least 3 and maybe 5TNm(-1); the insensitivity of contractional strain rates to those variations implies a lower bound of approximate to 10(22)Pas on the effective viscosity of the lithosphere. Seismic tomography shows variations in P and S wave speeds that imply large lateral temperature variations in crust and upper mantle of the region. If the ductile portion of the lithosphere were deforming by the mechanisms of high-temperature creep, such temperature variations would be accompanied by variations in strength of 1 or 2 orders of magnitude. The observed velocity profiles allow variations in lithospheric strength by no more than a factor of 2 to 4, implying that the strength-controlling portion of the lithosphere cannot be strongly sensitive to temperature. Lithospheric strength profiles that incorporate flow laws for low-temperature plasticity of olivine reproduce both the effective viscosity and the insensitivity to lateral temperature variation that are observed in the Tien Shan. Plastic deformation of dry, pyroxene-rich lower crust may also contribute to such a temperature-insensitive strength profile.