Rheology of the lithosphere beneath the central and western Tien Shan

Abstract

AbstractThe distribution of crustal velocity over a 800 by 400 km 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 ∼1 mm/yr, a constant rate of contractional strain. Gravitational potential energy per unit area varies along these profiles by at least 3 and maybe 5 TN m−1; the insensitivity of contractional strain rates to those variations implies a lower bound of ∼1022 Pa s 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.