Dynamics of the India-Eurasia collision zone

We present simple new dynamic calculations of a vertically averaged deviatoric stress field (over a depth average of 100 km) for Asia from geodetic, geologic, topographic, and seismic data. A first estimate of the minimum absolute magnitudes and directions of vertically averaged deviatoric stress is obtained by solving force balance equations for deviatoric stresses associated with gravitational potential energy differences within the lithosphere plus a first-order contribution of deviatoric stresses associated with stress boundary conditions. This initial estimate of the vertically averaged deviatoric stress field is obtained independent of assumptions about the rheology of the lithosphere. Absolute magnitudes of vertically averaged deviatoric stresses vary between 5 and 40 MPa. Assuming bulk viscous behavior for the lithosphere, the magnitudes of deviatoric stresses, together with the magnitudes of strain rates inferred from Quaternary fault slip rate and GPS data, yield vertically averaged effective viscosities for Tibet of 0.5-5 x 10(22) Pa s, compared with 1-2.5 x 10(23) Pa s in more rigid areas elsewhere in the region. A forward modeling method that solves force balance equations using velocity boundary conditions allows us to refine our estimates of the vertically averaged effective viscosity distribution and deviatoric stress field. The total vertically averaged deviatoric stress and effective viscosity field are consistent with a weak lower crust in Tibet; they are consistent with some eastward motion of Tibet and south China lithosphere relative to Eurasia; and they confirm that gravitational potential energy differences have a profound effect on the spatially varying style and magnitude of strain rate around the Tibetan Plateau. Our results for the vertically averaged deviatoric stress argue for a large portion of the strength of the lithosphere to reside within the seismogenic upper crust to get deviatoric stress magnitudes there to be as high as 100-300 MPa (in accord with laboratory and theoretical friction experiments indicating that stress drops in earthquakes are small fractions of the total deviatoric stress).