Dynamics of the India-Eurasia collision zone

We present simple new dynamic calculations of a vertically averaged deviato ric stress field (over a depth average of 100 km) for Asia from geodetic, g eologic, topographic, and seismic data. A first estimate of the minimum abs olute magnitudes and directions of vertically averaged deviatoric stress is obtained by solving force balance equations for deviatoric stresses associ ated with gravitational potential energy differences within the lithosphere plus a first-order contribution of deviatoric stresses associated with str ess boundary conditions. This initial estimate of the vertically averaged d eviatoric stress field is obtained independent of assumptions about the rhe ology of the lithosphere. Absolute magnitudes of vertically averaged deviat oric 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 elsewhe re in the region. A forward modeling method that solves force balance equat ions using velocity boundary conditions allows us to refine our estimates o f the vertically averaged effective viscosity distribution and deviatoric s tress field. The total vertically averaged deviatoric stress and effective viscosity field are consistent with a weak lower crust in Tibet; they are c onsistent with some eastward motion of Tibet and south China lithosphere re lative to Eurasia; and they confirm that gravitational potential energy dif ferences have a profound effect on the spatially varying style and magnitud e of strain rate around the Tibetan Plateau. Our results for the vertically averaged deviatoric stress argue for a large portion of the strength of th e lithosphere to reside within the seismogenic upper crust to get deviatori c stress magnitudes there to be as high as 100-300 MPa (in accord with labo ratory and theoretical friction experiments indicating that stress drops in earthquakes are small fractions of the total deviatoric stress).