Global Inversion of Lithospherical Stress and Deformation

We have derived a global model of lithospheric stress and deformation using numerical inversion. The lithosphere is divided into 14 plates and represented by thin spherical shells of Newtonian fluid in our finite element model. The relative velocity between two plates at divergent plate boundaries is prescribed according to the Nuvel-1A plate motion model, which provides good fit to observed sea-floor spreading rates and the azimuth of transform faults. Large transform boundaries are modeled as rheologic weak zones. A set of free parameters describing the regional variation of plate viscosity as well as the external forces at convergent plate boundaries and on the base of the plates are determined by least-square fitting the VLBI and principal stress orientation data. The optimal model predicts a global stress field and intraplate deformation that are generally consistent with geological observations. The modeling results indicate that continental lithosphere is about 5 times as viscous as oceanic lithosphere which, in turn, is 2 orders of magnitude more viscous than lithosphere at transform boundaries. The predicted convergent rates are ~20% slower for the India-Asia continental collision boundaries, and ~7 % slower for the Nazca-South American subduction boundary than those predicted by the Nuvel-1A model. Our model also suggests that dragging forces by the underlying mantle on the base of both continental and oceanic lithosphere are resistant to absolute plate motions, and basal drag under continental lithosphere is ~3 times stronger than that under oceanic lithosphere.