MECHANICAL STUDIES ON THE TIBETAN PLATEAU, ACCRETIONARY WEDGES, AND CONTINENTAL RIFTS (TECTONICS, CONVERGENT, DIVERGENT)

The elevation and crustal thickness of the Tibetan Plateau since the continental collision have been computed in a kinematic model based on plate tectonic reconstructions and conservation of crustal volume. The opening of the South China Sea affected the crustal material input into Tibet and delayed Tibetan uplift until early Miocene when its elevation was no higher than 1 km. This model predicts a steady uplifting of the plateau since Middle Miocene. A dynamical model, in which the stronger Indian crust injects into the weaker lower crust of Tibet, is then developed to explain the spatially uniform uplift of the Tibetan plateau. If the viscosity in the Tibetan lower crust is below a critical value (6 x 10('18) Pa-s), the continuous advancing of the Indian crust beneath Tibet produces negligible variation in stresses on the bottom of the brittle Tibetan upper crust and results in a spatially uniform uplift of the Tibetan Plateau. This model also predicts a viscosity contrast of at least 1:160 between the Tibetan and Tarim lower crust in order to maintain the current thickening rate of the Tibetan crust (1.2 mm/yr) and the virtually constant thickness of the Tarim crust.; The mechanics of accretionary wedges along convergent margins is investigated based on the critical taper theory developed by Davis, Suppe and Dahlen. We suggest that the convex shape of accretionary wedges is directly related to the process of sediment lithification. The weaker sediments near the toe must pile up to produce a relatively steeper bathymetric slope in response to the horizontal tectonic compression. The stronger sediments farther back in the wedge can maintain a relatively modest bathymetric slope in response to the same compressive force. We have looked into several possible causes of the increase in sediment strength with increasing compaction and conclude that the most likely cause is an increase in cohesive strength. The critical taper model is combined with empirical cohesion-porosity-velocity relations to predict the distributions of the porosity and seismic velocity within the Barbados accretionary wedge. Both the shape of the wedge taper and the observed orientations of thrust faults stepping up from the decollement are consistent with this inference.; Finally, the preferential rifting of continents is studied by comparing the total strength of continental and oceanic lithospheres. We found that continental lithosphere is weaker than oceanic lithosphere and a thickened crust can further decrease its total strength. Because the weakest area acts as a stress guide, any rifting near an ocean-continent boundary would prefer a continental pathway, producing slivers of continents and providing a possible source of worldwide displaced terranes.