HOW FAST CAN A PLATEAU LOWER CRUST FLOW?

Gravitational potential energy stored in an orogenic plateau can be strong enough to deform the foreland, hence contributing to both plateau growth and collapse. Gravity-driven channel flow from the plateau lower crust into the foreland lower crust, called channel extrusion, has been proposed as a main contributor to the eastward growth of the Tibetan plateau, possibly driving the lower crustal channel as far as 1500 km in 15 myr, at an average flow velocity of 10 cm/yr. However, isostasy-driven upward flow in response to either erosion focused on the plateau steep margins, or stretching of the plateau upper crust to produce domical structures called metamorphic core complexes, compete with horizontal channel flow extrusion. Here we show that there is a strong dynamic coupling among the various flow processes that take place during gravitational collapse, and therefore that channel flow extrusion cannot be considered in isolation to other gravitational collapse processes. In addition, we found that melt-dependent densities, as well as temperature- stress- and strain rate dependent viscosities impose severe limitations on the magnitude of channel extrusion. Cooling of the extruded channel, upward flow into core complexes favored by convective motion in the plateau channel and surface extension limit the channel extrusion velocity to 1 cm/yr or less and therefore limit the length scale of channel extrusion in eastern Tibet to 150 km. We propose that strain partitioning involving strike-slip faults in the upper crust and transpression and thickening in the lower crust more realistically account for the geomorphic evolution of, and seismic anisotropy in, southeast Tibet.