MORE THOUGHTS ON CONVERGENT CRUSTAL PLATEAU FORMATION AND MANTLE DYNAMICS WITH REGARD TO TIBET

Evidence suggests that some 10 m.y. ago a large portion of thickened m antle lithosphere below the Tibetan Plateau was rapidly removed. Remov al appears to have been abrupt and delayed from the time of substantia l thickening. Fluid dynamical models of convective thermal boundary la yer instability have had difficulties explaining abrupt lithospheric r emoval long after initial thickening. Such models have implied that ei ther removal should be synchronous with thickening or should not occur at all, due to boundary layer stabilization resulting from high litho spheric viscosity. We present simple thermal/chemical boundary layer c onvection models that suggest a compromise between previous end-member results. The models allow for delayed instability of a thickened ther mal boundary layer under geologically reasonable parameter conditions. Instability delay relies on high viscosity of thickened mantle lithos phere stabilizing it against free convective removal and on lateral ex tent of thickened crust exceeding that of mantle lithosphere. The latt er condition leads to a peripheral region of thickened crust and relat ively unthickened mantle lithosphere and an interior plateau region of both thickened crust and mantle lithosphere. If the thermal resistanc e and/or the rate of internal heat production of crust is greater than that of mantle, then mantle lithosphere at the peripheral region can be thermally eroded to the point that the base of the crust comes into contact with bulk mantle. This creates a peripheral weak zone that al lows thickened mantle lithosphere of the interior plateau region to be come mechanically detached and sink into bulk mantle. The instability is akin to delamination, as originally defined, and its delay time is roughly the thermal diffusion time across the thinnest portion of mant le lithosphere.