Rayleigh-Taylor instability of the upper mantle and its role in intraplateorogeny

We explore the possibility that intraplate orogeny is the result of gravita tional instability of the mantle lithosphere beneath the orogenic zone. We use a two-layered system overlying a half-space to represent a low-density crust overlying a high-density lithospheric mantle overlying a reference de nsity asthenosphere. A small harmonic perturbation is then imposed on the b ase of the high-density layer and the system is allowed to flow under the g ravitational instability described by the Rayleigh-Taylor instability. Visc osity and density are uniform within the layers and Newtonian rheology is a ssumed. We investigate the ability of the downwelling, high-density, lithos pheric layer to thicken the low-density crustal layer above the downwelling . We solve the system using two methods: a numerical solution to the full s et of 2-D viscous flow equations and a linearized approximation of the earl y growth of the instability, valid for small deflections. For typical physi cal parameters, our results show that the ratio of downward displacement at the Moho to that at the base of the lithosphere is similar to 6 per cent p rovided that the crust is weaker than the lithosphere. Our results show tha t a buoyant crustal layer overlying the higher-density lithospheric layer m ay be thickened and uplifted over a lithospheric downwelling, achieving a m aximum crustal thickening factor of similar to 1.4 (for typical lithospheri c parameters). This is enough to thicken a 35 km crust to 50 km and produce a significant intraplate mountain range. We find that thick, buoyant conti nental crust causes the instability to occur at a lateral wavelength of ord er 300 km regardless of whether a stress-free or rigid condition is used on the upper boundary. Thin, less buoyant crust, however, allows the instabil ity to occur at much longer wavelengths. For a stress-free upper boundary t he surface deflection over the downwelling is upwards due to crustal conver gence, unless the crust is very rigid (crustal viscosity >13 times mantle v iscosity), in which case the surface deflection over the downwelling is dow n, creating a topographic depression. For detachment of the downwelling blo b to occur within 30 Myr, without any external tectonic activity, an averag e lithospheric viscosity of similar to 10(21) Pa s is required. Once detach ment of the downwelling mass has occurred the system responds to flow drive n by a thickened crust. This causes the crust to flow back to its equilibri um position and a period of extension thus follows the period of compressio n. Such a cycle could conceivably be repeated if the thinned lithosphere is able to thermally equilibrate, cool and thicken again after extension.