Convective instability of thickening mantle lithosphere

Abstract

Mantle lithosphere, being colder and therefore denser than the underlying mantle, is prone to convective instability that can be induced by horizontal shortening. Numerical experiments on a cold layer with imposed horizontal shortening are carried out to examine the relative importance of mechanical thickening, thermal diffusion and gravitational instability in deforming the layer. This analysis is then used to develop a method for determining which of these styles dominates for a layer thickening at a given rate. If viscosity is non-Newtonian, the imposition of shortening decreases the lithospheric strength, which causes perturbations to the lithosphere’s temperature structure to grow exponentially with time. Once these perturbations become sufficiently large, they then grow super-exponentially with time, eventually removing the lithospheric base. Because lithospheric viscosity is highly temperature-dependent, at most only the lower 30 per cent of the lithosphere participates in the downwelling associated with this initial super-exponential growth event. After this event, however, a downwelling develops that removes material advected into the region of downwelling by horizontal shortening. The magnitude of this persistent downwelling depends on the rate and duration of shortening. If the total amount of shortening does not exceed 50 per cent (doubling of crustal thickness), then this downwelling extends to a depth three to four times the thickness of undeformed lithosphere and forms a sheet significantly thinner than the width of the region undergoing shortening. Once shortening stops, this downwelling is no longer replenished by the shortening process, and should then detach due to its inherent gravitational instability. The hottest 60 per cent of the mantle lithosphere could be removed in such an event, which would be followed by an influx of hot, buoyant asthenosphere that causes rapid surface uplift. Because more cold material is removed after the cessation of shortening than by the initial gravitational instability, the former has a potentially greater influence on surface uplift. The Tibetan interior is thought to have been shortened by about 50 per cent in ∼30Myr and afterwards, at ∼8Ma, experienced a period of rapid uplift that may have resulted from the removal of a large downwelling ‘finger’ of cold lithosphere generated by shortening.