The effects of buoyant crust on the gravitational instability of thickened mantle lithosphere at zones of intracontinental convergence

S U M M A R Y Using both numerical experiments and approximate linear theory, we examine the stability of a 2-D, three-layered viscous system that represents crust over mantle lithosphere, overlying asthenosphere. If horizontal shortening of lithosphere occurs, as in mountain building, the development of gravitational instability may follow one of two distinct paths. Localized downwelling occurs beneath the region of horizontal shortening and crustal thickening, if the layer representing crust has a large viscosity coefficient, is relatively dense or is thin, relative to the underlying layer that represents mantle lithosphere. Conversely, for a low viscosity ratio or buoyant crust, downwelling flow develops on the flanks of the zone of convergence, and the mantle lithosphere layer thins beneath the central region. Thinning in such a setting would facilitate volcanism and high-temperature metamorphism during the development of a mountain belt. A series of numerical experiments with different density and viscosity structures, including runs with non-Newtonian viscosity, show that this general description is insensitive to minor differences in the depth distributions of density and viscosity. The tendency of structures with relatively thick crustal layers (or relatively thin lithospheric mantle) to favour paired, marginal downwellings may account for sustained volcanism during intracontinental deformation where lithosphere initially is warm, thin and weak. The western part of the Tien Shan in Asia and the region including and surrounding the Alboran Sea illustrate some of the features associated with paired, marginal downwellings. The Western Transverse Ranges of California and the Southern Alps of New Zealand share features associated with single downwelling flow beneath the axes of the belts.