We explore the possibility that intraplate orogeny is the result of gravitational 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 density asthenosphere. A small harmonic perturbation is then imposed on the base of the high‐density layer and the system is allowed to flow under the gravitational instability described by the Rayleigh–Taylor instability. Viscosity and density are uniform within the layers and Newtonian rheology is assumed. We investigate the ability of the downwelling, high‐density, lithospheric layer to thicken the low‐density crustal layer above the downwelling. We solve the system using two methods: a numerical solution to the full set of 2‐D viscous flow equations and a linearized approximation of the early growth of the instability, valid for small deflections. For typical physical parameters, our results show that the ratio of downward displacement at the Moho to that at the base of the lithosphere is ∼6 per cent provided that the crust is weaker than the lithosphere. Our results show that a buoyant crustal layer overlying the higher‐density lithospheric layer may be thickened and uplifted over a lithospheric downwelling, achieving a maximum crustal thickening factor of ∼1.4 (for typical lithospheric 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 continental crust causes the instability to occur at a lateral wavelength of order 300 km regardless of whether a stress‐free or rigid condition is used on the upper boundary. Thin, less buoyant crust, however, allows the instability to occur at much longer wavelengths. For a stress‐free upper boundary the surface deflection over the downwelling is upwards due to crustal convergence, unless the crust is very rigid (crustal viscosity >13 times mantle viscosity), in which case the surface deflection over the downwelling is down, creating a topographic depression. For detachment of the downwelling blob to occur within 30 Myr, without any external tectonic activity, an average lithospheric viscosity of ∼1021 Pa s is required. Once detachment of the downwelling mass has occurred the system responds to flow driven by a thickened crust. This causes the crust to flow back to its equilibrium position and a period of extension thus follows the period of compression. Such a cycle could conceivably be repeated if the thinned lithosphere is able to thermally equilibrate, cool and thicken again after extension.

No abstract

Abstract. Recent high-resolution seismic experiments reveal that the crust beneath the San Gabriel Mountains portion of the Transverse Ranges thickens by 10-15 km (contrary to earlier studies). Associated with the Transverse Ranges, there is an anomalous ridge of seismically fast upper mantle material extending at least 200 km into the mantle. This high-velocity anomaly has previously been interpreted as a lithospheric downwelling. Both lithospheric downwelling and crustal thickening are associated with the oblique convergence of Pacific and North America plates across the San Andreas Fault, though it seems likely that the lithospheric downwelling is driven at least partly by gravitational instability of the cold lithospheric mantle. We show by means of numerical experiment that the balance between buoyancy forces that drive deforrnation and viscous stresses that resist deformation determines the geometry of crustal thickening and mantle downwelling. We use a simple two-layered lithospheric model in which dense lithospheric mantle overlies relatively inviscid and less dense asthenosphere and is overlain by buoyant crust. External plate motion drives convergence, which is constrained by boundary conditions to occur within a central convergent zone of specified width. A fundamental transition in the geometry of downwelling is revealed by our experiments. For slow convergence, or low crustal viscosity, downwelling occurs as multiple sheets on the margins of the convergent zone. For fast convergence or crust that is stronger than mantle lithosphere a single downwelling occurs beneath the center of the convergent zone. This complexity in the evolution of the system is attributed to the interaction of crustal buoyancy with the evolving gravitational instability. In order for a narrow downwelling slab to have formed beneath the Transverse Ranges within the last 5 Myr, the effective lithospheric viscosity of the convergent region is at most about 1020 Pa s.