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In studying orogenic processes, the mechanism of viscous Rayleigh-Taylor–type removal of gravitationally unstable lithosphere is often invoked to explain the behavior of the mantle lithosphere. Using numerical models, we consider this mechanism and explore alternate styles of deep lithospheric deformation during tectonic convergence. The numerical experiments incorporate a mix of viscous and plastic rheologies to model the mechanical evolution of the lithosphere-asthenosphere system. Our results suggest that there are a number of deformational modes of the model mantle lithosphere: (1) a dripping or Rayleigh-Taylor–type instability; (2) an asymmetric underthrusting or subduction; (3) symmetric, ablative plate consumption; (4) slab breakoff, the failure and detachment of the strong lithosphere; and (5) mixed modes with combinations of these processes. The development of the modes is controlled by the rate of convergence associated with the background tectonic regime, the density field, and the rheology of the mantle lithosphere. It is important to determine whether these modes occur in the Earth beneath collisional orogens.

Inverted metamorphic sequences are characterized by peak metamorphic temperatures that increase structurally upward, with isograds that are typically parallel to associated thrust faults. A fully coupled thermal-mechanical model for convergent orogens shows how an inverted sequence can be tectonically assembled in a crustal-scale ductile shear zone at moderate to high rates of synorogenic erosion. Model inverted sequences form by tectonic juxtaposition of points with widely differing initial positions that reach peak temperatures at different times and in different places within the model orogen, and thus do not represent metamorphic or thermal gradients. Inverted crustal isotherms are not required to produce model inverted isograds. Model results agree well with metamorphic pressure-temperature data from the Main Central Thrust zone of the central Nepal Himalayas.

A mechanical model of crustal shortening and deformation driven by the relative convergence of rigid, underlying mantle plates explains many features of convergent orogens. Results based on numerical models and supported by sandbox models show that a Coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth: (1) block uplift bounded by step-up shear zones; (2) development of a low-taper wedge over the underthrusting mantle plate; and (3) development of a low-taperwedge overlying the overthrusting mantle plate and verging in the opposite direction. When modified by isostasy, basal viscous flow, surface erosion and denudation, and sedimentation, the resultant model orogens exhibit a variety of styles with characteristics in common with small, rapidly denuded orogens, large orogens with plateaus and extensional characteristics, and active subduction margins with doubly vergent accretionary wedges and deformed fore-arc basins.