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.

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