Preliminary results from a three-dimensional, finite-element computer model of the dynamics of compressional orogens suggest that the deformation of the brittle crust is the superposition of two modes of deformation: a forced mode dictated by preexisting structures or plate-boundary geometry, and a natural mode resulting from the nonlinear elastoplastic rheology of the crust that may depend on the choice of a particular failure criterion. Natural modes of deformation exist in both the vertical (thrust planes) and horizontal (shear zones) directions, and there is a large degree of coupling between them (thrust planes accommodate vertical and horizontal shear). The obliqueness of convergence affects the geometry of thrusting; therefore, two-dimensional plane-strain models may be inappropriate to model orogens in which the direction of compression is oblique to the strike of the orogenic belt.

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