Analogue models of obliquely convergent continental plate boundaries

Analogue models are used to examine crustal-scale faulting at obliquely convergent continental plate boundaries. A uniform Coulomb material is deformed with basal kinematic boundary conditions to model two obliquely convergent lithospheric plates. The mantle part of one plate is assumed to detach from its overriding crust and then be subducted beneath the other plate. The obliquity of the collision is assumed to remain constant throughout the deformation. Experiments are run with obliquities ranging from pure convergence (low obliquity) to pure strike slip (high obliquity). Reverse faults are observed for all obliquities with a nonzero convergent component. By contrast, only collisions with a large amount of strike slip motion exhibit wrench faulting. In experiments dominated by their convergent component, the strike slip motion is totally accommodated by oblique slip along the reverse faults. Strain partitioning between reverse faults and wrench faults is only observed for experiments run above a certain critical partitioning obliquity. From the observed initial faults, we can deduce the change in orientation in the principal stress triad as the obliquity is changed. We propose that the initial direction of maximum compressive stress (sigma(1)) rotates horizontally as the obliquity is changed, which in turn affects the geometry of the initial faults formed in the material. In the case of reverse faults, the rotation increases their dip measured along the direction of pure convergence. The relative magnitude of the minimum horizontal stress and the vertical stress determine whether reverse faults or strike slip faults are the first to form. Although long term deformation is more difficult to analyze, a simple relationship for the angle at which strain partitioning occurs is derived.