Convergence Velocity Controls on the Structural Evolution of Orogens

The collision of continental crust results in the formation of orogenic wedges to accommodate convergence. We use 2D thermomechanical models to address the growth of viscoplastic orogenic wedges, focusing on the structural evolution over time. We find that when the shear stress at the base of the crust is below the yield stress then viscous behavior dictates the evolution, with the wedge structure and geometry influenced by the convergence velocity. In contrast, when the shear stress is equal to or above the yield stress across the entire wedge then plastic behavior controls the evolution, with the structural style and geometry independent of convergence velocity. The models highlight the controls of the basal décollement rheology on the deformation style of the orogenic wedge. We find that for increasing crustal thickness, the velocity required to transition from viscous-to-viscoplastic and then to plastic-dominated wedges increases. We determine empirically how the viscous deformation influences wedge strength and estimate an effective friction coefficient based on the geometry of the entire wedge. The models allow inferences on the dominant deformation mechanism currently accommodating convergence in the Zagros, Himalayas, and European Alps when estimating the effective friction coefficient from the geometry, showing the relevant role of rate-dependent viscous deformation in each orogenic wedge.