Coupling and decoupling of crust and mantle in convergent orogens: Implications for strain partitioning in the crust

Topographic relief, strain partitioning and crust-mantle coupling within convergent orogens may be systematically related to the distribution of strength as a function of depth within continental crust. This result is obtained by solving analytically for basally driven three-dimensional flow within an idealized crust with a laterally invariant, depth-dependent Newtonian viscosity. Two end-member flow states are shown to occur. In the first, crustal flow is directly coupled to the underlying mantle and is controlled by stresses transmitted vertically through the crust. In the second, upper crustal to midcrustal flow is decoupled from the underlying mantle and is controlled by stresses transmitted horizontally through the crust. This model is applied to deformation of continental crust above ''plate-like'' mantle that undergoes oblique convergence across a narrow ''suture.'' If no significant low-viscosity zone develops within the crust, crustal deformation is coupled to motion of the underlying mantle, and a triangular mountain range develops. Surface shortening occurs at the foot of the mountains, normal to the trend of the range, while strike slip and extension are partitioned along the axis of the orogen. If a low-viscosity zone is initially absent but develops during crustal thickening, a steep-sided, flat-topped plateau forms. Crustal deformation is coupled to mantle motions on the plateau flanks but may be strongly decoupled beneath the plateau. Surface shortening normal to the plateau occurs at the foot of the plateau, while surface extension normal to the plateau occurs at the plateau edge. If a low-viscosity zone is present in the lower crust prior to convergence, a wide orogen with low topographic relief develops. Crustal deformation is decoupled from the mantle except at the edges of the deforming region, and surface shortening is partitioned uniformly across the belt. This study shows that horizontal and vertical partitioning of strain occur even in the absence of lateral strength variations and that shortening and extension may exist simultaneously in different parts of an orogen. Similar morphology and strain partitioning can be observed in many real orogenic belts, including: the Eastern Alps, the Tibetan plateau, and the Lachlan (Tasman) fold belt.