Modeling Factors Controlling Retrowedge Evolution in Doubly Vergent Orogens

Abstract:

Doubly vergent orogens have fold and thrust wedges on both sides: The prowedge (lower plate) and the retrowedge (upper plate). Models have shown that retrowedges behave differently from prowedges but what influences retrowedge behavior is still poorly understood. We use high-resolution numerical models to investigate several factors that influence the evolution of retrowedges and the mechanisms by which they do so. Because pro- and retrowedges are strongly influenced by each other, we model a complete doubly vergent orogen. We investigate the relative importance of 1) inherited extensional shear zones, 2) décollement horizons in the sedimentary cover, 3) sedimentation and erosion, and 4) mantle lithosphere strength. The influence of inherited structural weaknesses is investigated by creating a rift and then inverting it. Our numerical models demonstrate that this facilitates shortening in the retrowedge by reactivation of the rift margin resulting in thrusting of basement blocks on top of the upper plate, and also allows thrusting of lower crust onto the upper plate, such as observed in the Pyrenean and Alpine mountain belts. The décollement horizon in the sedimentary cover controls thin-skinned tectonics. Sedimentation influences rift inversion as well as the length of both thick-skinned and thin-skinned thrust sheets. Efficient erosion facilitates exhumation in the core of the orogen inhibiting transfer of shortening into the retrowedge.