Modeling the Rheological Dependence of Strain Partitioning in Oblique Wedges During Active Collision and "Post-Tectonic" Relaxation

We have observed the partitioning of deformation in the doubly-vergent wedges produced during oblique convergence in analogue models with frictional, Newtonian viscous, and mixed (frictional over viscous) rheologies. In each of these experiments, we quantify the margin-normal velocity, strain rates, and topography, as well as the plane-strain, shear and rotation fields, during and after active convergence. The convergence obliquity in these experiments is large, 50&deg° to plate motion, so significant partitioning of deformation was expected in all wedges, regardless of rheology. The pure frictional wedge is characterized by numerous discrete thrust faults in the pro-wedge and a zone of shear between the pro-wedge and the retro-wedges. The highest rate of compressional deformation is at the thrust front, while the highest rate of shear is isolated on near-vertical faults at the back of the pro-wedge. The zones of active deformation are narrow compared to the cross-sectional width of the frictional wedge. In the purely viscous experiment, the deformation is more diffusely distributed within the wedge, with compression and shear spread across wider zones and bulk deformation of the wedge. At slow strain rates, the dual rheology wedge is essentially a wedge with a weak basal detachment. It has discrete structures on which strain is accommodated but the relative width of the actively deforming zone is substantially wider than that for pure friction, with several faults in the pro-wedge actively slipping at any given time. Because it allows shear strain to be isolated largely behind the pro-wedge, it is the dual rheology wedge that most fully partitions strain into margin-normal and margin-parallel components. After the end of active convergence in these experiments the wedges were examined for signs of gravitational collapse. The rates of collapse were highest in the pure viscous experiment and were non-existent in the pure frictional experiment. However the mixed rheological model yielded the most interesting results as collapse and extension of the topographic high associated with the pro-wedge resulted in ongoing compression well forward of the deformation fronts of both the foreland or hinterland foldbelts.