Calibrating the bulk rheology of active obliquely convergent thrust belts and forearc wedges from surface profiles and velocity distributions

Abstract

The geometrical profiles and surface velocity distributions in active forearc wedges depend on the bulk rheology and the surface tractions. Analytical and numerical solutions of the force balance and corner flow circulation equations indicate that the mechanical parameters should, in principle, all be directly determinable from the observational variables. The analysis suggests the following preliminary conclusions. (1) The surface profiles of forearc wedges are consistent with either a bulk plastic rheology or a viscous rheology with a transition to plastic or Coulomb behavior in the upper few kilometers. (2) Corner flow circulation in the absence of accretion is diagnostic of wedges with a bulk viscous rheology. The surface strike‐normal velocity and the surface profile both depend on the bulk viscosity and the viscous coupling at the base, allowing these mechanical variables to be determined: current estimates of this velocity in some wedges suggest the bulk viscosity is not less than 3×1019 Pa s. (3) Obliquely converging nonaccreting plastic and Coulomb wedges show no distributed strike‐parallel shear. If the obliquity is above a critical angle, the wedge is separated from the upper plate by a strike‐slip fault, defining a forearc sliver. The critical angle of obliquity, the velocity of the forearc sliver, and the wedge geometry allow the boundary tractions to be determined. In obliquely convergent viscous wedges the strike‐parallel velocity is distributed throughout the wedge, decaying exponentially from the rear to the front. Its value at the rear and the length scale for the decay allow the bulk viscosity and the coupling at the rear to be determined. Slip vectors in some forearc wedges and thrust belts suggest complete partitioning of the strike‐parallel component of motion, characteristic of a bulk viscous rheology. Others show partitioning above a limiting angle of obliquity, suggesting bulk plastic behavior. There are no documented cases of active margins where there is an absence of partitioning for obliquity ∼45°, which appears to rule out Coulomb behavior as a general description for the bulk rheology of forearc wedges.