Dynamic and kinematic growth and change of a Coulomb wedge

Abstract

Deformation and structural relationships in accretionary prisms and fold and thrust belts are the result of dynamic changes in the size, geometry, or strength of the deforming wedge and its boundary conditions. The concepts of critical slope or taper that have been successful in explaining the static geometry and state of stress in a Coulomb wedge can be expanded through the use of finite element models to consider the kinematics and dynamics of a deforming Coulomb wedge. The numerical technique adopts a Coulomb failure criterion and isotropic plastic flow in a velocity-based Eulerian formulation. This formulation allows for very large deformation to be accommodated by a numerical mesh that remains fixed in space, deforming only to follow the movement of the upper surface. Critical wedge theory defines deformational domains bounded by the critical wedge solutions. Imposed changes in boundary conditions or geometry can move the mechanical state of a wedge off a critical line into either the sub-critical or stable domain, in which a wedge is unstable during accretion, leading to transient deformation as the wedge adjusts to a new critical geometry. With steady boundary conditions the accretion process leads to self-similar growth. A zone of high strain rate representing the frontal step-up thrust and the decollement develops and separates underthrust sediment from the deforming wedge. An increase in basal strength produces large internal deformation as the wedge increases its taper. A decrease in basal strength concentrates deformation at the toe of the wedge. A large decrease in basal strength may lead to extensional collapse. A complex geological history involving repeated cycles of growth and collapse could produce tectonic exhumation of the deeply buried interior of the wedge, even in the absence of erosion.