Numerical modeling of crustal scale faulting using the distinct and boundary element methods

Crustal scale faulting in the upper crust, including the formation of structures associated with detachments and reactivated basement normal faults, is not well understood. Experimentalists have used sandbox models to gain insight into the kinematics of their formation and progressive development. Although the structures observed in some models resemble those observed in nature, boundary conditions and constitutive properties of these models have rarely been measured precisely so the experiments are difficult to interpret.;We use a numerical approach based on the distinct element method (DEM) to model structures that form above reactivated basement normal faults and detachments. With DEM, the overburden is represented by hundreds of individual elements that interact mechanically through contact forces. Forces are transmitted between elements at contact points. Assemblages of elements can simulate different kinematic behavior from localized faulting to distributed flow by changing element size ratios. The faults that form in an assemblage of elements resemble those observed in nature. The boundary conditions are prescribed so the numerical experiments are easily interpreted.;DEM is a tool for solving a "forward" problem in which forces are applied to a body with known boundary conditions and the resulting structure is observed. Another method for understanding geologic structures is to consider "inverse" problems. In this case, the goal is to deduce the stress state from a geologic structure.;Specifically, stress inversions methods, which estimate a regional stress tensor from populations of faults containing slickenlines, rely on the assumption that slip on each fault occurs in the direction of resolved shear stress. This premise ignores directional differences in fault compliance caused by fault shape and the earth's surface and perturbations caused by interactions with nearby faults. Differences in compliance and stress perturbations may result in a difference between the direction of resolved shear stress and the direction of fault slip. Mechanical modeling of common fault geometries in an elastic halfspace using the computer code DIS3D, based on the boundary element method, provides a means for evaluating this difference. Our goal is to distinguish those circumstances under which the inversion techniques are reliable from those which violate the basic assumptions.