MECHANICAL MODELING OF FAULTS WITH WING CRACKS: EFFECTS ON FLUID FLOW AND LOCAL PERMEABILITY

Mechanical interaction between a fault and various structures in the damage zone and beyond may increase the local permeability, thereby providing pathways for ore-depositing fluids. We use mechanical models to show how, and under what conditions, these interactions lead to opening of the fault and associated vein formation. The 2D Displacement Discontinuity Method (DDM) is combined with a complementarity algorithm to model the quasi-static propagation and opening of wing cracks that emanate from regions of tensile stress concentration along a fault in an otherwise homogeneous and isotropic elastic material. Because stress states and geometry change with wing crack propagation, one can not specify the boundary conditions a priori. That is, the fault may slip, stick, open or close as the wing cracks propagate. Under these conditions complementarity is superior to other well-known contact algorithms. We focus on meter scale phenomena where mineralogical heterogeneity (common to laboratory scale) and 3D geometry (common to crustal scale) reasonably can be ignored. Analytical solutions to the elastic boundary value problem of the closed fault include those that assume fully drained conditions, no friction, uniform friction, and a cohesive end zone (CEZ). Here we generalize the problem to consider both drained and undrained conditions, uniform or linearly varying friction in the CEZ and allow the fault to open when mechanical interaction with the wing crack dictates that it should. Opening of wing cracks facilitates sliding along the fault by diminishing the resolved compressive stress. The magnitude of the local tensile stress induced by this mechanical interaction may exceed the resolved compressive stress resulting in opening along the fault. Such opening along the initially closed fault surfaces, as well as associated single or multiple wing crack opening creates potential leaking points, facilitates fluid flow, and enhances the local permeability, possibly leading to the formation of ore deposits.