The architecture and permeability structure of brittle fault zone

The time dependent architecture of a brittle fault zone is a framework for understanding its permeability structure. Conceptual models for fault-related permeability structures are derived from field investigations, laboratory permeability measurements, in situ flow tests, and numerical models of fluid flow in and around fault zones in many geologic settings. Conceptual models lead to a set of indices that describe distinct structural and hydrogeological components defined as a fault core and damage zone. Two idealized end-members are used to shed light on the myriad combinations of these components and how they control whether a fault zone will act as a fluid flow conduit, barrier, or combined conduit-barrier system. Indices help to understand, compare, and correlate the properties of individual and different fault zones in various geologic settings.;Because brittle fault zones are typically composed of macroscopic fracture networks, determining the patterns and rates of fault-related fluid flow is a three-dimensional problem. A series of finite element simulations, in a set of three-dimensional discrete fracture models, aids in identifying the primary factors controlling flow. Idealized fault zone architectural models are based on field data, fault mechanics, and the end-member models. The relative permeability contrasts between components and their internal hydraulic anisotropy are identified as the major controlling factors. In the context of natural fault zone evolution, model results demonstrate that small changes in architecture and hydraulic parameters of individual components can cause variations of up to five orders of magnitude in permeability structure.;Detailed mapping and laboratory analyses of fault rocks from the Stillwater fault zone (SFZ) in Dixie Valley, Nevada, provide a natural example of the processes associated with deformation-related fluid flow. The SFZ is associated with a heterogeneous and anisotropic fracture network that leads to a combined conduit-barrier permeability structure. Interrelated episodic deformation and fluid flow processes are inferred at a host of observational scales. The SFZ appears to be a relatively strong fault where fluid assisted dilatancy was a dominant deformation mechanism. The evolution of permeability anisotropy and heterogeneity likely control the location, propagation, and arrest of bedrock rupturing and the distribution of deformation associated with major earthquakes.