FAULTS AND FLUID FLOW IN THE CRUST: ADVANCES YET TO BE MADE

During the past 25 years, the geologic community has developed a much stronger and integrated approach to questions regarding fluid flow in the crust, and how fault zones impact that flow. Fault zones appear to be one of the major pathways for fluids, something that mining geologists had long recognized. Structural geology has played a curious role in these advances. A survey of literature of 1980 shows that the authors of the few papers that addressed the questions of fault-influenced fluid flow were primarily geochemists, geophysicists, a rare hydrogeologist, and petrologists, with only a handful of structural geologists contributing. In some ways, we have come full circle, with new work using isotopes, geochemical tracers, flow modeling, and geophysical methods to determine how and when fluids and faults interact. Numerous important contributions from the structural geologic community have contributed to our understanding that faults act as complex conduit/barrier systems depending on their geometry, composition, state of evolution, state of stress, and where they lie in the hydrologic system, but the practitioners of the dark arts of hydrologic analysis of fault and fracture systems are still too few, and our ability to communicate with other communities at times seems wanting. We might consider how to better pose structural geologic studies of faulted and fractured rocks to address broad questions, such as: What are (or were) the rates and volumes of flow? What was the chemical composition and evolution of the fluids and rock during faulting? What are the mechanical processes of fluid-fault processes? What is the periodicity of flow and how well have we tested the Sibson models of fault-related flow and the earthquake cycle? What are the data needed for scientific and engineering models of flow in faulted crust, and do we provide these data in a timely manner?