Fluid Flow Accompanying Faulting: Field Evidence and Models 论文

摘要

Direct evidence that channel flow of aqueous fluids accompanies shallow crustal faulting, in some instances at least, comes from the observation of transitory surface effusions following some moderate to large earthquakes in consolidated rocks, and the textural characteristics of the hydrothermal vein systems often found associated with ancient, exhumed faults. These phenomena are examined in relation to two alternative models. In the first, the transitory post-seismic flow results from the collapse of pre-failure dilatant fractures in accordance with the dilatancy/fluid-diffusion hypothesis, so that the fault system can be regarded as a ‘pump'. In the second model, the fault/fracture system functions as a ‘valve' on a fluid reservoir. Rising fluid pressure induces fault slip, creating a temporary fracture permeability which allows partial draining of the reservoir. As the fluid pressure drops, self-sealing of the fracture system occurs by deposition of hydrothermal minerals, and the whole cycle can then be repeated. In some instances there is convincing evidence that extensive fracture dilatancy has developed close in to faults (usually of normal dip-slip character), and that dilatancy pumping has occurred as a result of post-slip collapse of these fracture systems. Dilatant extension fractures can only form or re-open by hydraulic fracturing adjacent to faults provided they retain some cohesive strength, but an important upper limit is thereby placed on the permissible level of differential stress. The more frequent and intense development of extension vein arrays adjacent to normal faults in comparison with thrusts is in accord with theory, which suggests that hydraulic fracturing should occur at significantly lower fluid pressures around the former than the latter.