Field, laboratory, and modeling studies of faulted rock yield insight into the hydraulic character of thrust faults. Late-stage faults comprise foliated and subparallel faults, with clay-rich gouge and fracture zones, that yield interpenetrating layers of low-permeability gouge and higher-permeability damage zones. Laboratory testing suggests a pereability constrast of two orders of magnitude between gouge and damage zones. Layers of differing permeability lead to overall permability anisotropy with maximum permeability within the plane of the fault and minium permeability perpendicular to the fault plane. Numerical modeling of regional-scale fluid flow and heat transport illustrates the impact of fault zone hydrogeology on fluid flux, fluid pore pressure, and temperature in the vicinity of a crystalline thrust sheet. ¿American Geophysical Union 1991