Fluid pressure at the base of the seismogenic zone influences fault strength and slip behavior. However, data on transport properties of faults at this depth remain lacking. Here, we conducted permeability experiments using samples exhumed from mid-crustal depths along the Red River Fault (RRF), which is a large-scale strike-slip fault located in the southeastern Tibetan Plateau. Experiments were performed at effective pressures ranging from 5 MPa to 165 MPa. The permeability, porosity, and specific storage of samples exhibit overall low values and display systematic variations depending on rock types and their deformation microstructures. Mylonites display 1 to 2 orders of magnitude anisotropy in permeability parallel and normal to foliation. The hydraulic architecture of the RRF at the base of the seismogenic zone is thus characterized by low permeabilities in cataclasite slip zones and higher permeabilities in mylonites, with fluid flow facilitated along mylonitic foliation. Microstructural observations indicate that microfractures, grain boundaries, and phyllosilicate cleavages are potential fluid pathways. The high temperature and pressure conditions promote viscous creep, mineral recrystallization, and fluid-associated dissolution-precipitation, which seal fractures and reduce fluid pathways. Despite low permeabilities, the relatively higher porosities and widespread presence of hydrothermal veins and fluid alteration signatures suggest that cataclasite slip zones are potential fluid reservoirs and pathways. We interpret that dynamic porosity generated by creep cavitation and grain boundary sliding during shearing, along with permeability enhancement during transient frictional slip, are major fluid flow mechanisms at the base of the seismogenic zone. The low permeability and specific storage imply that fluids in cataclasites are sensitive to deformation and thus prone to overpressurization during creep compaction and depressurization during transient slips. We further discuss the implications of our findings for the evolution of fluid pressure and transport properties during different modes of fault slip at the base of the seismogenic zone.