Abstract Fluid transport properties at the base of the seismogenic zone exert a critical control on fault strength, slip behavior, and fluid circulation. However, quantitative constraints on permeability in deep fault rocks remain limited. We present new laboratory measurements of permeability, porosity, and specific storage on cataclasite‐ and pseudotachylyte‐bearing mylonitic rocks along the Red River Fault (RRF), China. Experiments conducted at effective pressures up to 165 MPa reveal systematically low permeabilities, with mylonitized cataclasites and pseudotachylyte‐bearing rocks exhibiting the lowest values (10 −22 –10 −21 m 2 ), while mylonites display relatively higher permeabilities, up to 10 −19 m 2 . A key finding is the pronounced permeability anisotropy, where permeability parallel to foliation is up to two orders of magnitude greater than perpendicular values. This anisotropy reflects the microstructural alignment of phyllosilicate minerals and cavitation bands, which provides interconnected flow pathways along foliation while impeding cross‐foliation transport. The hydraulic architecture of the RRF at the base of the seismogenic zone is thus characterized by overall low permeabilities with fluid flow facilitated along mylonitic foliation. Such fabric‐controlled anisotropy persists even under mid‐crustal pressure conditions, indicating that fault zones at the base of seismogenic depths can retain directional fluid pathways despite overall low permeabilities. Our results provide critical constraints on the hydraulic architecture of the RRF and offer broader insights into the role of anisotropy in controlling fluid flow and deformation at the base of the seismogenic zone.
Liang et al. (Thu,) studied this question.