Both the sources and pathways of fluid circulation are key factors to understanding the evolution of low-angle normal fault (LANF) systems and the distribution of mineral deposits in the upper crust. In recent years, several reports have shown the presence of meteoric waters in mylonitic LANF systems at mid-crustal conditions. However, a mechanism for meteoric water infiltration to these mid-crustal depths is not well understood. Here we report paired δ¹⁸O and δ²H isotopic values from dated, neoformed clays in fault gouge in major detachments of the southwest United States. These isotopic values demonstrate that brittle fault rocks formed from exchange with pristine to weakly evolved meteoric waters at multiple depths along the detachment. ⁴⁰Ar/³⁹Ar dating of these same neoformed clays constrains the Pliocene ages of fault-gouge formation in the Death Valley area. The infiltration of ancient meteoric fluids to multiple depths in LANFs indicates that crustal-scale normal fault systems are highly permeable on geologic timescales and that they are conduits for efficient, coupled flow of surface fluids to depths of the brittle-plastic transition.