The San Ysidro fault is a spectacularly exposed normal fault located in the northwestern Albuquerque Basin of the Rio Grande Rift. This intrabasin fault is representative of many faults that formed in poorly lithi fi ed sediments throughout the rift. The fault is exposed over nearly 10 km and accommodates nearly 700 m of dip slip in sub hori zontal, silici clastic sediments. The extent of the exposure facilitates study of along-strike variations in deformation mechanisms, archi tecture, geochemistry, and permea bility. The fault is composed of structural and hydrogeologic components that include a clay-rich fault core, a calcite-cemented mixed zone, and a poorly developed damage zone primarily consisting of deformation bands. Structural textures suggest that initial defor ma tion in the fault occurred at low temperature and pressure, was within the paleosaturated zone of the evolving Rio Grande Rift, and was dominated by particulate fl ow. Little geochemical change is apparent across the fault zone other than due to secondary processes. The lack of fault-related geochemical change is interpreted to reflect the fundamental nature of water-saturated, particulate fl ow. Early mechanical entrainment of lowpermeability clays into the fault core likely caused damming of groundwater fl ow on the up-gradient , footwall side of the fault. This may have caused a pressure gradient and fl ow of calcite-saturated waters in higherpermeability , fault-entrained siliciclastic sediments, ultimately promoting their cementa tion by sparry calcite. Once developed, the cemented and clay-rich fault has likely been, and continues to be, a partial barrier to cross-fault groundwater fl ow, as suggested by petrophysical measurements. Aeromagnetic data indicate that there may be many more unmapped faults with similar lengths to the San Ysidro fault buried within Rio Grande basins. If these buried faults formed by the same processes that formed the San Ysidro fault and have persistent low-permeability cores and cemented mixed zones, they could compartmentalize the basin-fi ll aquifers more than is currently realized, particularly if pumping stresses continue to increase in response to population growth.