No abstract

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.

The characteristics and statistical range of fault zone features appear to be predictable and consistent throughout the basin; this predictability can be used in groundwater flow simulations that consider the influence of faults.

High‐resolution aeromagnetic data acquired over the Albuquerque basin show widespread expression of faults that offset basin fill and demonstrate that the aeromagnetic method can be an important hydrogeologic and surficial mapping tool in sediment‐filled basins. Aeromagnetic expression of faults is recognized by the common correspondence of linear anomalies to surficial evidence of faulting across the area. In map view, linear anomalies show patterns typical of extensional faulting, such as anastomosing and en echelon segments. Depths to the tops of faulted magnetic layers showing the most prominent aeromagnetic expression range from 0 to 100 m. Sources related to subtler fault expressions range in depths from 200 to 500 m. We estimate that sources of the magnetic expressions of the near‐surface faults likely reside within the upper 500–600 m of the subsurface. The linear anomalies in profile form show a range of shapes, but all of them can be explained by the juxtaposition of layers having different magnetic properties. One typical anomaly differs from the expected symmetric fault anomaly by exhibiting an apparent low over the fault zone and more than one inflection point. Although the apparent low could easily be misinterpreted as representing multiple faults or an anomalous fault zone, geophysical analysis, magnetic‐property measurements, and geologic considerations lead instead to a “thin‐thick model” in which magnetic layers of different thickness are juxtaposed. The general geometry of this model is a thin magnetic layer on the upthrown block and a thick magnetic layer on the downthrown block. The thin‐thick model can be represented geologically by growth faulting and syntectonic sedimentation, where relatively coarse‐grained sediment (which is more magnetic than fine‐grained material) has accumulated in the hanging wall. This implies that the aeromagnetic data have potential for mapping growth faults and locating concentrations of coarse‐grained material that may have high hydraulic transmissivity. Although cementation along fault zones is common in portions of the area, we found no evidence that this secondary process results in measurable aeromagnetic anomalies. This observation differs from the findings in other sedimentary basins suggesting that magnetic anomalies arise from secondary magnetization along fault planes.