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We noninvasively detected the characteristics and location of a regional fault in an area of poor bedrock exposure complicated by karst weathering features in the subsurface. Because this regional fault is associated with sinkhole formation, its location is important for hazard avoidance. The bedrock lithologies on either side of the fault trace are similar; hence, we chose an approach that capitalized on the complementary strengths of very low frequency (VLF) electromagnetic, resistivity, and gravity methods. VLF proved most useful as a first-order reconnaissance tool, allowing us to define a narrow target area for further geophysical exploration. Fault-related epikarst was delineated using resistivity. Ultimately, a high-resolution gravity survey and subsequent inverse modeling using the results of the resistivity survey helped to further constrain the location and approximate orientation of the fault. The combined results indicated that the location of the fault trace needed to be adjusted 53 m south of the current published location and was consistent with a north-dipping thrust fault. Additionally, a gravity low south of the fault trace agreed with the location of conductive material from the resistivity and VLF surveys. We interpreted these anomalies to represent enhanced epikarst in the fault footwall. We clearly found that a staged approach involving a progression of methods beginning with a reconnaissance VLF survey, followed by high-resolution gravity and electrical resistivity surveys, can be used to characterize a fault and fault-related karst in an area of poor bedrock surface exposure.

Field and microstructural analyses of faults with millimeters to hundreds of meters of displacement within welded tuffs at Yucca Mountain, Nevada, has led us to identify four different fault zone architectures. We designate these fault zones as Classes A–D. We conclude that Classes A, C, and D are genetically related and that observed differences in their morphology and deformation mechanisms are related to differences in displacement magnitude and changes in the rheology of faulted rocks. We show that Class A, C, and D fault zones were formed by progressive cataclasis, leading to the development of a foliated gouge core and wide damage zone at the highest displacement magnitudes. By inference, displacement variations across individual high displacement faults are expected to produce significant lateral changes in fault zone architecture. The broad range of fault rock textures and architectures found on the Class A, C, and D continuum may produce important corresponding variability in hydrologic properties in ignimbrites. By contrast, Class B fault zones have unique mineralogies and microtextures relative to other Yucca Mountain fault zones. Most notable is the occurrence of distinctive jigsaw puzzle texture within fault core breccias, which consists of angular wall rock fragments floating in a coarsely crystalline secondary calcite cement matrix. The calcite cement constitutes more than 65% of the Class B fault zones. Comparisons with other occurrences of secondary minerals within open lithophysal cavities, which appear to have formed in unsaturated conditions, indicate that the secondary mineralization history at Yucca Mountain was complex and likely polygenetic. The age and origin of the secondary calcite minerals in the Class B fault zones remain undetermined.