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The Edwards Aquifer, the major source of water for many communities in central Texas, is threatened by population growth and development over its recharge zone. The location of the recharge and confined zones and the flow paths of the aquifer are controlled by the structure of and deformation processes within the Balcones fault system, a major system of predominantly down-to-the-southeast normal faults. We investigate the geologic structure of the Edwards Aquifer to assess the large-scale aquifer architecture, analyze fault offset and stratigraphic juxtaposition relationships, evaluate fault-zone deformation and dissolution and fault-system architecture, and investigate fault-block deformation and scaling of small-scale (intrablock) normal faults. Characterization of fault displacement shows a pattern of aquifer thinning that is likely to influence fault-block communication and flow paths. Flow-path constriction may be exacerbated by increased fault-segment connectivity associated with large fault displacements. Also, increased fault-zone deformation associated with larger-displacement faults is likely to further influence hydrologic properties. Overall, faulting is expected to produce strong permeability anisotropy such that maximum permeability is subhorizontal and parallel to fault-bedding intersections. At all scales, aquifer permeability is either un-changed or enhanced parallel to faults and in many cases decreased perpendicular to faults. Keywords: Edwards Aquifer, Edwards Group, Balcones fault zone, faults and faulting, normal faults.

An orthogonal system of tube-bearing joints constitutes the oldest fractures in the Tiva Canyon Tuff at Yucca Mountain, Nevada. The joints formed within a month of ignimbrite deposition, prior to major degassing. The system consists of (1) narrow, persistent, northeast-striking joint swarms with trace lengths typically greater than 5 m and between-joint spacings of less than 20 cm and (2) northwest-striking swarms that have a more en echelon geometry and greater between-joint spacings compared to the northeast-striking swarms. Between-swarm spacing for both trends is ~50 m. Questions concerning the joints include the following: (1) What was the origin of driving stress(es) for formation of the joints, particularly as their orientations were not consistent with the regional stress geometry at the time of their formation? (2) What mechanism caused the horizontal principal stresses to be reoriented so as to yield an orthogonal geometry? (3) What insights can be developed for predicting joint geometry in unexposed rock volumes by understanding joint origin? These questions are important because the joints and other fractures may affect the performance of a proposed high-level nuclear waste repository within Yucca Mountain. To address these questions, we use new and existing field data about joint geometry and the relationships of joints to degassing structures, numerical modeling of fault behavior, and work by previous authors. Our interpretation begins with the initial ignimbrite eruption and deposition during caldera collapse. The ignimbrites were deposited over a preexisting topography that possibly included a shallow northwest-trending basin in the Yucca Mountain area. During initial ignimbrite cooling, joint swarms formed as elements of orthogonal fumarolic ridge systems where degassing was associated with vertical dilation. Both joint sets have unusual tubes that are interpreted to have formed during dilation and segmentation of joint faces resulting from lithophysae inflation in the cooling ignimbrite deposit. Modeling supports the interpretation that a combination of regional stresses and stress related to slip on local normal faults controlled the orientation of the joint swarms and favored the formation of the northeast-striking joints first. The faults might have moved in a stress field already perturbed by caldera collapse. Formation of the northwest-striking joints occurred after a local 90 [degrees] switch of horizontal principal stress directions due to the presence of the northeast-striking swarms, possibly aided by differential compaction across the northwest-trending basin. Tube-bearing joints occur in all lithophysae-bearing lithostratigraphic units of the Topopah Spring Tuff, which is the stratigraphic interval for the proposed nuclear waste repository within Yucca Mountain. We conclude that the tube-bearing joints formed in the same manner and share similar geometric characteristics, adding a persistent joint population to the overall fracture system that influences hydrological and mechanical properties. Keywords: orthogonal joints, welded tuffs, cooling joints, degassing tubes, perturbed stress fields, normal faults.

Paleomagnetic investigations of Tertiary dikes and Ordovician carbonate rocks at Bare Mountain and in the surrounding region of southwestern Nevada were conducted to further constrain possible rotations related to Walker Lane dextral shear and Basin and Range detachment faulting. Demagnetization experiments revealed two distinct remanence components in the Ordovician carbonate rocks: (1) a dual polarity Permian-Triassic magnetization carried by magnetite that was acquired after Paleozoic or Mesozoic folding but prior to Tertiary tilting; and (2) a dual polarity Miocene magnetization carried by hematite that was acquired after folding and tilting. The Tertiary dikes also contain a dual polarity magnetization carried by magnetite that can be interpreted either as a primary thermoremanent magnetization or a secondary chemical remanent magnetization acquired shortly after dike intrusion. Overall, mean directions for all three magnetizations roughly coincide with corresponding reference directions for Nevada. The largest declination discordance of 16 degrees +/- 7 degrees (clockwise) comes from comparison of the Permian-Triassic magnetite magnetization to the Late Permian reference direction for Nevada. Thus, although structural and geothermal data, including zircon fission-track ages, indicate that northern Bare Mountain was tilted 20 degrees -40 degrees northeastward about a horizontal axis during early Tertiary time, we find no evidence for large-scale (i.e., >30 degrees) vertical-axis rotation. On the basis of these results, we suggest that Bare Mountain forms an autochthonous or parautochthonous footwall within the Basin and Range and Walker Lane, in a region where many of the surrounding upper crustal rocks are detached and significantly rotated about vertical and steeply inclined axes. In addition, the nature and timing of footwall uplift indicated by our paleomagnetic results do not support tectonic models that reconstruct Bare Mountain and the Striped Hills as adjacent segments of a continuous fold and thrust belt system. Finally, we suggest that the postfolding Permian-Triassic magnetization at Bare Mountain coupled with Paleozoic zircon fission-track ages indicate that the Meiklejohn Peak thrust predates Jurassic-Cretaceous (Sevier) deformation and may have instead resulted from a Permian or earlier contractional event.

Analyses of alluvial fan sedimentation around Bare Mountain, Nevada, indicate differential slip along the Bare Mountain fault during the Pleistocene and Holocene. Analyses show that the ratios of fan area to sediment source area and the degree of Holocene sedimentation at the fan heads can be correlated with dip and displacement of the fault. The well-developed bajada and Holocene deposits near the fan toes along the southwestern flank of Bare Mountain contrast with the location of Holocene deposits near the fan heads and significantly smaller individual fan lobes on the eastern flank of the mountain (adjacent to the Bare Mountain fault). In addition, ratios of fan area to source area along the eastern flank of the mountain decrease from north to south, which we interpret to be caused by increased Quaternary slip (from north to south) along the fault, with relatively little Holocene throw at Tarantula Canyon and maximum throw near Wildcat Peak. The dip of the fault also changes from north to south, with relatively shallow dip angles (45 [degrees] to 50 [degrees]) at Tarantula Canyon and steeper ([approximately]70 [degrees]) dips near Wildcat Peak. Geometric constraints dictate that, for any given increment of horizontal extension, vertical offsets will be smaller on those segments of the fault that have shallower dip angles. On the basis of these observations, we conclude that the recent slip rate of about 0.02 mm/yr derived from trenching studies in the Tarantula Canyon fan should be considered a minimum value that may not represent the slip rate of the Bare Mountain fault as a whole.

Slip-tendency analysis is a new technique that permits rapid assessment of stress states and related potential fault activity. The tendency of a surface to undergo slip in a given stress field depends on its frictional characteristics (primarily controlled by rock type) and the ratio of shear to normal stress acting on the surface, here defined as slip tendency (determined by orientation of the surface within the stress field). An interactive computer tool displays the stress tensor in terms of its associated slip-tendency distribution and the relative likelihood and direction of slip on surfaces of all orientations. The technique provides easy visualization and rapid evaluation of stress in terms of its potential for causing slip on individual faults or fault populations for use in seismic-risk and fault-rupture-risk assessment, exploration for high-risk and earthquake-prone blind faults, selection of likely earthquake focal mechanism solutions, and for use in analysis of compatibility of geologic structures.

Ferrill et al. (1996) contended that they can generate constraints on the slip history of the Bare Mountain fault other than those included in detailed field studies by examining literature and topographic maps for the age and distribution of alluvial fan sediments around the mountain. However, it is asserted that this approach is flawed because it is founded on misinterpreted data, incomplete assumptions and an unverified sedimentation model. Nevertheless, Ferrill et al. argue that their alternative analysis is useful for evaluating trenching study results.