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Fault Zone Deformation Overprints Permeability of Nonwelded Ignimbrite: Chalk Cove Fault, Bishop Tuff, Bishop, California

^a Center for Nuclear Waste Regulatory Analyses, Geosciences and Engineering Division of Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238
^b Utah Faults, Fractures, and Fluids (UF³), Innovation Campus, Utah State Univ., 1770 Research Park Way, Ste. 183, North Logan, UT 84341
^c now at Dept. of Earth, Material, and Planetary Sciences, Geosciences and Engineering Division of Southwest Research Institute
^d now at U.S. Nuclear Regulatory Commission

View larger version (46K): [in a new window]   Fig. 1. Stratigraphic comparison between the Paintbrush nonwelded hydrogeologic unit and the Bishop Tuff.

View larger version (141K): [in a new window]   Fig. 2. Topography and geographic location of the research area near Bishop, California, in relation to that of Yucca Mountain, Nevada. Painted, shaded-relief map generated from USGS 7.5' quad DEMs, UTM coordinates, Zone 11. Resolution is 30 m, NAD 27 datum.

View larger version (118K): [in a new window]   Fig. 3. Aerial photograph of the Chalk Cove fault exposure at the erosional southern margin of the Volcanic Tableland; view to the north.

View larger version (81K): [in a new window]   Fig. 4. Chalk Cove fault: (a) photograph annotated to show the fault trace, the locations of Transects CCHW1 and CC2/CC3, and lithologic subunits; view to the north-northeast; (b) photograph illustrating wedge-shaped system of fractures in the hanging wall; (c) schematic illustration of the wedge-shaped architecture of the fault deformation zone.

View larger version (22K): [in a new window]   Fig. 5. Geologic map of Chalk Cove fault study site in map view, illustrating main fault trace and hanging-wall fracture systems. Detailed fracture mapping areas of Fig. 6 and 7 are shaded in gray. Transect CCHW1 is downsection and Transect CC2/CC3 is upsection (see Fig. 4). Red shading and traces represent fault core gouge, small slip surfaces, or iron-oxide stained zones. Sample locations are identified.

View larger version (41K): [in a new window]   Fig. 6. Fracture pavement map of Transect CCHW1, located at x = 0 m in Fig. 5. Sample locations for gas permeability studies, liquid permeability studies, and laboratory samples are identified.

View larger version (38K): [in a new window]   Fig. 7. Fracture pavement map of Transect CC2/CC3, located at x = 17 m in Fig. 5; mapping extends to x = 22 m. Sample locations for gas permeability studies, liquid permeability studies, and laboratory samples are identified.

View larger version (127K): [in a new window]   Fig. 8. (a) Small-drillhole minipermeameter probe. (b) Drilled test holes for gas permeability measurement at Transect CC2/CC3. (c) Constant-head permeameter near Transect CC2/CC3; view to the south-southwest.

View larger version (144K): [in a new window]   Fig. 9. Outcrop photos from the Chalk Cove fault site: (a) 2-mm-thick fault contact near station 0 along Transect CCHW1 (Fig. 6); (b) sample locations and fault zone architecture at x = 9 m (Fig. 5); (c) fault zone architecture at x = 22 m (Fig. 5 and 7); view to south; (d) 5-cm-thick central fault core gouge in CC2/CC3 map area (Fig. 5 and 7). F = central fault core or contact; HW = hanging-wall block; FW = footwall block.

View larger version (127K): [in a new window]   Fig. 10. Photomicrographs showing: (a) flaking and rotation of quartz clasts, and open fractures that follow grain boundary contacts near HW Fracture 3 (Fig. 6, Sample BT-99); (b) a zone of cataclasis and an opening-mode microfracture in the footwall mixed zone and the fault core contact (Fig. 5, Sample BT-74), viewed in plane polarized light; (c) indistinct micro-scale layering within the internal fault core where deformation is localized in narrow cataclastic bands of aligned angular shards and phenocrysts, with an opening mode microfracture that parallels cataclastic layering (Fig. 5, Sample BT-82); (d) sintering in the hanging-wall zone at approximately 0.5 m west of the Chalk Cove fault (Fig. 5, Sample BT-82); (e) locally decreased grain size (zone of comminution) within the central fault zone core (Fig. 5, Sample BT-85). Deformation within the fault-zone core includes dilational microfractures that parallel bands of comminuted host rock. Note how the extension fracture locally follows the margin of a pumice clast.

View larger version (29K): [in a new window]   Fig. 11. Gas and liquid permeability data measured along (a) Transect CCHW1; (b) Transect CC2/CC3. The fault is at y = 2.0 m (6.6 ft) along Transect CC2/CC3.

View larger version (25K): [in a new window]   Fig. 12. Spatial profiles of fracture density and gas permeability data using the same vertical scales for (a) Transect CCHW1, and (b) Transect CC2/CC3. (c) Scatterplot to assess how well the gas permeability data may be correlated with available fracture intensity data.