Figure 3 Block diagram of structural units of Punchbowl fault zone.
Related Figures (17)
Mechanical Properties of the Punchbowl Fault Zone “eons Canyon exposure of Punchbowl fault zone within Devil’s Punchbowl, Los Angeles County Park. Fault juxtaposes basement rocks and Punchbowl Formation along continuous gouge layer. Location and orientation of gouge Jayer shown by arrow. Canyon wall approximately 75 m high. “orm” Photomicrographs of subsidiary faults and microfractures in damaged Punchbowl Formation, showing textural evidence of fluid-assisted processes. Scale bar applies to all photos and is 0.4 mm in (b) and (c) and 0.13 mm in (a) and (d); views are under crossed polarizers in (a) and (b) and under plane-polarized light in (c) and (d). (a) Quartz-sealed fracture adjacent to quartz-cemented subsidiary fault. (b) Dilation of quartz grain by extension on network of fractures; uniform extinction of quartz fragments shows fragments were not significantly rotated, ic, pure dilation. Cement is calcite. (c) Concentration of opaques and elongate shape of quartz grains in subsidiary fault gouge (sample DP10F). (d) Close-up of subsidiary fault in (c) showing embayed and interlocking grain boundaries. Features in (c) and (d) suggest dissolution of silica during faulting. Mineralogy’ of Undeformed and Deformed Punchbowl Formation Sandstones? Table 1 atom ft Excavations of main gouge zone at DP4. Pencil is 14cm long. (a) Nearly horizontal exposure of gouge zone with basement at top and Punchbowl Formation at bottom. Contacts with host are distinct (white arrows). Punchbowl Formation directly adjacent to gouge is intensely fractured. Both ends of pencil point to prominent surfaces in gouge layer, interpreted as sites of shear. (b) View of gouge and Punchbow! Formation contact, showing characteristic irregular geometry. Host embayed at point of intersection with subsidiary fault (black arrow). PAGEOPH, PAGEOPH, 4p ee™= Vv Texture of Punchbowl fault gouge. Scale bar applies to all photos and is 0.4 mm; views are under crossed polarizers in (a) and under plane-polarized light in (b), (c), and (d). (a) Symmetric bands of fibrous quartz cement in sealed fracture, which is offset by microscopic shear bands. (b) Quartz-cemented microbreccia. (c) Layering defined by distribution of microcrystalline quartz porphyroclasts. Each layer composed of porphyroclasts that are sheared and offset remnants of ancient, once continuous sealed fractures. (d) Example of typical foliation present in gouge. Mineralogy' of the Punchbowl Fault Gouge? Table 2 Map of subsidiary faults and fractures in multiple-fault specimens (DP10C) used in triaxial compression tests. Sides of block are ‘unfolded’ onto page; ends of block not shown. Heavy lines indicate faults open by separation; thin lines indicate closed faults. Block 2 used in dry test and block 4 in wet test. Mechanical Properties of the Punchbowl Fault Zone Gouge Samples Used in Triaxial Friction Experiments Mechanical Properties of the Punchbowl Fault Zone 2.5(10)-® cm/sec in a large-specimen triaxial apparatus described by HANDIN et al. (1972). To simulate grossly the environmental conditions in the fault zone during natural deformation, the experiments were conducted at an effective confining pres- sure of 50 MPa and at room temperature. Several additional experiments were conducted at 100 and 150 MPa. All the fault rocks were deformed under nominally dry and water-saturated conditions. PAGEOPH, Differential stress versus axial strain for triaxial compression of undeformed-sandstone and single-fault samples under wet and dry conditions. Differential stress versus axial strain for triaxial compression of multiple-fault and intact-gouge samples under wet and dry conditions. Mechanical Properties of the Punchbowl Fault Zone gouge increases with increasing connining pressure (rig. 1V). All samples deformed at an effective pressure of 50 MPa show a consistent differ- ence in behavior between wet and dry conditions (Figs. 11 and 12). Under wet conditions all four rock types show a reduction in elastic modulus (except for the single fault sample), a reduction in ultimate strength and, for experiments with stress drops, a lower negative slope for the post-failure portion of the curve of differential stress versus axial strain. The strain at failure also varies slightly between samples (Figs. 11, 12, and 13). The stress drops are sudden under dry conditions, but they take several hours when the rocks are saturated. The preéxisting natural faults in the samples were reactivated during the experimental deformation. Tha wnmrnt farlhrien cteanmathan Af tha thenen naw Deter weet mw or Iw cw J. Jw MVE TEI. The different fault gouges in shear have similar sliding behavior under wet and iry conditions, and the dry gouges have similar strengths (Fig. 14). Under wet sonditions the initial sliding strengths of the gouges are much lower than those of the corresponding dry tests. The four wet gouges strain-harden at slightly different ‘ates, producing a greater variation in sliding strengths at high shear strains under wet conditions than under dry conditions. No correlation between behavior and slay composition of the gouge is demonstrated (Table 2 and Fig. 14). a Schematic diagram of structure and relative properties of idealized brittle fault zone based on observations of Punchbowl fault. Block diagram indicates structural-mechanical units of fault. Surfaces indicate relative variation in properties along and across fault. Note that increasing direction of particular properties may be up or down, as shown on axes. Mesoscopic or macroscopic scale intended.
