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Faulting and Fracturing of Nonwelded Bishop Tuff, Eastern California

Deformation Mechanisms in Very Porous Materials in the Vadose Zone

Department of Geology, Utah State University, Logan, UT, 84322-4505

View larger version (72K): [in a new window]   Fig. 1. Geology of the study area. (A) Location and regional geology of the Bishop study area, Bishop, CA. Location of the Volcanic Tablelands between the Sierra Nevada to the west and the White Mountains to the east. Normal faulting changes polarity from the east-dipping Owens Valley fault to the west-dipping White Mountain fault. Qbt, Quaternary Bishop Tuff; Kg, Cretaceous granitoids of the Sierra Nevada; Pzu, undifferentiated Paleozoic sedimentary rocks of the White Mountains; WMF, White Mountain Fault. The Long Valley caldera is the source of the Bishop Tuff. (B) Traces of the major normal faults in the study area, along with the locations of the study sites, as mapped from air photos and the work of Ferrill et al. (1999).

View larger version (31K): [in a new window]   Fig. 2. Stratigraphic section of the Bishop Tuff in the study area.

View larger version (81K): [in a new window]   Fig. 3. Outcrop photographs of undeformed Bishop Tuff. (A) Entire sequence from near Chalk Cove site. Lower white sequence is the nonwelded airfall deposits, overlain by Units B–D. (B) Close-up photographs of the basal thin-bedded airfall deposits. (C) Massive nonwelded sequence. (D) Upper portion of Ig2Ea and Ig2eb of Wilson and Hildreth (1997). Note variation in fracture density upsection, which corresponds to variations in welding.

View larger version (35K): [in a new window]   Fig. 4. (A) Sketch map of the Borrow Pit site and fracture characterization zones. (B) Fracture pavement map illustrates spacing and trace style of fractures within the poorly to slightly welded Bishop Tuff. Persistent fracture network mapped in pavement area have a horsetail-splay geometry to the south-southwest and begin to die out west of Pit 1.

View larger version (80K): [in a new window]   Fig. 5. Details of the Borrow Pit. (A) Vertical outcrop face of the southern Borrow Pit site and (B) corresponding fracture trace maps underlying test pit sites of dye and infiltration tests at the Borrow Pit site (Fedors et al., 2001). Persistent fracture network lies within the immediate hanging wall of two macroscopic faults. Two major high-angle sets are characterized by thin discrete surfaces filled with calcite or wider distributed zones of hairline fractures stained with iron oxides. Slickenlines were observed on several fracture surfaces. Hydrologic tracer tests (Pits 1 and 2) by Fedors et al. (2001) suggest fractures enhance matrix flow vertically by constraining lateral flow. (C) North-northeast view of the floor of the Borrow Pit (shown in Fig. 4b) and its vertical faces. (D) Vertical fault at the northwest corner of the Borrow Pit site. Smooth mineralized fault surface consists of calcite and has nearly pure dip-slip slickenlines.

View larger version (53K): [in a new window]   Fig. 6. Photomosaic and projected vertical fault map for the Crucifix site. (A) Exposure of west-dipping normal fault that offsets thinly bedded, highly porous and nonwelded, pumice-rich hybrid to streaky fallout tephra deposits (Wilson and Hildreth, 1998) interbedded with minor thin ignimbrite sequences pf the Crucifix site. Conjugate normal faults and numerous centimeter-scale displacement faults occur within the footwall damage zone. Ferrill et al. (2000) suggested these faults formed by sequential rather than simultaneous slip. (B) We estimated a minimum offset of 8 m from measurements along the Crucifix fault. The hanging wall damage zone extends about 5 m to the west, whereas the footwall damage zone extends about 10 m to the east. Fault core is heterogeneous and is filled with calcite and/or coated with clay, or consists of a comminuted gouge material with abundant pumice clasts (Fig. 8). Vertical fractures are closely spaced in the hanging wall and typically get deflected as they intersect less-welded and/or coarser pumice-rich layers. Subsidiary fractures are open or have millimeter to 1-cm-thick calcite and silica coatings. Numbers indicate sample locations for X-ray diffraction and thin-section analyses.

View larger version (89K): [in a new window]   Fig. 7. Photographs of the Crucifix fault zone and sampling localities denoted by numbers. Photos highlight fault zone architecture. (A) Note drag geometry and deformation of fault gouge appears similar to that of a clay smear process (Yielding et al., 1997) parallel to the main slip surfaces. (B) Close-up view of fault core. In hand sample we observed a decrease in grain size in the central fault core gouge and numerous small-scale en echelon slip surfaces. X-ray diffraction of Sample BT-43-01 yields calcite mineralization on the outer slip surface and quartz and feldspars comprise the central gouge zone. (C) Closely spaced fractures between two faults that show the relationship between grain size and degree welding and fracture density. The white ash-rich beds (A) have numerous fractures that do not persist into the nonwelded pumice-rich layers (P).

View larger version (30K): [in a new window]   Fig. 8. X-ray diffraction patterns for undeformed and deformed samples from the Crucifix site. (A, B) samples from the thick pumice-rich unit (see Fig. 7b for location) show marked contrast in the amount of noncrystalline (glass) material in the primary deposit vs. a quartz–calcite rich fault zone. (C, D) Analysis of a thinner ash-rich sequence shows the presence of feldspar phenocrysts and glass, which in the fault zone becomes a more ordered quartz–feldspar cataclasite.

View larger version (73K): [in a new window]   Fig. 9. Chalk Cove study site and fault zone. (A) View of fault showing fault zone architecture of a single fault that flares upward into a wide zone of fractures in the densely welded tuff. (B) Fault core composed of 1-mm to 5-cm-thick clay smear or gouge; heterogeneous mixed zones with varying degrees of deformation. Immediate damage zones on either side of fault have fracture intensity values of 11 fractures per meter. Within first 3 m of hanging wall, fracture intensity values increase to 33 fractures per meter. (C) Slip surface is very narrow in the lower part of the exposure.

View larger version (29K): [in a new window]   Fig. 10. Schematic sketch of the Chalk Cove fault illustrating interpretation of a "funnel-shaped" geometry. Open fractures in welded units narrow to a discrete slip surface. Predominate strike orientations of fault and hanging wall fracture surfaces are north–south to northeast–southwest. Numerous high-angle slip vectors were measured along the trace of the main fault.

View larger version (111K): [in a new window]   Fig. 11. Horton Creek outcrop. (A) The Horton Creek fault and fault traces interpreted on the photo. (B) Exposed fault contact between older lacustrine deposits and younger air-fall sequences of the Bishop Tuff. (C) Nonwelded ignimbrite and air fall deposits about 30 m north of the fault trace. Pumice fragments and glass have an average grain size of 0.9 to 1.9 m or coarse to very coarse sand particle size, show inverse grading, and are overall very poorly sorted, angular grains.

View larger version (78K): [in a new window]   Fig. 12. Mosaic of backscattered scanning electron micrograph of nondeformed nonwelded tuff from the Horton Creek site. Large feldspar phenocrysts (F), vesiculated pumice lapilli (P), and quartz grains (Q) lie in a matrix of irregularly shaped glass shards and ash. Intragranular fractures in the quartz may be the result of rapid cooling from the caldera. Images acquired with 10 KV accelerating voltage.

View larger version (115K): [in a new window]   Fig. 13. Optical photomicrographs of textures of nonwelded tuffs. (A) Plane-polarized light image of undeformed tuff from the Horton Creek site. Pumice (P) at right contains elongate vesicles and numerous smaller pumice lapilli in a matrix of glass shards and ash. (B) Plane-polarized light image of open fractures (F) in a feldspar phenocryst that appear to be truncated at the pumice grain boundary at center of image. (C, D) Fragmentation of tuff samples from the Crucifix site. Fractured quartz grains (Q) lie next to relatively undeformed pumice (P) and a matrix of cataclastically deformed grains (C). (E) Cross-polarized light image of cataclastically deformed sample at the Crucifix site (BT-50). Birefringent material is calcite filling the fault. (F) Calcite-filled cataclasite in a fault zone from the Crucifix site. CC, calcite.

View larger version (83K): [in a new window]   Fig. 14. Single fractures in nonwelded tuff. (A) Mosaic of plane-polarized light photomicrographs of a sample parallel to and 1 cm from the Chalk Cove fault. Fracture (f) that traverses the sample cuts dense ash and glass-shard matrix with quartz (Q) phenocryrsts, lithic fragments (L), and an ash matrix (M). Fabric density is due to compaction and deformation associated with the fault. (B) Mosaic of backscattered scanning electron micrograph of natural fracture from the Crucifix site. Fracture (f) lies along the phenocryst and lapilli boundaries. Irregular fracture boundary is produced by the interaction of the fracture and the primary pores. Note that the matrix pores are in general much smaller than the fracture aperture, and the pore spaces (black regions) are not well connected. Q, quartz; F, feldspar; P, pumice.

View larger version (155K): [in a new window]   Fig. 15. Cataclastic deformation and development of deformation band fault in the nonwelded tuff. (A) Mosaic of backscattered scanning electron image of a fault zone comprised of feldspar and pumice fragments in a calcite-ash-glass shard matrix. Fault zone lies below a large pumice grain (P) at the top of image. Note the lack of internal deformation of the pumice grain, despite the large volume of pore space (black) and the thin walls between the pores. Fault zone has fractures (f) along either side of fault; Q is quartz, (B) Thin zone of cataclasite with calcite (C) along the surface. Matrix consists of nonwelded tuff.

View larger version (115K): [in a new window]   Fig. 16. Evidence for fluid flow associated with faults. (A) Open fracture filled with calcite. Cross-polarized light image with quarter-wave plate in. (B) Calcite-filled vein (C), with iron oxide fill (dark material) subparallel to the fine-grained fault gouge of the Horton Creek fault. P is pumice. Cross-polarized light image. (C) Open fractures (f) cutting pumice (P) and feldspar (F). Red iron-oxide staining lies in a fault zone. Cross-polarized light image with reflected light. (D) Fine-grained fault gouge that cuts calcite (C) at the Crucifix site. Cross-polarized light image.