HR: 16:00h
AN: T24B-01    [Abstracts]
TI: Mineralogy of the SAFOD Main Hole: Detailed characterization of fault and country rocks
AU: * Solum, J G
EM: jsolum@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, CA 94025 United States
AU: Hickman, S H
EM: hickman@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, CA 94025 United States
AU: Lockner, D A
EM: dlockner@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, CA 94025 United States
AU: Moore, D E
EM: dmoore@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, CA 94025 United States
AB: Cuttings and core collected from across the San Andreas Fault Zone in the SAFOD Main Hole provide a unique opportunity to characterize the composition and structure of a major plate-boundary fault system. X-ray diffraction techniques provide a means to determine fault rock mineral assemblages, as well as help to identify the sedimentary packages that were encountered during drilling. These analyses indicate that there are multiple types of mineral assemblages in the fault rocks encountered by the SAFOD hole. Two zones of laumontite (zeolite) mineralization are present, one in the granodiorite encountered from ~244-1923 m (800 to 6310 ft) measured depth (MD), and one in a deeper arkose at ~2682 to 3158 m (~8800 to 10,360 ft) MD. Trace amounts of laumontite are associated with a possible shear zone at ~3338 m (10,950 ft) MD, but no clear relation between zeolite mineralization and other shear zones penetrated by SAFOD has been observed. The main hole entered a sequence of shale/siltstone/fine sandstones at ~3158 m (10360 ft) MD, indicating the presence of a significant fault. Below this depth the chlorite (001) XRD peak widths exhibit little variability (~0.35-0.4 compared to a broad range of ~0.15-0.6 above that depth), indicating a major change in lithology. There is also a very pronounced change in the clay mineral assemblages at ~3353 m (11000 ft) MD, below which clays (chlorite, illite, and a possible mixed-layer phase) exhibit little variation both in abundance and crystallinity, indicating the presence of a significant fault at approximately that depth. This is broadly consistent with the first appearance of serpentinite minerals at ~3322 m (10900 ft) MD, and a sudden increase in the concentration of methane dissolved in the drilling mud at 3338-3344 m (10950-10970 ft) MD. A mixed-layer illite-smectite phase is present in a major fault zone at ~2554 m (8380 ft) MD. This phase is present both in bulk cuttings as well as in plucked grains of fault rocks, indicating that it is not a contaminant from the drilling fluid. A minor illite-rich mixed-layer illite-smectite phase is also present in a clay-rich shear zone at ~3067 m (10,062 ft) MD, sampled during a coring run in 2004. The restricted occurrence of these phases in and adjacent to shear zones suggests that they are syn-deformational, fault-related phases. The generally low coefficient of friction of smectites suggests that the formation of these phases may weaken the fault zone. Two shear zones are enriched in iron oxides, one at 1923 m (6310 ft) MD, where the hole leaves granodiorite and enters sedimentary rocks, and one at 3158 m (10360 ft) MD, where the hole leaves a series of arkoses and enters a sequence of siltier rocks. The absence of iron oxides in other fault zones sampled by SAFOD indicates that the fluid-rock interactions in these two fault zones were different from the rest of the San Andreas fault system at this location. These results provide preliminary characterizations of several of the shear zones encountered during drilling of the SAFOD Main Hole. The relation of these shear zones to the active trace(s) of the San Andreas System will be determined as casing deformation is monitored and locations of microseismicity are refined in preparation for the continuous coring phase of SAFOD in the summer of 2007.
DE: 3699 General or miscellaneous
DE: 8010 Fractures and faults
DE: 8034 Rheology and friction of fault zones (8163)
DE: 8106 Continental margins: transform
DE: 8111 Continental tectonics: strike-slip and transform
SC: Tectonophysics [T]
MN: Fall Meeting 2005