Project Title: Extracting Information on Fault Properties from the Mineral Assemblages of SAFOD
Mendenhall Fellow: John G. Solum, (650) 329-4857, firstname.lastname@example.org
Duty Station: Menlo Park, CA
Start Date: January 10, 2005
Education: Ph.D., 2005, University of Michigan, Geology
Research Advisors: David Lockner, (650) 329-4826, email@example.com; Steve Hickman, (650) 329-4807, firstname.lastname@example.org; Diane Moore, (650) 329-4825, email@example.com
Project Description: The San Andreas Fault Observatory at Depth (SAFOD) is one of three components of the National Science Foundation's EarthScope initiative (http://www.earthscope.org/), conducted in partnership with the USGS and NASA. Near the town of Parkfield, California (http://quake.wr.usgs.gov/research/parkfield/) a 3.2 km-deep deviated drill hole will cross the San Andreas Fault, targeting an area of the fault where repeating M2 earthquakes nucleate. Phase I drilling of the hole was conducted in the summer of 2004, with the hole reaching a vertical depth of 2.5 km, driving the hole within 0.7 km of the main San Andreas Fault. Phase II drilling, in which the hole will penetrate the fault zone, will begin in the summer of 2005. Cuttings and spot cores collected during drilling of SAFOD provide the unparalleled means to gain insight into the behavior of large faults like the San Andreas.
Study of clay minerals in fault zones is vital as these phases influence both the mechanical (strength and frictional behavior) and hydrologic (permeability structure) of fault zones, and therefore provide a means to shed light on explaining the apparent weakness of major faults. Clay mineral assemblages vary systematically in many exhumed (San Gabriel, Punchbowl) and active fault zones (Nojima, Chelungpu), as well as in the SAFOD Pilot Hole, drilled during the summer of 2002. In addition, studies of fault-related minerals provide a means to constrain fault properties (fluid sources and abundances, timing, etc.) if the signature of faulting contained within fault zone mineral assemblages can be distinguished from non-fault-related sources. We plan to study bulk cuttings and clay separates principally using X-ray diffraction (XRD), and to collaborate with other researchers using other techniques to characterize the mineralogy and deformation microstructures of San Andreas Fault rocks.
The bulk mineralogy of washed cuttings from SAFOD Phase 1 rotary drilling is being determined using powder XRD at 100 ft (30.5 m) intervals from 110 to 10025 ft (33.5 to 3055.6 m) measured depth (MD) (Figure 1), and at 10 ft (3 m) intervals across alteration and shear zones. Grain mounts (thin sections) were made from the same washed cuttings used for XRD. Point counts conducted on these thin sections using an optical microscope will allow detection of phases that are present in concentrations too low to be detected using XRD, which is approximately a few weight percent.
Clay separates will be extracted from samples of shear zones, which will be used to quantify clay mineral assemblages using XRD. Separates will be produced using an environmental chamber, which uses repeated freeze/thaw cycles to disaggregate a rock. This approach minimizes grain size reduction of non-clay phases, which results in cleaner clay separates than are provided by other crushing methods. This aids measurement of geochemical properties of clays, such as stable isotopic composition (e.g., δ18O) and 40Ar/39Ar dating.
To ascertain the degree to which formation clays were removed during the cuttings washing process (along with the drilling mud) unwashed cuttings were collected every 10 ft during drilling of SAFOD. Unwashed cuttings from selected intervals of the SAFOD borehole, particularly sedimentary/metasedimentary sections and alteration/shear zones, will be analyzed using XRD from the point at which the hole was deviated toward the fault zone (1450 m) to the bottom of the hole. XRD studies of washed cuttings from the SAFOD Pilot Hole have shown that the clays in the drilling mud (dominantly montmorillonite) can be distinguished from formation clays on samples solvated with ethylene glycol, and this technique will also be used in analyzing the SAFOD Main Hole samples.
Cuttings collected during coring are also being analyzed. Analyses of thin sections from the core indicate that the shallower interval (1462.4-1470.4 m MD) is composed of hornblende-biotite granodiorite, while the deeper interval (3055.6-3067.2 m) is composed of a combination of granitic rocks and meta-arkose. Our XRD and optical observations on cuttings from these cored intervals will help us to "calibrate" our interpretation of cuttings from the rotary-drilled intervals of the hole and will allow the few spot cores that were obtained in Phase 1 to be placed into a broader lithological context.
These analyses will be continued as we drill (and conduct spot coring) across the San Andreas Fault Zone in the summer of 2005, providing a critical tool for recognizing fault- and non-fault-related phyllosilicates and determining how clays and other alteration products might influence the mechanical behavior of faults like the San Andreas.
Figure 1. Qualitative clay mineral assemblages based on relative areas on XRD patterns from SAFOD Phase 1 drilling from 1829 m measured depth (MD) to the bottom of the hole at 3048 m MD (2.5 km vertical depth) based on washed cuttings at 100' intervals. Clay-rich alteration/shear zones, such as those in the gray box, will be investigated using cuttings at 10' intervals. Quantitative clay mineralogy (weights percent, separation of drilling mud from 10 Å formation clays, identification of 10 Å clays, identification of formation mixed-layer clays like illite-smectite or chlorite-smectite) is in progress. Geologic features noted on the right side of the figure are based on analyses of geophysical logs by Naomi Boness and Mark Zoback at Stanford University.
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