HR: 0800h
AN: T21A-0456    [Abstracts]
TI: Phyllosilicate mineral assemblages, elemental compositions and microstructures from the SAFOD Main Hole
AU: * van der Pluijm, B A
EM: vdpluijm@umich.edu
AF: Department of Geological Sciences, University of Michigan, 1100 N University, Ann Arbor, MI 48109 United States
AU: Schleicher, A M
EM: Anja.Schleicher@urz.uni-heidelberg.de
AF: Department of Geological Sciences, University of Michigan, 1100 N University, Ann Arbor, MI 48109 United States
AU: Solum, J G
EM: jsolum@usgs.gov
AF: Earthquake Hazards Team, U.S. Geological Survey, 345 Middlefield Road MS977, Menlo Park, CA 94025 United States
AU: Tourscher, S N
EM: stours@umich.edu
AF: Department of Geological Sciences, University of Michigan, 1100 N University, Ann Arbor, MI 48109 United States
AU: Warr, L N
EM: warr@illite.u-strasbg.fr
AF: Centre de Geochimie de la Surface (CGS/EOST), 1 rue Blessig, Strasbourg, 67084 France
AB: Mineral transformations and fabrics, particularly those involving phyllosilicates, can critically affect the mechanical behavior of shallow faults and may be more important than generally assumed. Thus, detailed phyllosilicate mineralogy, mineral transformation history and fabric development are central to the study of fault rocks and our understanding of fault properties. The SAFOD hole provides the opportunity to study the in situ change from undeformed protolith to fault rock, thereby establishing a reference for the characterization of the type and magnitude of changes in active fault zones. The SAFOD hole also allows for direct comparison with exhumed SAFOD analogs, which have been used as representatives for fault zones at depth. In exhumed faults several populations of discrete and mixed-layer phyllosilicates were observed, including a protolithic population (chlorite and mica), a syn-faulting population (chlorite-rich chlorite-smectite and illite-rich illite-smectite), and a post-faulting population (smectite-rich chlorite-smectite). In the SAFOD Pilot Hole multiple populations of phyllosilicates were also observed, including mixed-layer clays in the shallow sedimentary rocks and chlorite in deeper granitic rocks. These findings indicate that there are variations in phyllosilicate mineralization with (1) depth (2) strain energy/deformation mechanism/position in fault zone and (3) time. There are several preliminary zones of phyllosilicates in the SAFOD Main Hole, including: (1) a zone at 7800-8100 ft MD, marked by an increase in the amount of illite, and the appearance of a mixed-layer illite-smectite (I-S) phase; (2) a zone at 8400-8800 ft MD that includes at least one large shear zone marked large a increase in the amount of I-S and illite relative to surrounding protolith; (3) a zone at 11050-12400 ft MD marked by a large increase in the amount of chlorite that is fairly constant to the bottom of the hole, and the appearance of a mixed-layer clay. Three significant problems with the use of cuttings are (1) mixing of cuttings as they travel from the drill face to the surface, (2) "dilution" of phyllosilicate signatures from shear/alteration zones that are thinner than the 10 ft sampling interval and (3) possible contamination from montmorillonite in the bentonite drill mud. These difficulties can be overcome in part through the analysis of plucked grains of fault rock and protolith, guided by analyses of bulk cuttings. Our current research on SAFOD main hole samples consists of a suite of techniques than can be applied to the relatively small cuttings (less than a few mm). We will report on preliminary observations on cuttings, including mineralogical and elemental data (XRD and ICP), and electron beam (SEM and TEM) characterization of mineralized surface polishes in fault horizons, collectively offering a beginning framework for (1) interpreting fault zone mineral transformations and precipitation in seismically active settings, (2) testing the relevance of processes inferred from the study of nearby exhumed fault rocks, and, ultimately, (3) providing constraints on the thermo-mechanical behavior of faults.
DE: 8000 STRUCTURAL GEOLOGY
DE: 8010 Fractures and faults
DE: 8100 TECTONOPHYSICS
SC: Tectonophysics [T]
MN: Fall Meeting 2005