HR: 11:35h
AN: T42C-06    [Abstracts]
TI: Clay Neomineralization and its Role in a Weak San Andreas Fault: new Insights From the SAFOD Drillhole in Parkfield/California
AU: * Schleicher, A M
EM: aschleic@umich.edu
AF: University of Erlangen, Geological Institute, Schlossgarten 5, Erlangen, 91054, Germany
AU: van der Pluijm, B A
EM: vdpluijm@umich.edu
AF: University of Michigan, Department of Geological Sciences, 1100 University Ave, C.C.Little Building, Ann Arbor, 48109, United States
AU: Warr, L N
EM: laurencewarr@gmail.com
AF: University of Strasbourg, Centre de G¨¦ochimie de la Surface (CNRS-ULP), 1 rue Blessig, Strasbourg, 67000, France
AB: Neomineralization in fault zones and its influence on deformation and shear strength has been suggested as a possible explanation for a weak fault behavior. However, studies often lack the localization and characterization of the minerals along the slip surfaces, which is crucial for understanding the role of mineralization in fault zone processes. In this study, a detailed examination of illite minerals has been conducted from three sedimentary core samples of the SAFOD drill hole (3066m, 3436m and 3992m), showing different degrees of shearing and deformation. Based on X-ray diffraction and electron microscopy (SEM, TEM), we measured the crystal structure and fabric, as well as the shape and size of illite, their chemical composition, water content and polytypes. All samples contain detrital quartz, feldspar, chlorite, muscovite and biotite, with authigenic illite, illite-smectite (I-S), chlorite-smectite and smectite. The I-S phase at 3066m contains ca 75% of illite with a long-range ordering (R¡Ý3), the I-S at 3436m consists of ca 70% illite with randomly ordered smectite layers (R0), and illite at 3992m reveal thicker crystals without distinct smectite interlayers. The chemical composition ranges from a muscovitic composition (~1.0pu) at 3992m to depleted interlayer values of 0.5pu at 3436m. However, all illite phases show mixtures of 2M1 and 1M polytypes, with the coarse-grained (~150nm thick) pseudohexagonal 2M1 phase dominating and the very fine grained (10-15nm thick) 1M illite occurring as fibrous or flaky pore-fillings. The fine grained mineral phases were likely formed during the circulation of aqueous fluids along permeable fractures and veins by dissolution-precipitation reactions, partly at the expense of the detrital packets. Based on CALCMIX modeling, the clays contain different amounts of interlayer water with the highest values recorded at 3436m. Also heat flow analyses indicate the highest volatiles content at 3436m (6.5% total volatiles with 2% interlayer water), and the lowest volatiles content (2.7% with 0% interlayer water) at 3992m. The timing of illite growth in the matrix is considered to be coeval or older than the growth reported along open fractures, which are notably smectite rich and are an indication of more extensive dissolution associated with enhanced fluid flow. The localization of clay indicates that these phyllosilicates may dominate the fault behavior at shallow depth, as it is the dominant phase in these fault rocks.
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8010 Fractures and faults
DE: 8030 Microstructures
DE: 8034 Rheology and friction of fault zones (8163)
DE: 8045 Role of fluids
SC: Tectonophysics [T]
MN: 2007 Fall Meeting