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_Journal of Structural Geology 32 (2010) 1899-1911_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Fault-related clay authigenesis along the Moab Fault: Implications for calculations of fault rock composition and mechanical and hydrologic fault zone properties John G. Solum a,*, Nicholas C. Davatzes b, David A. Lockner c a Shell International Exploration and Production, Inc., Bellaire Technology Center, 3737 Bellaire Blvd., Houston, TX 77025, USA b Department of Earth and Environmental Science, Temple University, 326 Beury Hall, 1901 N. 13th Street, Philadelphia, PA 19122, USA c U.S. Geological Survey, Earthquake Hazards Team, 345 Middlefield Rd MS 977, Menlo Park, CA 94025, USA Article history: Received 10 August 2009; Received in revised form 13 July 2010; Accepted 28 July 2010; Available online 26 October 2010 Abstract The presence of clays in fault rocks influences both the mechanical and hydrologic properties of clay-bearing faults, and therefore it is critical to understand the origin of clays in fault rocks and their distributions. Field mapping shows that layers of clay gouge and shale smear are common along the Moab Fault, from exposures with throws ranging from 10 to >1000 m. Elemental analyses of four locations along the Moab Fault show that fault rocks are enriched in clays at R191 and Bartlett Wash, but this clay enrichment occurred at different times and was associated with different fluids. Fault rocks at Corral and Courthouse Canyons show little difference in elemental composition from adjacent protoliths, suggesting that formation of fault rocks at those locations is governed by mechanical processes. Friction tests show that these authigenic clays result in fault zone weakening, and potentially influence the style of failure along the fault (seismogenic vs. aseismic) and potentially influence the amount of fluid loss associated with coseismic dilation. Scanning electron microscopy shows that authigenesis promotes continuity of slip surfaces, thereby enhancing seal capacity. The occurrence of the authigenesis, and its influence on the sealing properties of faults, highlights the importance of determining the processes that control this phenomenon. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Clays are common constituents of fault rocks, and affect both the mechanical and hydrologic properties of the faults that contain them. Most clay minerals are mechanically weak, and so the type and amount of clay minerals in fault rocks can influence the frictional fault strength (Morrow et al., 2007; Tembe et al., 2006). The frictional strength of rock can be reduced from nominal values of 0.6-1 in quartzo-feldspathic rocks to <0.3 with as little as 18% clay (Tembe et al., 2006). In addition, clay minerals have a range of rate dependences of the coefficient of friction, ranging from positive values that favor creep to negative values that favor seismic slip (Saffer and Marone, 2003; Lockner et al., 2006). Similarly, permeability can be reduced as much as seven orders of magnitude due to the incorporation of clay in fault rocks (Crawford et al., 2002; Davatzes and Hickman, 2005). Thus the occurrence of clay-rich fault rocks tends to dominate fault zone hydrologic properties such as permeability and transmissibility and their anisotropy. Given the role clays play in influencing fault behavior, their concentration in fault zones and the governing mechanisms controlling that concentration must be quantified. Two commonly used empirical methods of estimating the clay concentration in fault rocks are the shale gouge ratio (SGR) (Yielding et al., 1997) and the clay smear potential (CSP) (Fulljames et al., 1997). The SGR approach calculates clay concentrations based on the assumption that the gouge composition at a particular point along a fault is equivalent to the geometrically averaged composition of all of the host rock that has moved past that point, implying mixing of the various rock bodies offset by the fault. The CSP approach is based on the assumption that clay-rich horizons will be smeared into a fault, with the offset over which the smear remains continuous and its thickness are taken as a function of the thickness of the clay beds and the offset of the fault. J.G. Solum et al. Journal of Structural Geology 32 (2010) 1899-1911 Studies over the past decade have shown that fault-related clay authigenesis is a common process in fault zones in a variety of geologic settings: the San Andreas Fault (Solum et al., 2006; Schleicher et al., 2006; Tembe et al., 2006; Morrow et al., 2007), thrust faults in the North American Cordillera (Vrolijk and van der Pluijm, 1999; Yan et al., 2001; van der Pluijm et al., 2001, 2006; Solum and van der Pluijm, 2007), a small normal fault in northern California (Eichhubl et al., 2006), and along the Moab Fault in Utah (Solum et al., 2005; Pevear et al., 1997; Anyamele et al., 2009). In addition to the generation of fault rock, fault zone hydrology is greatly impacted by small-scale structures within the damage zone (Caine et al., 1996), which are distributed adjacent to the fault rock and vary by position (Davatzes et al., 2005a,b) and lithology. The damage zone consists of a region of enhanced deformation adjacent to the core of fault rock. Its impact on fault zone hydrology depends on the type and distribution of structures within the zone (including joints, fractures, deformation bands, and folds), their geometric and age relationships, and the spatial variation in the width of the damage zone along the fault. It is not possible to adequately image either the attributes of the damage zone and the fault rock through seismic methods or even with wells due to limited sampling. Therefore, their nature must be predicted, but predictive relationships are generally lacking. In addition, both the damage zone and the fault rock attributes vary in width along faults related to a number of parameters including stratigraphic heterogeneity, early fault geometry (Childs et al., 2009), and the local state of stress, which are difficult to predict from the fully developed fault geometry in the subsurface visible in reflection seismic data. However, the physical mechanisms that account for the formation of clay-rich fault rocks, and the conditions under which such mechanisms operate, can be effectively investigated from outcrop analogues, providing a sound basis for predicting fault rock attributes. The Moab Fault is one such analogue. The purpose of this study is to 1) describe the occurrence of clay-rich fault rocks along four exposures of the Moab Fault; 2) determine as best as possible the degree of authigenesis at each site; 3) speculate on more generally applicable ways in which this authigenesis may affect hydrologic and mechanical fault properties. 2. Geologic setting The Moab Fault, located in eastern central Utah (Fig. 1), is a large normal fault that cuts a heterogeneous series of dominantly clastic sedimentary rocks (Fig. 2). The fault is composed of three main components: a poorly exposed southern section, a central section where the greatest throws (up to >1 km) are found, and a complex branching northern section that tips out to the northwest. The fault was initially activated in the Early Mesozoic in response to motion of thick accumulations of salt deposited in the Pennsylvanian (Foxford et al., 1996, 1998; Doelling, 2001). The fault was reactivated at ~60 Ma (Solum et al., 2005), likely due to reinitiated salt movement during the Laramide Orogeny. The architecture of this fault zone is variable and has been studied by several authors. The first systematic study was conducted by Foxford et al. (1996, 1998), which classified architectural elements into slip band zones, shaley gouge zones, and sandstone cataclasites and breccias. While that study described many locations along the fault, it did not describe the geometry of the architectural. Fig. 1. Simplified geologic map of the Moab Fault system (Davatzes and Aydin, 2005) with sample locations noted. Locations are R191 (throw of ~960 m), Corral Canyon (throw of ~500 m), Courthouse Canyon (throw of ~10 m), and Bartlett Wash (throw of ~250 m). 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