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_Journal of Structural Geology 32 (2010) 1792–1805_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Relationship between fault growth mechanism and permeability variations with depth of siliceous mudstones in northern Hokkaido, Japan Eiichi Ishii a,*, Hironori Funaki a, Tetsuya Tokiwa a, Kunio Otaba Horonobe Underground Research Unit, Japan Atomic Energy Agency, Hokushin 432-2, Horonobe-cho, Hokkaido 098-3224, Japan b Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency, Muramatsu 4-33, Tokai-mura, Ibaraki 319-1144, Japan Article info Article history: Received 26 January 2009; Received in revised form 6 October 2009; Accepted 30 October 2009; Available online 11 November 2009 Keywords: Fault Shear Tensile Stress Permeability Siliceous mudstone Abstract In order to assess the influence of remote mean stress correlated with depth of burial on the principal mode of failure at fault tips during fault slip in a lithologically homogeneous, fractured rock mass, the growth mechanisms of strike-slip faults have been studied at outcrop-scale in the siliceous mudstones of northern Hokkaido, Japan. We take a multifaceted approach combining i) geological characterization of fractures by fracture mapping in outcrop and fracture logging of boreholes (drilling depth: 1020 m), ii) rock mechanical characterization by laboratory tests on core samples, and iii) theoretical analyses using the Griffith–Coulomb criterion. These suggested that the principal mode of failure in the mudstones is dependent not only on rock strength but also on remote mean stresses. During and/or after uplift and erosion, the faults grew mainly by linking with adjacent faults via numerous splay cracks, formed by tensile failure above roughly 400 m depth. In contrast, below this depth, the faults grew predominantly by shear failure. Such growth mechanisms are consistent with the fact that hydraulic tests performed in boreholes show that highly permeable sections (hydraulic transmissivity: >10−5 m2 s) are restricted to depths of less than 400 m. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Decreasing permeability with depth in fractured rock masses (e.g., at depths of less than 1 km), which is consistent with closure of pore apertures with increasing pressure (e.g., Wei et al., 1995; Ohman et al., 2005), has been widely recognized in deep drilling programs worldwide (e.g., Rhein et al., 2006; Andersson et al., 2007; Nordqvist et al., 2008). However, clear exceptions to this trend exist and numerous authors have suggested that permeability distributions in fractured rock masses are fundamentally dependent on mechanisms of their brittle deformation (e.g., Caine et al., 1996; Mazurek et al., 1996, 1998, 2003; Evans et al., 1997; Gutmanis et al., 1998; Mazurek, 1998, 2000; Bossart et al., 2001). Sheared-joint-based faulting associated with the numerous tensile splay cracks that propagate from the tip of faults when faults slip is a well-known mechanism of brittle deformation (e.g., Segall and Pollard, 1983; Martel and Pollard, 1989; Flodin and Aydin, 2004; Myers and Aydin, 2004). This style of faulting is known to significantly increase the permeability of a fractured rock mass (e.g., Mazurek et al., 1998, 2003; Dholakia et al., 1998; d’Alessio and Martel, 2004; Lunn et al., 2008; Eichhubl et al., 2009). Mazurek et al. (1998) showed that the transmissivity of groundwater inflow points in boreholes penetrating the highly consolidated and fractured marls in the central Swiss Alps decreases with depth, and indicated that the tensile splay cracks forming the fault linkages may be the actual inflow points. This is an example of an earlier study that implied a relationship between splays and permeability variations with depth. However, the relationship has not been examined in detail. In order for tensile splay cracks to form, tensile failure is required near fault tips when fault slip occurs. However, such tensile failure may not necessarily occur under all stress states; it is possible that shear failure could occur. The principal mode of failure is also known to depend on lithology, i.e., tensile failure tends to occur in stronger rocks while shear failure tends to occur in weaker rocks (e.g., Gross, 1995; Eichhubl and Boles, 2000; Welch et al., 2009). Also concerning the principal mode of failure near fault tips during sheared-joint-based faulting, the control of failure mode by lithology is shown by field observations on the Monterey Formation of California (Dholakia et al., 1998). However, it is indicated by numerical simulations using the Griffith–Coulomb criterion that the principal mode of failure near fault tips due to fault slip depends not only on lithology but also on remote mean stress at the time of fault slip (Bourne and Willemse, 2001), E. Ishii et al. Journal of Structural Geology 32 (2010) 1792–1805 1793 which suggests that tensile failure tends to occur under lower remote mean stress and shear failure tends to occur under higher remote mean stress. Therefore, remote mean stress is also an important factor in failure mode, and considering that remote mean stress is generally correlated with burial depth (e.g., Twiss and Moores, 2007), the difference in the principal failure mode with increasing burial depth may also have a direct relationship to variations in permeability with depth, as observed in fractured rock masses. In the Horonobe area, northern Hokkaido, Japan, folded Neogene siliceous mudstone, the Wakkanai Formation (Figs. 1 and 2), is massive and lithologically homogeneous (Iijima and Tada, 1981) with very weakly developed bedding planes (Ishii et al., 2006). Faults crosscutting bedding planes at a high angle (FCBs) and bedding faults parallel to bedding are observed at outcrop-scale (Ishii and Fukushima, 2006). The FCBs are mainly strike-slip faults oblique to fold axes. Geological and hydrogeological data indicate that some of the FCBs are the main flow paths (Ishii and Fukushima, 2006; Kurikami et al., 2008; Funaki et al., 2009). Furthermore, hydraulic packer tests performed in twelve deep, vertical boreholes (drilling depth: 1020 m) show that the sections with highest permeability (hydraulic transmissivity: >10−5 m2 s) are restricted to depths of less than about 400 m in the formation (Fig. 3). In this study, a case study addressing the influence of remote mean stress (i.e., burial depth) on the principal mode of failure near fault tips during fault slip in a lithologically homogeneous, fractured rock mass is discussed. Taking it one step further, the relationship between failure mode and permeability, that is, the relationship between the growth mechanisms of the FCBs and the permeability variations with depth of the Wakkanai Formation is discussed. For this study, related to the parameter burial depth, vertical borehole data are important. However, information on the faults reliant solely on restricted information from borehole core data is not enough. Hence, a multifaceted approach combining outcrop surveying, borehole investigations, and theoretical analyses is applied for the Wakkanai Formation. The study combines i) geological characterization of fractures by fracture mapping in outcrop and fracture logging of boreholes, ii) rock mechanical characterization by laboratory tests on core samples for tensile strength and angle of internal friction, and iii) theoretical analyses using the Griffith–Coulomb criterion. The Horonobe area is located on the eastern margin of a Neogene to Quaternary sedimentary basin located on the western side of northern Hokkaido, in a Quaternary, active foreland fold-and-thrust belt near the boundary between the Okhotsk and Amurian plates (e.g., Yamamoto, 1979; Wei and Seno, 1998; Ikeda, 2002; Fig. 1). The basin grades upward stratigraphically from the Wakkanai Formation (massive siliceous mudstones including opal-CT), to the Koetoi Formation (massive diatomaceous mudstones including opal-A, but not opal-CT), to the Yuchi Formation (fine to medium grained sandstones), and lastly to the Sarabetsu Formation (alternating beds of conglomerate, sandstone and mudstone, intercalated with coal seams), overlain by late Pleistocene to Holocene deposits (Figs. 1 and 2). The Wakkanai and Koetoi Formations have the following characteristics: i) they have fairly uniform muddy facies with rare intercalation of pyroclastic sediments (Mitsui and Taguchi, 1977; Tada and Iijima, 1982; Ishii et al., 2008), ii) bedding planes are difficult to recognize due to their homogeneity, though planes can be recognized, albeit weakly by electrical micro-imaging (Ishii et al., 2006). 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