Description:

_Journal of Structural Geology 32 (2010) 1557-1575_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Review Article A review of recent developments concerning the structure, mechanics and fluid flow properties of fault zones D.R. Faulkner a,*, C.A.L. Jackson b, R.J. Lunn c, R.W. Schlische d, Z.K. Shipton e, C.A.J. Wibberley f, M.O. Withjackd a Department of Earth and Ocean Sciences, University of Liverpool, Liverpool, UK b Department of Earth Science and Engineering, Imperial College London, UK c Department of Civil Engineering, University of Strathclyde, Glasgow, UK d Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey, USA e Department of Geographical and Earth Sciences, University of Glasgow, Glasgow, UK f Total France EP, CSTJF, Av. Larribau, Pau, France Article info Article history: Received 11 December 2009 Received in revised form 1 June 2010 Accepted 16 June 2010 Available online 11 August 2010 Keywords: Faults Structure Fluid flow Mechanics Earthquakes Abstract Fault zones and fault systems play a key role in the development of the Earth’s crust. They control the mechanics and fluid flow properties of the crust, and the architecture of sedimentary deposits in basins. We review key advances in the study of the structure, mechanics and fluid flow properties of fault zones and fault systems. We emphasize that these three aspects of faults are intimately related and cannot be considered in isolation. For brevity, the review focuses on advances made primarily in the past 10 years, and also to fault zones in the brittle continental crust. Finally, the paper outlines some key areas for future research in this field. © 2010 Published by Elsevier Ltd. 1. Introduction Fault zones control a wide range of crustal processes. Although fault zones occupy only a small volume of the crust, they have a controlling influence on the crust’s mechanical and fluid flow properties. Much recent work has concentrated on describing and understanding the importance of the structure, mechanics and fluid flow properties of fault zones. This has involved field observations, laboratory experiments, seismology, hydrogeology, and analytical and numerical modeling. Brittle fault zones are lithologically heterogeneous, anisotropic and discontinuous. Faults are complex zones composed of linked fault segments, one or more high strain slip surfaces nested within regions of high and low strain (often called fault core and damage zone), Riedel shears, splay faults, dilational and contractional jogs, and relay ramps (Rawling et al., 2001; Shipton and Cowie, 2001; Faulkner et al., 2003; Childs et al., 2009). Individual fault zones * Corresponding author. E-mail address: faulkner@liv.ac.uk (D.R. Faulkner). 0191-8141 $ e see front matter © 2010 Published by Elsevier Ltd. doi:10.1016/j.jsg.2010.06.009 commonly show significant variation in complexity along strike or down dip, even over relatively short distances (Schulz and Evans, 2000; Shipton and Cowie, 2001; Kirkpatrick et al., 2008; Lunn et al., 2008). Fault zone structure, mechanics and permeability can vary strongly both over geological time (e.g. Eichhubl et al., 2009) and at timescales relevant to a variety of industrial applications. The strength of the lithosphere varies with depth, temperature and mineralogy (Kohlstedt et al., 1995), but a major load-bearing region is likely present at the base of the seismogenic zone at ~15 to 20 km depth for normal continental crust. The mechanical properties of faults at this depth are thus inferred to control to some extent the strength of the entire crust. This inference is supported by observations of crustal stress magnitudes at shallower crustal levels that appear to be limited by the typical frictional strength of faults (Townend and Zoback, 2000). A related goal to characterizing the mechanical properties of faults at depth is to understand the earthquake process from nucleation and propagation to arrest. Faults play an important role in controlling the migration of crustal fluids. One example of this is hydrocarbon migration, accumulation and leakage in sedimentary basins. At the basin scale, 1558 D.R. Faulkner et al. Journal of Structural Geology 32 (2010) 1557-1575 faults and fault-related folds control subsidence patterns and hence the distribution of thermally mature zones (e.g. Brister et al., 2002). Faults are also a key component of many hydrocarbon plays; they may also control discrete subsurface pressure cells (e.g. Grauls et al., 2002). At a smaller scale, a better understanding of the role of faults in compartmentalizing fields will yield better estimates of hydrocarbon production (e.g. Manzocchi et al., 1999). Increasingly, the recognition of high transient permeability along faults induced by hydrocarbon production (Losh and Haney, 2006) has focused interest on the temporal variation of fault-zone permeability. Ore deposits are also commonly related to fault zones due to episodic, localized hydrothermal flows that occur during and immediately after periods of fault movement (e.g. Cox et al., 2001; Sibson, 2001). Characterizing the fluid flow properties of the crust is necessary to facilitate the development of deep-waste storage repositories (e.g. Ferrill et al., 1999; Douglas et al., 2000), to allow sequestration of industrially-produced greenhouse gases (Streit and Hillis, 2004; Dockrill and Shipton, 2010) and to realize the potential of geothermal energy in appropriate locations (Rowland and Sibson, 2004; Fairley, 2009). The physical characteristics and properties of faults will play an important role in regional crustal fluid flow that might affect such applications. This paper concentrates on three primary aspects of fault zones and fault systems: their structure, mechanics and fluid flow properties. We emphasize that these three aspects are inextricably coupled and this is highlighted in Fig. 1. Much recent research reflects efforts to understand the nature and processes behind this coupling. For instance, fault rocks are commonly altered (Evans and Chester, 1995) and are not simply a granulated product of their protolith. In fact, in the upper crust, many fault rocks may be viewed as low-to-medium-grade metamorphic rocks, with authigenic growth of clays and other minerals. A close-knit coupling exists between deformation, mechanics and fluid flow in fault zones by deformation-and reaction-driven changes in porosity and permeability, and fluids causing changes in deformation mechanisms through fluid-rock interactions in fault zones (e.g. Rutter and Brodie, 1995; Wibberley and McCaig, 2000; Eichhubl et al., 2005; Holyoke and Tullis, 2006; O’Hara, 2007). The impact on fault rheology, such as by fluid-enhanced reaction softening, may be considerable (Imber et al., 2001; Gueydan et al., 2003; Jefferies et al., 2006a). Given the importance of fault zones and fault systems, a vast body of work covers their development and properties. In this paper we review important classical concepts but concentrate on Structure Permeability Reaction Alteration Fluid Flow Mode of Failure Segmentation Roughness Fault Zone Composition Mode of Failure Dilatancy Compaction Fig. 1. Flow diagram showing inter-relationships among the three main topics of structure, mechanics and fluid flow. Mode of failure refers to whether or not seismic slip occurs. key advances in our understanding of fault zones over the past ~10 years. Further, we limit our discussion to upper crustal faults developed in the continental crust (i.e., those in the top 20 km of the crust). The review is perhaps biased towards the interests of the authors and concentrates on what we see as important issues reading fault zones. A number of excellent review articles have been published that refer to earlier work and the reader will be directed to these for not only a more historical view on the development of studies in this field, but more detail regarding the different disciplines involved. This review concentrates first on the structure of fault zones and fault systems, including scaling relationships that can help in deciphering how fault zones develop and grow. Second, we discuss the mechanics of fault zones; this involves topics ranging from laboratory experimentation to modeling of earthquake rupture. Last, we address the topic of fluid-flow properties of faults, involving studies from in situ observations of fluid flow, laboratory experiments and modeling. We conclude by highlighting some key outstanding questions. 2. Structure and development of fault zones and fault systems 2.1. Typical fault zone structure A simple conceptual model for fault zone structure, developed over the past 20 years, involves strain that is localized in a fault core surrounded by a distributed zone of fractures and faulting in the damage zone (Fig. 2; see references in Wibberley et al., 2008). The fault core generally consists of gouge, cataclasite or ultracataclasite (or a combination of these), and the damage zone generally consists of fractures over a wide range of length scales and subsidiary faults. Strain may be homogeneously distributed across the fault core (Rutter et al., 1986; Faulkner et al., 2003) or may be highly localized onto discrete slip surfaces (Chester and Logan, 1986; De Paola et al., 2008). Brittle fault rocks have previously been considered chaotic, but detailed observations show that they are highly structured, commonly containing P foliations, Riedel shears and Y shears (Logan et al., 1979; Chester et al., 1985; Rutter et al., 1986; Jefferies et al., 2006b). Additionally, fault breccias, such as those described by Caine et al. (2010), can occur as part of the fault core in areas where movement involving fault jogs results in dilatation. Ключевые слова: laboratory, seismological society, field observation, frictional property, scholz, compaction band, velocity, tectonophysics, experimental, hickman, physical characteristic, dynamic, large fault, solid earth, shear, data, faulkner journal, crust, seismogenic zone, carboneras fault, fault zone, porosity, sufcient quantity, key area, rate, strain, science, seismological data, mechanic, behaviour, measurement, environmental condition, geological, surface, strike-slip fault, evolution, general applicability, slip, rice, quartz, experiment, uid, median tectonic, moore, damage zone, tectonics, property, earthquake, toro, lockner, fracture, rock collected, integrated approach, geophysical, hydrocarbon, direct observation, study, aapg, solid, width, cowie, punchbowl fault, frictional process, damage, host rock, normal fault, letters, structure, rock, frictional melting, frictional strength, geophysics, earth, tullis, mesoscopic structure, seismic interpretation, experimental deformation, schlische, structural, rutter, slip-pulse model, fault gouge, zuccale fault, temporal variation, porosity rock, holdsworth, geological society, surface trace, structural control, geophysical letters, physical property, surrounding, reviews, geology, normal, common factor, safod borehole, microfracture density, journal geophysical, process, fault creep, mechanism, review, clay, hydrocarbon migration, range, compaction characteristic, fault-related fold, mechanical, dynamic rupture, structural geology, experimental study, basin, deformation, pore space, geophysical solid earth, stress, byerlees law, displacement, uid-ow property, press, underlying physic, wiltschko vermilye, fault core, marone, property fault, background level, depth, development, permeability reduction, transport properties, core zone, frictional, normal stress, friction coefcients, weakening mechanism, logan, permeability, mechanical property, faulkner, geophysical solid, seismic slip, ha, cocco, journal geophysical solid, weak, shear strain, fault journal, bulletin, logelog plot, ?uid ?ow, clay takahashi, scale-invariant behaviour, pressure, relay zone, gouge, zone, sibson, evans, scale, mitchell, ?uid, high, shipton, zoback, off-fault damage, journal structural, wide scatter, overpressure pulse, material, growth, upper bound, nature, weaken fault, stress eld, situ measurement, friction, along-fault, deformation band, signicant proportion, collettini, granular friction, triaxial compression, american association, grain contact, analysis, surface energy, weakening, chester, fault developed, temporal evolution, model, natural, faulting, laboratory measurement, seismic, slip pulse, shimamoto, pressure solution, strength, byerlee, hydraulic property, internal structure, slip occurs, ?ow, observation, core, journal geophysical solid earth, rupture, yielding, clay fabric, journal structural geology, uid pressure, fault, journal, upper crust, anders kim, individual fault, condition, wibberley, ?eld, energy