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1 Institut fuer Angewandte Geowissenschaften, Technische Geologie und Mineralogie, Technische Universitaet Graz, Rechbauerstrasse 12, A-8010 Graz, Austria
2 Reactivation Research Group, Dept of Earth Sciences, University of Durham, South Road, Durham DH1 3LE, UK
3 TOTAL, CSTJF, Avenue Larribau, 64018 PAU Cedex, France(e-mail: christopher.wibberley{at}total.com)
4 Department of Earth Sciences, University of Durham, South Road, Durham, DH1 3LE, UK
5 Dipartimento di Scienze della Terra, Universita di Perugia, Piazza dell'Universita 1, 06100, Perugia, Italy
Faults are important controls on hydrocarbon migration and ore mineralization and, in areas of active deformation, are the most important source of seismic hazard. However, faults are rarely discrete surfaces and the internal structure of fault zones (e.g., the thickness, nature and continuity of the fault rocks, the distribution and segmentation of slip surfaces, and the orientation, distribution and connectivity of subsidiary faults and fractures) is a key control on their bulk fluid flow and mechanical properties. This Special Publication was inspired by two sessions held at the European Geosciences Union General Assembly in Vienna during 2005 and 2006 and contains 19 original papers divided into three sections. Part I addresses the controls on fault zone evolution, whilst Parts II and III focus, respectively, on the mechanical behaviour and fluid flow properties of fault zones.
The introductory paper (Wibberley et al.) addresses each theme of the Special Publication: fault zone evolution, the permeability structure of ancient and active fault zones, the impact of faults on hydrocarbon sealing and migration, and the implications of fault zone geometry and material heterogeneity for seismogenic processes. In each section, Wibberley et al. identify important recent findings and suggest areas in which new conceptual advances in our understanding of fault zones are likely to occur.
A key theme highlighted by many of the papers in Part I is the importance of pre-existing mechanical heterogeneities (e.g., bedding, joints) in controlling the internal structure of faults in sedimentary sequences. Johanssen & Fossen consider the control of bed thickness and fault displacement on the geometry, orientation and distribution of minor fractures and deformation bands (i.e., the ‘damage zone’) that surround faults cutting aeolian sandstones, siltstones and shales in the western United States. They conclude that the highest concentrations of deformation bands occur close to the main faults, favouring fluid flow within the damage zone in a direction parallel to the principal fault surfaces. Next, van der Zee et al. examine the influence of layering and pre-existing joints on the internal structure of normal fault zones exposed in southern France. They show that fault zone complexity increases, first as fault displacement exceeds the thickness of the competent beds and second, in response to syn-faulting tilting of the strata. The role of mechanical anisotropy is also central to Brosch & Kurz's study of brittle shear zones within layered marbles and foliated quartzites in the eastern Alps. These authors demonstrate that pre-existing fault-parallel fractures and foliation planes are important controls on the development of fault breccias. In the following paper, Putz-Perrier & Sanderson investigate how strain is partitioned between fault zones, minor faults and veins that cut a carbonate-mudstone sequence exposed in southern England. They show that faults and veins follow different scaling relationships, thus the observed heterogeneity of extensional strain appears to be scale dependent. The contribution by Ferrill et al. returns to the question of damage zone evolution. In a study of neotectonic ruptures in California, Ferrill et al. conclude that damage zone width is established early during fault propagation, although the active portion of the fault zone will likely narrow as faulting continues and a through-going slip surface accommodates the bulk of the displacement. The final paper in this section (Micarelli & Benedicto) examines the role played by mechanical layering in accommodating strains at the tips of normal faults, and the implications for the fluid flow properties of normal faults. Echoing one of the conclusions reached by van der Zee et al., Micarelli & Benedicto show that the ratio between displacement magnitude and bed thickness strongly influences fault tip architecture.
Part II deals with the mechanical consequences of fault zone architecture, with emphasis on the relationships between the mechanical behaviour, seismicity and internal structure of fault zones. Faulkner et al. present observations from two well-exposed strike-slip faults that have been passively exhumed from seismogenic depths. They demonstrate cases in which internal structures of these fault zones are consistent with the strain hardening or strain weakening behaviour of the country rock predicted by laboratory deformation experiments. In the following paper, Imber et al. review the geology of large displacement intra-plate faults exhumed from <15 km depth and conclude that many such faults display evidence for frictional–viscous deformation within foliated, phyllosilicate rich cores. Comparison with seismological data suggest that some faults with phyllosilicate rich cores are likely to generate large earthquakes, which is contrary to the findings of some previous rock deformation experiments. The study by Collettini et al. focuses on CO2 degassing in the Northern Apennines. This region provides an important opportunity to investigate slip and microseismicity along an active low-angle normal fault, and to compare these geophysical data with geological observations of equivalent exhumed structures. Collettini et al. propose that over geological timescales, fluid–rock interaction within fault cores give rise to aggregates of weak, phyllosilicate rich fault rocks that deform by frictional–viscous creep at sub-Byerlee friction values (µ<0.3). Fluids can be stored in structural and stratigraphic traps (e.g., beneath mature fault cores and regionally extensive Triassic evaporites) giving rise to short-term cycles of fluid pressure build-up and release. In a study of strike-slip earthquake sequences in southern Italy, Boncio also highlights the role of rheological layering in allowing the build-up of fluid overpressures and in controlling the vertical extent of seismicity. The final paper in this section (Wölfler et al.) presents a case study of structures produced during sinistral transpression along the southwestern margin of the Tauern Window. They consider the role of fluids in reducing fault shear strength and promoting shear localization on geological timescales.
The final section considers the fluid flow properties of fault zones. The first contribution (Lunn et al.) uses numerical simulations of fluid flow through fault zones mapped in outcrop to demonstrate that across fault flow is controlled by tortuous high permeability pathways. Lunn et al. argue that predicting the bulk hydraulic properties of faults in the subsurface depends upon a statistical characterization of the likelihood and frequency of such pathways. In the following paper, Zhang et al. also use numerical modelling to explore the interactions between faulting, fluid flow and chemical processes in dilatant jogs. In these regions, the precipitation rates of gold and quartz depend on the local fluid velocity and chemical concentration gradients generated by fluid mixing. Benedicto et al. investigate changes in the nature of fluid–rock interactions during the growth of a major normal fault system on the southern margin of the Corinth rift, Greece. They show that the evolution from distributed deformation (brecciation) to localized slip during progressive exhumation of the footwall was accompanied by a change from a geochemically closed system characterized by fluid–rock equilibrium to a more open system characterized by influx of meteoric waters. Developing this focus on extensional faulting, Agosta examines the fluid flow properties of major (basin-bounding) faults within platform carbonates of the Fucino Basin, central Italy. He uses measurements of porosity, pore-throat radii and elastic moduli to compute the permeability of the host rocks and carbonate-rich fault cores to explain the combined conduit-barrier behaviour of the normal faults. In the following contribution, Rolland et al. highlight the application of syn-kinematic phyllosilicates (similar to those described by Imber et al. and Collettini et al.) to dating deformation and fluid-rock interaction within ductile shear zones under low-grade metamorphic conditions. Returning to the theme of near-surface fluid flow, Baietto et al. use three-dimensional thermohydraulic models to investigate the control of fault geometry on thermal circulation and fluid outflow at the tip of a major strike-slip fault in the western Alps. In the final paper, Boutareaud et al. examine the effect of secondary splay faults on the hydrodynamic behaviour of fault zones. They conclude that splaying of a rupture into surrounding microbreccias or into newly generated splay faults of higher permeability will release co-seismic fluid pressures, or inhibit the generation of excess fluid pressures by thermal pressurization.
Several points emerge from the contributions to this Special Publication, all of which illustrate the importance of the internal structure of fault zones to understanding the hydraulic, seismogenic and mechanical behaviour of faults. The first is the critical role of mechanical anisotropy – in particular stratigraphic layering – in influencing not only the detailed structural evolution of fault zones, but also the distribution of fluid overpressure and fault-related seismicity (Boncio, 2008). These findings highlight the need to investigate the influence of mechanical stratigraphy on the development and scaling of fault zone structures such as stepovers (e.g., relay zones; Peacock 2003), which are known to be important controls on both the fluid-flow/sealing properties of faults (e.g., Childs et al. 1995; Zhang et al. 2008) and on earthquake rupture processes (e.g., Sibson 1989). Second, the results of laboratory deformation experiments appear to be consistent with field and microstructural observations of some, but not all natural large displacement faults. The possible mismatch between field and laboratory observations seems to arise where fluid–rock interaction within fault zones has a significant chemical in addition to mechanical effect (e.g., Axen 2004). Future laboratory studies should therefore investigate deformation under hydrothermal conditions where chemical and metamorphic processes may be of critical importance (e.g., Mariani et al. 2006; Niemeijer & Spiers 2007). Finally, it is clear that the hydrodynamic behaviour of fault zones is a major control on processes that operate on widely differing timescales and under different stress conditions. These include dynamic weakening during seismic slip (e.g., Wibberley & Shimamoto 2005), fluid flow in hydrocarbon reservoirs and aquifers (e.g., Manzocchi et al. 1999), ore mineralization and fault sealing (e.g., Yielding et al. 1997). The internal structure of faults is likely to evolve to a greater or lesser extent during each of these processes and future work should aim to better describe and quantify the temporal in addition to spatial variations in fault zone structure and permeability (e.g., Sheldon & Micklethwaite 2007). The breadth of topics discussed here has meant that we have relied heavily upon the expertise and professionalism of the peer reviewers. We are therefore grateful to the following colleagues for their timely and constructive reviews:
F. Agosta, S. Barba, A. Billi, P. Bonsio, C. Bonson, W. Brueckmann, C. Childs, P. Connolly, N. Davatzes, I. Davison, K. Decker, N. De Paola, O. Dor, J. Fairley, H. Fritz, N. Froitzheim, B. Fugenschuh, J. Genser, M. Handy, G. Hirth, J. Imber, K. de Jong, K. McCaffrey, T. Manzocchi, A. McCaig, S. Micklethwaite, S. Miller, T. Needham, E. Nelson, F. Neubauer, D. Peacock, G. Roberts, D. Sanderson, J. Selverstone, S. Shapiro, H. Sheldon, S. Sherlock, Z. Shipton, R. Soliva, A. Tsutsumi, P. Vanucchi, C. Vita-Finzi, S. Wilkins, R. Wilson, D. Wiltschko, N. Woodcock.
Finally, we would like to thank Angharad Hills and Jonathan Turner for handling this project on behalf of the Geological Society.
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Axen, G. J. 2004. Mechanics of low-angle normal faults. In: Karner, G. D., Taylor, B., Driscoll, N. W. & Kohlstedt, D. L. (eds) Rheology and Deformation of the Lithosphere at Continental Margins. Columbia University Press, New York, 46–91.
Boncio, P. 2008. Deep-crust strike–slip earthquake faulting in southern Italy aided by high fluid pressure: insights from rheological analysis. In: Wibberley, C. A. J., Kurz, W., Imber, J., Holdsworth, R. E. & Collettini, C. (eds) The Internal Structure of Fault Zones: Implications for Mechanical and Fluid-Flow Properties, Geological Society, London, Special Publications, 299, 195–210.
Childs, C., Watterson, J. & Walsh, J. J. 1995. Fault overlap zones within developing normal fault systems. Journal of the Geological Society, London, 152, 535–549.
Manzocchi, T., Walsh, J. J. Nell, P. & Yielding, G. 1999. Fault transmissibility multipliers for flow simulation models. Petroleum Geoscience, 5, 53–63.
Mariani, E., Brodie, K. H. & Rutter, E. H. 2006. Experimental deformation of muscovite shear zones at high temperatures under hydrothermal conditions and the strength of phyllosilicate-bearing faults in nature. Journal of Structural Geology, 28, 1569–1587.[CrossRef][Web of Science][GeoRef]
Niemeijer, A. R. & Spiers, C. J. 2007. A microphysical model for strong velocity weakening in phyllosilicate-bearing fault gouges. Journal of Geophysical Research, 112, B10405, doi:10.1029/2007JB005008.[CrossRef]
Peacock, D. C. P. 2003. Scaling of transfer zones in the British Isles. Journal of Structural Geology, 25, 1561–1567.[CrossRef][Web of Science][GeoRef]
Sheldon, H. A. & Micklethwaite, S. 2007. Damage and permeability around faults: implications for mineralization. Geology, 35, 903–906.
Sibson, R. H. 1989. Earthquake faulting as a structural process. Journal of Structural Geology, 11, 1–14.[CrossRef][Web of Science][GeoRef]
Wibberley, C. A. J. & Shimamoto, T. 2005. Earthquake slip weakening and asperities explained by thermal pressurization. Nature, 436, 689–692.[CrossRef][Medline]
Yielding, G., Freeman, B. & Needham, D. T. 1997. Quantitative fault seal prediction. AAPG Bulletin, 81, 897–917.[Abstract][GeoRef]
Zhang, Y., Schaubs, P. M. Zhao, C. Ord, A. Hobbs, B. E. & Barnicoat, A. C. 2008. Fault-related dilation, permeability enhancement, fluid flow and mineral precipitation patterns: numerical models. In: Wibberley, C. A. J., Kurz, W., Imber, J., Holdsworth, R. E. & Collettini, C. (eds) The Internal Structure of Fault Zones: Implications for Mechanical and Fluid-Flow Properties, Geological Society, London, Special Publications, 299, 239–255.
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