This essential reference for graduate students and researchers provides a unified treatment of earthquakes and faulting as two aspects of brittle tectonics at different timescales. The intimate connection between the two is manifested in their scaling laws and populations, which evolve from fracture growth and interactions between fractures. The connection between faults and the seismicity generated is governed by the rate and state dependent friction laws - producing distinctive seismic styles of faulting and a gamut of earthquake phenomena including aftershocks, afterslip, earthquake triggering, and slow slip events. The third edition of this classic treatise presents a wealth of new topics and new observations. These include slow earthquake phenomena; friction of phyllosilicates, and at high sliding velocities; fault structures; relative roles of strong and seismogenic versus weak and creeping faults; dynamic triggering of earthquakes; oceanic earthquakes; megathrust earthquakes in subduction zones; deep earthquakes; and new observations of earthquake precursory phenomena.

The Mechanics of Earthquakes and Faulting

The validity of the cubic law for laminar flow of fluids through open fractures consisting of parallel planar plates has been established by others over a wide range of conditions with apertures ranging down to a minimum of 0.2 µm. The law may be given in simplified form by Q/Δh = C(2b)3, where Q is the flow rate, Δh is the difference in hydraulic head, C is a constant that depends on the flow geometry and fluid properties, and 2b is the fracture aperture. The validity of this law for flow in a closed fracture where the surfaces are in contact and the aperture is being decreased under stress has been investigated at room temperature by using homogeneous samples of granite, basalt, and marble. Tension fractures were artificially induced, and the laboratory setup used radial as well as straight flow geometries. Apertures ranged from 250 down to 4µm, which was the minimum size that could be attained under a normal stress of 20 MPa. The cubic law was found to be valid whether the fracture surfaces were held open or were being closed under stress, and the results are not dependent on rock type. Permeability was uniquely defined by fracture aperture and was independent of the stress history used in these investigations. The effects of deviations from the ideal parallel plate concept only cause an apparent reduction in flow and may be incorporated into the cubic law by replacing C by C/ƒ. The factor ƒ varied from 1.04 to 1.65 in these investigations. The model of a fracture that is being closed under normal stress is visualized as being controlled by the strength of the asperities that are in contact. These contact areas are able to withstand significant stresses while maintaining space for fluids to continue to flow as the fracture aperture decreases. The controlling factor is the magnitude of the aperture, and since flow depends on (2b)3, a slight change in aperture evidently can easily dominate any other change in the geometry of the flow field. Thus one does not see any noticeable shift in the correlations of our experimental results in passing from a condition where the fracture surfaces were held open to one where the surfaces were being closed under stress.

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VALIDITY OF CUBIC LAW FOR FLUID FLOW IN A DEFORMABLE ROCK FRACTURE - eScholarship

The concept of strength envelopes, developed in the 1970s, allowed quantitative predictions of the strength of the lithosphere based on experimentally determined constitutive equations. Initial strength envelopes used an empirical relation for frictional sliding to describe deformation along brittle faults in the upper portion of the lithosphere and power law creep equations to estimate the plastic flow strength of rocks in the deeper part of the lithosphere. In the intervening decades, substantial progress has been made both in understanding the physical mechanisms involved in lithospheric deformation and in refining constitutive equations that describe these processes. The importance of a regime of semibrittle behavior is now recognized. Based on data from rocks without added pore fluids, the transition from brittle deformation to semibrittle flow can be estimated as the point at which the brittle fracture strength equals the peak stress to cause sliding. The transition from semibrittle deformation to plastic flow can be approximated as the stress at which the pressure exceeds the plastic flow strength. Current estimates of these stresses are on the order of a few hundred megapascals for relatively dry rocks. Knowledge of the stability of sliding along faults and of the onset of localization during brittle fracture has improved considerably. If the depth to the bottom of the seismogenic zone is determined by the transition to the stable frictional sliding regime, then that depth will be considerably more shallow than the depth of the transition to the plastic flow regime. Major questions concerning the strength of rocks remain. In particular, the effect of water on strength is critical to accurate predictions. Constitutive equations which include the effects of water fugacity and pore fluid pressure as well as temperature and strain rate are needed for both the brittle sliding and semibrittle flow regimes. Although the constitutive equations for dislocation creep and diffusional creep in single-phase aggregates are more robust, few data exist for plastic deformation in two-phase aggregates. Despite the fact that localization is ubiquitous in rocks deforming both in brittle and plastic regimes, only a limited amount of accurate experimental data are available to constrain predictions of this behavior. Accordingly, flow strengths now predicted from laboratory data probably overestimate the actual rock strength, perhaps by a significant amount. Still, the predictions are robust enough that uncertainties in geometry, mineralogy, loading conditions and thermodynamic state are probably the limiting factors in our understanding. Thus, experimentally determined rheologies can be applied to understand a broad range of topical problems in regional and global tectonics both on the Earth and on other planetary bodies.

Strength of the lithosphere: Constraints imposed by laboratory experiments

A review of recent developments concerning the structure, mechanics and fluid flow properties of fault zones

Brittle microtectonics: principles and practice

Evidence is presented to support the conclusion that pore fluid pressures tresses PF during regional metamorphism are generally greater than or equal to the minimum principal compressive stress S3. The resultant very low effective stresses σ lead to significantly increased porosity and permeability, even at moderate to high metamorphic temperatures. Permeabilities between 10−18 and 10−15 m2 and are considered to be common, resulting in rapid fluid migration and the dominance of advective (infiltrative) over diffusive mass transport, even over relatively small distances. In view of the importance of intergranular mass transport to rock deformation during metamorphism, a mobile, high-pressure fluid will have substantial rheological effects, especially in polyphase rocks. The fluid is capable of influencing the rate of dislocation creep in a number of ways. More importantly, advective mass transport along fluid pressure gradients can give rise to a solution transfer deformation mechanism that competes with conventional pressure solution. The rate of deformation by advective mass transport could be controlled by a number of processes, including dissolution kinetics, advective transport rates, and the rate of crack growth. A specific deformation model, based on advective transport rate control,is developed, which can produce strain at competitive rates but with stress and temperature dependences of unusual form.

High fluid pressures during regional metamorphism and deformation: Implications for mass transport and deformation mechanisms

Principles of Structural Control on Permeability and Fluid Flow in Hydrothermal Systems

The internal structures of the Wattle Gully Fault provide insights about the mechanics and dynamics of fault systems exhibiting fault valve behavior in high fluid pressure regimes. This small, high-angle reverse fault zone developed at temperatures near 300°C in the upper crust, late during mid-Devonian regional crustal shortening in central Victoria, Australia. The Wattle Gully Fault forms part of a network of faults that focused upward migration of fluids generated by metamorphism and devolatilisation at deeper crustal levels. The fault has a length of around 800 m and a maximum displacement of 50 m and was oriented at 60° to 80° to the maximum principal stress during faulting. The structure was therefore severely misoriented for frictional reactivation. This factor, together with the widespread development of steeply dipping fault fill quartz veins and associated subhorizontal extension veins within the fault zone, indicates that faulting occurred at low shear stresses and in a near-lithostatic fluid pressure regime. The internal structures of these veins, and overprinting relationships between veins and faults, indicate that vein development was intimately associated with faulting and involved numerous episodes of fault dilatation and hydrothermal sealing and slip, together with repeated hydraulic extension fracturing adjacent to slip surfaces. The geometries, distribution and internal structures of veins in the Wattle Gully Fault Zone are related to variations in shear stress, fluid pressure, and near-field principal stress orientations during faulting. Vein opening is interpreted to have been controlled by repeated fluid pressure fluctuations associated with cyclic, deformation-induced changes in fault permeability during fault valve behavior. Rates of recovery of shear stress and fluid pressure after rupture events are interpreted to be important factors controlling time dependence of fault shear strength and slip recurrence. Fluctuations in shear stress and transient rotations of near-field principal stresses, indicated by vein geometries, are interpreted to indicate at least local near-total relief of shear stress during some rupture events. Fault valve behavior has important effects on the dynamics of fluid migration around active faults that are sites of focused fluid migration. In particular, fault valve action is expected to lead to distinctly different fluid migration patterns adjacent to faults before, and immediately after, rupture. These fluid migration patterns have important differences with those predicted by models for dilatancy-diffusion effects and for poroelastic responses around reverse faults.

Faulting processes at high fluid pressures: An example of fault valve behavior from the Wattle Gully Fault, Victoria, Australia

Deformational and metamorphic processes in the formation of mesothermal vein-hosted gold deposits — examples from the Lachlan Fold Belt in central Victoria, Australia

At depths greater than several kilometers in the crust, elevated temperature, elevated confining pressure, and the presence of reactive pore fluids typically drive rapid destruction of permeability in fractured and porous rock. Ongoing deformation is required to regenerate permeability and facilitate the high fluid flux necessary to produce hydrothermal ore systems. A dominant influence on the development of fluid pathways in hydrothermal systems is provided by stress states, fluid pressures, and reactions that drive permeability enhancement and compete with permeability destruction processes. Fluid redistribution within hydrothermal systems at depth in the crust is governed largely by hydraulic gradients between upstream fluid reservoirs and the downstream regions of permeable networks of active faults, shear zones, and related structures that drain reservoirs. Pressure-driven flow leads to generally upward migration of fluids, although permeability anisotropy and tortuous flow paths may cause a significant along-strike component to fluid migration. Devolatilization reactions in prograding metamorphic regimes play a key role, not only in fluid production but also in generating transitory elevated permeability in deep crustal reservoirs. Active deformation and the development of high pore-fluid factors (the ratio of fluid pressure to vertical stress) in fluid reservoirs also drive permeability enhancement via grain-scale microfracturing and pervasive development of mesoto macroscale hydraulic fracture arrays. In the upstream, high-temperature parts of hydrothermal systems, pervasive fluid flow through the crust may occur via episodic migration of fluid-pressure pulses. Recent observations suggest that propagating fluid-pressure pulses may create transient permeabilities as high as 10–13m2 in the deep crust. Flow focusing occurs wherever networks of active, high-permeability shear zones, faults, or other permeable structures, penetrate pressurized fluid reservoirs. These structures drain reservoirs and provide pathways for fluid redistribution to higher crustal levels. Contrasting styles of flow are expected between flow pathways in the aseismically deforming lower half of the crust and pathways within the seismogenic regime in the upper half of the crust. Below the seismic-aseismic transition, steady-state creep processes favor near-constant permeabilities and continuous fluid flow. In the seismogenic regime, large changes in fault permeability during the seismic cycle produce episodic flow regimes. In particular, large earthquake ruptures that propagate down from the upper crust into deeper level fluid reservoirs generate major, transitory perturbations to fluid pressure gradients. Episodic fluid redistribution from breached, overpressured (i.e., suprahydrostatic) reservoirs has the potential to generate large fluid discharge and high fluid/rock ratios around the downstream parts of fault systems after large rupture events. Hydrothermal self-sealing of faults, together with drainage of the hydraulically accessible parts of reservoirs between earthquakes, progressively shuts off flow along fault ruptures. Permeability enhancement due to rupture events may also drive transitory flow of fluids, derived from shallow crustal reservoirs, deep into fault zones after earthquakes. As earthquakes migrate around fault systems in the upper, seismogenic part of the crust, permeability distribution and fluid pathways can evolve in complex ways. To achieve the necessary time-integrated fluid fluxes, the formation of large ore systems in this regime requires redistribution of fluid batches predominantly through small segments of fault systems during numerous rupture cycles. Sustained localized flow at the ore field scale is favored by development of long-lived, actively deforming, high-permeability structures such as fault jogs or fault intersections on high displacement faults. These structures can produce pipelike pathways linking deep reservoirs and shallower crustal levels. The generation of aftershock networks also influences fluid redistribution and discharge around the downstream ends of main-shock rupture zones. The distribution of these networks, and their repeated reactivation, is influenced by stress changes caused by main-shock rupture and by postseismic migration of fluid-pressure pulses away from the downstream ends of main-shock rupture zones. At the deposit scale, in fracture-controlled hydrothermal systems, the highest fluid flux occurs where the apertures, densities, and connectivities of fractures are greatest. The locations and geometries of these sites are governed by fluid-driven permeability enhancement in structurally controlled sites such as jogs, bends, and terminal splays, typically in low displacement faults and shear zones, as well as by fault intersections, competence contrasts, and fold-related dilation. Permeability anisotropy in structural pathways can influence deposit-scale flow directions and shapes of ore shoots. † E-mail, Stephen.Cox@anu.edu.au ©2005 Society of Economic Geologists, Inc. Economic Geology 100th Anniversary Volume pp. 39–75

Stephen F. Cox

Papers

High fluid pressures during regional metamorphism and deformation: Implications for mass transport and deformation mechanisms

Principles of Structural Control on Permeability and Fluid Flow in Hydrothermal Systems

Faulting processes at high fluid pressures: An example of fault valve behavior from the Wattle Gully Fault, Victoria, Australia

Deformational and metamorphic processes in the formation of mesothermal vein-hosted gold deposits — examples from the Lachlan Fold Belt in central Victoria, Australia

Coupling between Deformation, Fluid Pressures, and Fluid Flow in Ore-Producing Hydrothermal Systems at Depth in the Crust