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

[1] The slip behavior of major faults depends largely on the frictional and hydrologic properties of fault gouge. We report on laboratory experiments designed to measure the strength, friction constitutive properties, and permeability of a suite of saturated clay-rich fault gouges, including: a 50:50% mixture of montmorillonite-quartz, powdered illite shale, and powdered chlorite schist. Friction measurements indicate that clay-rich gouges are consistently weak, with steady state coefficient of sliding friction of <0.35. The montmorillonite gouge (μ = 0.19–0.23) is consistently weaker than the illite and chlorite gouges (μ = 0.27–0.32). At effective normal stresses from 12 to 59 MPa, all gouges show velocity-strengthening frictional behavior in the sliding velocity range 0.5–300 μm/s. We suggest that the velocity-strengthening behavior we observe is related to saturation of real contact area, as documented by the friction parameter b, and is an inherent characteristic of noncohesive, unlithified clay-rich gouge. Permeability normal to the gouge layer measured before, during, and after shear ranges from 8.3 × 10−21 m2 to 3.6 × 10−16 m2; permeability decreases dramatically with shearing, and to a lesser extent with increasing effective normal stress. The chlorite gouge is consistently more permeable than the montmorillonite and illite gouge and maintains a higher permeability after shearing. Permeability reduction via shear is pronounced at shear strains ≲5 and is smaller at higher strain, suggesting that shear-induced permeability reduction is linked to fabric development early in the deformation history. Our results imply that the potential for development of excess pore pressure in low-permeability fault gouge depends on both clay mineralogy and shear strain.

Frictional and hydrologic properties of clay‐rich fault gouge

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1468-8123.2004.00091.x

Structural controls on hydrothermal flow in a segmented rift system, Taupo Volcanic Zone, New Zealand

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

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

[1] We compare the frictional strengths of 17 sheet structure mineral powders, measured under dry and water-saturated conditions, to identify the factors that cause many of them to be relatively weak The dry coefficient of friction μ ranges upward from 02 for graphite, leveling off at 08 for margarite, clintonite, gibbsite, kaolinite, and lizardite The values of μ (dry) correlate directly with calculated (001) interlayer bond strengths of the minerals This correlation occurs because shear becomes localized along boundary and Riedel shears and the platy minerals in them rotate into alignment with the shear planes For those gouges with μ (dry) < 08, shear occurs by breaking the interlayer bonds to form new cleavage surfaces Where μ (dry) = 08, consistent with Byerlee's law, the interlayer bonds are sufficiently strong that other frictional processes dominate The transition in dry friction mechanisms corresponds to calculated surface energies of 2–3 J/m2 Adding water causes μ to decrease for every mineral tested except graphite If the minerals are separated into groups with similar crystal structures, μ (wet) increases with increasing interlayer bond strength within each group This relationship also holds for the swelling clay montmorillonite, whose water-saturated strength is consistent with the strengths of nonswelling clays of similar crystal structure Water in the saturated gouges forms thin, structured films between the plate surfaces The polar water molecules are bonded to the plate surfaces in proportion to the mineral's surface energy, and μ (wet) reflects the stresses required to shear through the water films

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003JB002582

Crystallographic controls on the frictional behavior of dry and water-saturated sheet structure minerals

Permeability anisotropy and pressure dependency of permeability in experimentally sheared gouge materials

The effects of fault slip and effective mean stress on permeability and permeability anisotropy have been investigated during long displacement shearing of synthetic quartz, feldspar, muscovite and granitic gouges under high pressures. In quartzo-feldspathic gouges, permeability anisotropies up to one-and-a-half orders of magnitude were developed by heterogeneous grain crushing associated with shear sliding. In muscovite gouge, a permeability anisotropy of about one order of magnitude was first developed during initial pressurization of gouge materials due to a preferred alignment of mica basal planes. During shear sliding, rearrangement of packing and rotation of mica basal planes lead to a slight decrease in permeability anisotropy. The pressure dependency of permeability of gouge materials changes systematically with shear displacements. In quartzo-feldspathic gouges, the pressure dependency of permeability first decreased with shear sliding and then increased to a value typical for rocks containing microcracks. In contrast, the pressure dependency of permeability in muscovite gouge decreases continuously with increasing shear displacement. The evolution of permeability anisotropy and pressure dependency of permeability for different gouge materials during shear sliding implies that the distribution of fluid pressure in natural fault zones could be very heterogeneous.

Shuqing Zhang

Papers

Permeability anisotropy and pressure dependency of permeability in experimentally sheared gouge materials

Permeability anisotropy and pressure dependency of permeability in experimentally sheared gouge materials

The effect of fault slip on permeability and permeability anisotropy in quartz gouge

Implications of permeability and its anisotropy in a mica gouge for pore pressures in fault zones