Ronald L. Bruhn

Introduction to special section: Mechanical involvement of fluids in faulting

A growing body of evidence suggests that fluids are intimately linked to a variety of faulting processes. These include the long term structural and compositional evolution of fault zones; fault creep; and the nucleation, propagation, arrest, and recurrence of earthquake ruptures. Besides the widely recognized physical role of fluid pressures in controlling the strength of crustal fault zones, it is also apparent that fluids can exert mechanical influence through a variety of chemical effects. The United States Geological Survey sponsored a Conference on the Mechanical Effects of Fluids in Faulting under the auspices of the National Earthquake Hazards Reduction Program at Fish Camp, California, from June 6 to 10, 1993. The purpose of the conference was to draw together and to evaluate the disparate evidence for the involvement of fluids in faulting; to establish communication on the importance of fluids in the mechanics of faulting between the different disciplines concerned with fault zone processes; and to help define future critical investigations, experiments, and observational procedures for evaluating the role of fluids in faulting. This conference drew together a diverse group of 45 scientists, with expertise in electrical and magnetic methods, geochemistry, hydrology, ore deposits, rock mechanics, seismology, and structural geology. Some of the outstanding questions addressed at this workshop included the following: 1. What are fluid pressures at different levels within seismically active fault zones? Do they remain hydrostatic throughout the full depth extent of the seismogenic regime, or are they generally superhydrostatic at depths in excess of a few kilometers? 2. Are fluid pressures at depth within fault zones constant through an earthquake cycle, or are they time-dependent? What is the spatial variability in fluid pressures? 3. What is the role of crustal fluids in the overall process of stress accumulation, release, and transfer during the earthquake cycle? Through what mechanisms might fluid pressure act to control the processes of rupture nucleation, propagation, and arrest? 4. What is the chemical role of fluids in facilitating fault creep, including their role in aiding solid-state creep and brittle fracture processes and in facilitating solution-transport deformation mechanisms? 5. What are the chemical effects of aqueous fluids on constitutive response, fractional stability, and long-term fault strength? 6. What are the compositions and physical properties of faultfluids at different crustal levels? 7. What are the mechanisms by which porosity and permeability are either created or destroyed in the middle to lower crust? What factors control the rates of these processes? How should these effects be incorporated into models of time-dependent fluid transport in fault zones? 8. What roles do faults play in distributing fluids in the crust and in altering pressure domains? In other words, when and by what mechanisms do faults aid in or inhibit fluid migration? What are the typical fluid/rock ratios, flow rates, and discharges for fault zones acting as fluid conduits? 9. Are fluids present in the subseismogenic crust, and by what transformation and/or transport processes are they incorporated into the shallower seismogenic portions of faults?

Fracturing and hydrothermal alteration in normal fault zones

Large normal fault zones are characterized by intense fracturing and hydrothermal alteration. Displacement is localized in a slip zone of cataclasite, breccia and phyllonite surrounding corrugated and striated fault surfaces. Slip zone rock grades into fractured, but less comminuted and hydrothermally altered rock in the transition zone, which in turn grades abruptly into the wall rock. Fracturing and fluid flow is episodic, because permeability generated during earthquakes is destroyed by hydrothermal processes during the time between earthquakes.Fracture networks are described by a fracture fabric tensor (F). The permeability tensor (k) is used to estimate fluid transport properties if the trace of F is sufficiently large. Variations in elastic moduli and seismic velocities between fault zone and wall rock are estimated as a function of fracture density (ε). Fracturing decreases elastic moduli in the transition zone by 50–100% relative to the country rock, and similar or even greater changes presumably occur in the slip zone.P-andS-wave velocity decrease, andVp/Vs increases in the fault zone relative to the wall rock. Fracture permeability is highly variable, ranging between 10−13 m2 and 10−19 m2 at depths near 10 km. Changes in permeability arise from variations in effective stress and fracture sealing and healing.Hydrothermal alteration of quartzo-feldspathic rock atT>300°C creates mica, chlorite, epidote and alters the quartz content. Alteration changes elastic moduli, but the changes are much less than those caused by fracturing.P-andS-wave velocities also decrease in the hydrothermally altered fault rock relative to the country rock, and there is a slight decrease inVp/Vs, which partially offsets the increase inVp/Vs caused by fracturing.Fracturing and hydrothermal alteration affect fault mechanics. Low modulus rock surrounding fault surfaces increases the probability of exceeding the critical slip distance required for the onset of unstable slip during rupture initiation. Boundaries between low modulus fault rock and higher modulus wall rock also act as rupture guides and enhance rupture acceleration to dynamic velocity. Hydrothermal alteration at temperatures in excess of 300°C weakens the deeper parts of the fault zone by producingphyllitic mineral assemblages. Sealing of fracture in time periods between large earthquakes generates pods of abnormally pressured fluid which may play a fundamental role in the initiation of large earthquakes.

Deformation during terrane accretion in the Saint Elias orogen, Alaska

The Saint Elias orogen of southern Alaska and adjacent Canada is a complex belt of mountains formed by collision and accretion of the Yakutat terrane into the transition zone from transform faulting to subduction in the northeast Pacific. The orogen is an active analog for tectonic processes that formed much of the North American Cordillera, and is also an important site to study (1) the relationships between climate and tectonics, and (2) structures that generate large- to great-magnitude earthquakes. The Yakutat terrane is a fragment of the North American plate margin that is partly subducted beneath and partly accreted to the continental margin of southern Alaska. Interaction between the Yakutat terrane and the North American and Pacific plates causes significant differences in the style of deformation within the terrane. Deformation in the eastern part of the terrane is caused by strike-slip faulting along the Fairweather transform fault and by reverse faulting beneath the coastal mountains, but there is little deformation immediately offshore. The central part of the orogen is marked by thrusting of the Yakutat terrane beneath the North American plate along the Chugach–Saint Elias fault and development of a wide, thin-skinned fold-and-thrust belt. Strike-slip faulting in this segment may be localized in the hanging wall of the Chugach–Saint Elias fault, or dissipated by thrust faulting beneath a north-northeast–trending belt of active deformation that cuts obliquely across the eastern end of the fold-and-thrust belt. Superimposed folds with complex shapes and plunging hinge lines accommodate horizontal shortening and extension in the western part of the orogen, where the sedimentary cover of the Yakutat terrane is accreted into the upper plate of the Aleutian subduction zone. These three structural segments are separated by transverse tectonic boundaries that cut across the Yakutat terrane and also coincide with the courses of piedmont glaciers that flow from the topographic backbone of the Saint Elias Mountains onto the coastal plain. The Malaspina fault–Pamplona structural zone separates the eastern and central parts of the orogen and is marked by reverse faulting and folding. Onshore, most of this boundary is buried beneath the western or “Agassiz” lobe of the Malaspina piedmont glacier. The boundary between the central fold-and-thrust belt and western zone of superimposed folding lies beneath the middle and lower course of the Bering piedmont glacier.

Laboratory characterization of hydromechanical properties of a seismogenic normal fault system

Abstract The Stillwater seismogenic normal fault in Dixie Valley, Nevada has been historically active and is located in an area of high heat flow and hydrothermal activity. Three primary structural elements are identified in the fault zone: a relatively wide fault core (with breccia pods embedded in cataclasites), a damage zone (with arrays of mesoscopic fractures), and protolith. Hydromechanical properties of representative core samples were characterized in the laboratory, and microstructural analyses were conducted using optical and scanning electron microscopy. When deformed in conventional triaxial compression, dilatancy and brittle fracture were observed in each sample. Samples from the core of the fault were relatively weak, with strengths similar to that of unconsolidated fault gouge, whereas granodiorite samples from the protolith were as weak as the core and damage zone samples were stronger. Permeability is dependent on effective pressure, porosity and connectivity of the pore space, with values ranging over four orders of magnitude among the core samples. The lowest permeability of 3×10 −20 m 2 was measured in a fault core sample with a microstructure indicative of implosion brecciation. In conjunction with field measurements, the laboratory data suggest that fluid flow and changes in fluid storage are concentrated in the damage zone, with permeability several orders of magnitude higher than the protolith and fault core. Permeability contrast (one order of magnitude) at the core sample scale exists between the cataclasite and implosion breccia in the fault core. Because of dilatancy and poor drainage in the breccia pods, anomalously low pore pressures may develop in localized clusters due to dilatancy hardening during the preseismic period. These clusters of low pore pressure can act similarly to fault jogs, locally inhibiting fault rupture and inducing brecciation when the delayed failure finally occurs by catastrophic implosion.

Potential seismic hazards and tectonics of the upper Cook Inlet basin, Alaska, based on analysis of Pliocene and younger deformation

The Cook Inlet basin is a northeast-trending forearc basin above the Aleutian subduction zone in southern Alaska. Folds in Cook Inlet are complex, discontinuous structures with variable shape and vergence that probably developed by right-transpressional deformation on oblique-slip faults extending downward into Mesozoic basement beneath the Tertiary basin. The most recent episode of deformation may have began as early as late Miocene time, but most of the deformation occurred after deposition of much of the Pliocene Sterling Formation. Deformation continued into Quaternary time, and many structures are probably still active. One structure, the Castle Mountain fault, has Holocene fault scarps, an adjacent anticline with flower structure, and historical seismicity. If other structures in Cook Inlet are active, blind faults coring fault-propagation folds may generate M w 6–7+ earthquakes. Dextral transpression of Cook Inlet appears to have been driven by coupling between the North American and Pacific plates along the Alaska-Aleutian subduction zone, and by lateral escape of the forearc to the southwest, due to collision and indentation of the Yakutat terrane 300 km to the east of the basin.

Fluid pressure transients on seismogenic normal faults

Abstract Fluid inclusions in hydrothermally altered footwall rocks of the Dixie Valley fault, Nevada, and the Wasatch fault, Utah, indicate that pore fluid pressure fluctuated. Minimum entrapment pressures for fluid inclusions consisting of H2O-CO2-NaCl ranged from 295 MPa to 60 MPa in the temperature range 350° to 170 ° C on the Wasatch fault, and from 158 to 35 MPa in the temperature range 350° to 200 ° C on the Dixie Valley fault. Scatter in the pressure estimates at constant temperature is interpreted as paleo-fluid pressure transients at depths of up to 11 km on the Wasatch fault and 3 to 5 km on the Dixie Valley fault. Observed pressure transients range from 5 MPa, within the limits of error in pressure determination, to 120 MPa on the Wasatch fault and 7 to 126 MPa on the Dixie Valley fault. The pressure transients are greatest on both faults in the temperature range 270 ° to 310 ° C. The fluids represented by fluid inclusions play a key role in nucleation and propagation of earthquake ruptures. High fluid pressures may initiate rupture, then dilatancy, pore-pressure reduction, and dilatant hardening may arrest the rupture. However, decompression of the fluids and phase separation produces a decrease in fluid bulk modulus of 41 to 90% which reduces the dilatant hardening effect and may permit ruptures to propagate.

Structural anisotropy of normal fault surfaces

Abstract Precise description of natural fault surfaces is indispensable to understanding the geometry, mechanics and fluid transport properties of faults. Profiles of fault surfaces in the Wasatch fault zone and Oquirrh Mountains, Utah, are measured at 30° increments within the fault plane to determine the directional anisotropy of surface roughness at wavelengths between 10 −3 m and 30 m, and then compared with profiles of larger-scale fault surfaces. Surface anisotropy and an increasing ratio of surface amplitude to wavelength are consistent with self-affine fault topography at wavelengths between 1 mm and approximately 5 km. Fractal dimension of surface profiles generally decreases systematically as the angle to the slip direction increases. Directional anisotropy is described by an azimuthal scaling function γ φ = K sin( φ ) + γ 0 or AF φ = ( AF max −1) sin( φ ) + 1, where γ φ and AF φ are the amplitude to wavelength ratio and anisotropy factor respectively at azimuth φ, measured clockwise relative to slip direction within the fault surface, and γ 0 is the amplitude to wavelength ratio parallel to slip direction. K = ( γ 90 − γ 0 ) is an anisotropy coefficient and increases systematically with spatial wavelength on the fault surface. Characterization of natural fault surfaces provides parameters such as fractal dimension ( D ), intercept (log( C )) of power spectra, profile variance, and variation in anisotropy factor ( AF ), which are needed to generate fractal models of natural fault surfaces using spectral synthesis. We generate sample models which illustrate the differences between fault surfaces characterized by constant versus azimuthally varying fractal dimension. The latter model surfaces contain low amplitude corrugations superimposed on elongate ridges which parallel slip direction. This surface texture resembles that of natural fault surfaces that refract across lithologic layering or are cut by secondary faults such as R and R ′ shears.

Fluid inclusion evidence for minimum 11 km vertical offset on the Wasatch fault, Utah

The footwall of the Wasatch fault in the Corner Creek area near Salt Lake City is hydrothermally altered and deformed quartz monzonite of the Oligocene Little Cottonwood stock. Secondary fluid inclusions are associated with hydrothermal alteration minerals and structural deformation. Thermometric measurements of fluid inclusion characteristics indicate entrapment of a CO2-H2O-NaCl fluid at minimum temperatures of 223–353 °C and minimum fluid pressures of 900–2800 bar. The 2800-bar fluid pressure is near lithostatic pressure at a depth of 11 km, the minimum displacement of the fault that is required for exhumation of the observed alteration and fluid inclusions and more than three times greater than most previous estimates. The large displacement estimate is supported by the occurrence of pyrophyllite in shales in the footwall, by the petrology of metamorphosed shale surrounding an unroofed pluton in the footwall, and by geologic reconstruction of the eroded footwall.

Fluid permeability of deformable fracture networks

We consider the problem of defining the fracture permeability tensor for each grid block in a rock mass from maps of natural fractures. For this purpose we implement a statistical model of cracked rock developed by M. Oda, where the permeability tensor is related to the crack geometry via a volume average of the contribution from each crack in the population. In this model, tectonic stress is implicitly coupled to fluid flow through an assumed relationship between crack aperture and normal stress across the crack. We have included three enhancements to the basic model. (1) A realistic model of crack closure under stress has been added along with the provision to apply tectonic stresses to the fracture system in any orientation. The application of compressive stress results in fracture closure, and consequently, a reduction in permeability. (2) The fracture permeability can be linearly superimposed onto an arbitrary anisotropic matrix permeability. (3) The fracture surfaces are allowed to slide under the application of shear stress, causing fractures to dilate and result in a permeability increase. Through two examples we demonstrate that significant changes in permeability magnitudes and orientations are possible when tectonic stress is applied to fracture systems.

Structural and fluid-chemical properties of seismogenic normal faults

Abstract Structures in extensional fault zones can be classified using the configuration of fault branch lines and tip lines. Important classes include the segment bend, segment termination, segment branch, cross-fault intersection, and segment offset. The effect of these structures on rupture history is not necessarily consistent, neither between individual earthquakes nor between different fault zones. Rupture behavior is dependent on several other factors including loading conditions (regional and localized rupture tip stress fields), fluid-mechanical processes, and chemical processes. Fracture toughness is partly controlled by the angular discordance between slip directions on adjacent fault segments. Greater discordance between slip directions and intersection lines of fault segments results in greater strain incompatibility. An internal fracture network generally evolves within a segment boundary to maintain compatibility and transfer slip between the segments. The dimensions and structure of this fracture network may also partly control rupture propagation. Presumably, activation of a fault network with large angular discordances between slip directions and intersection lines will generate numerous asperities as the subsidiary faults mutually interact and offset each other. The geometry of a segment boundary may change with depth and the three-dimensional nature of the structure may be important in controlling rupture history. Fluids influence rupturing via fluid-pressure effects and time-dependent chemical processes. Fracture propagation by stress corrosion may favor instability, and chemical alteration may produce minerals of lower strengths, allowing time-dependent creep. Sealing and healing of fractures, however, may remove damage and increase strength. Elementary computations indicate that representative times for sealing and chemical alteration are between 1 and 1000 years for reasonable physical conditions, well within the recurrence intervals of most large earthquakes. Time to failure for stress corrosion cracking is more highly variable and strongly sensitive to applied stress and fluid pressure.