John M. Logan

Implications for mechanical properties of brittle faults from observations of the Punchbowl fault zone, California

Field observations of the Punchbowl fault zone, an inactive trace of the San Andreas, are integrated with results from experimental deformation of naturally deformed Punchbowl fault rocks for a qualitative description of the mechanical properties of the fault and additional information for conceptual models of crustal faulting. The Punchbowl fault zone consists of a single, continuous gouge layer bounded by zones of extensively damaged host rock. Fault displacements were not only localized to the gouge layer, but also to discrete shear surfaces within the gouge. Deformation in the exposure studied probably occurred at depths of 2 to 4 km and was dominated by cataclastic mechanisms. Textural data also suggest that significant amounts of pore fluids were present during faulting, and that fluid-assisted mechanisms, such as dissolution, diffusion, and precipitation, were operative.The experimental data on specimens collected from the fault zone suggest that there is a gradual decrease in strength and elastic modulus and an increase in relative ductility and permeability toward the main gouge zone. The gouge layer has fairly uniform mechanical properites, and it has significantly lower strength, elastic modulus, and permeability than both the damaged and the undeformed host rock.For the Punchbowl fault and possibly other brittle faults, the variations in loading of the gouge zone with time are primarily governed by the morphology of the fault and the mechanical properties of the damaged host rock. In addition, the damaged zone acts as the permeable unit of the fault zone and surrounding rock. It appears that the gouge primarily governs whether displacements are localized, and it therefore may have a significant influence on the mode of slip.

Composite planar fabric of gouge from the Punchbowl Fault, California

Abstract The structure of clay-bearing gouge from the Punchbowl Fault zone, a brittle fault of the San Andreas system in southern California, is examined at the microscopic scale. The right-lateral, oblique-slip fault consists of a single, continuous gouge zone bounded by a zone of damaged host-rock. The gouge has a statistically homogeneous, composite planar fabric that consists of a foliation defined by the preferred orientation of clay, porphyroclasts and compositional lamination, and a planar anisotropy defined by zones of localized high shear strain (shear-bands). The foliation appears to be related to the accumulation of finite strain and to rotate towards the shear-band orientation with an increase in shear strain. In the Punchbowl gouge the sense of shear on individual shear-bands, and the asymmetric disposition of the planar fabrics with respect to each other and to the boundaries of the zone, appear to be valid indicators of the overall sense and direction of shear. Local variation in preferred orientation of fabric elements exists in the gouge, and therefore statistically based sampling and analysis of the fabric is necessary to infer shear sense and direction.

Frictional dependence of gouge mixtures of quartz and montmorillonite on velocity, composition and fabric

Abstract The velocity of frictional sliding was studied experimentally using 35° -precut cylinders of Tennessee sandstone with a layer of saturated quartz-montmorillonite gouge along the precut. Experiments were performed at room temperature, confining pressures of 25, 50, and 70 MPa and displacement rates varying during the tests from 10−3 to 102 μm s −1 Frictional coefficients range from 0.08 to 0.14 for montmorillonite, increasing to 0.49 to 0.62 for pure quartz, with 70% or more of an end member controlling the coefficient. Mid-range compositions show monotonic changes with composition. Pure quartz gouge exhibits only stick-slip, however, with as little as 5% montmorillonite added, sliding is stabilized. Mixtures of coarse-grained quartz containing 15 to 50% montmorillonite strain harden, while other mixtures achieve steady-state sliding within the available displacement of 15 mm. Steady-state behavior is promoted by a decrease in quartz grain size and gouge thickness, as well as precompaction, large displacements, and long periods of time under a normal stress. Negative velocity dependency is shown by 100% quartz while all other gouges show a slight to pronounced positive velocity dependency. The latter show stable sliding, but gouges with 5% and 100% montmorillonite show incipient unstable behavior at the slowest rates. The characteristic sliding distance, dc, is found to be independent of gouge thickness, grain size and the magnitude of the velocity step, but does increase with slip velocity. Petrofabric analyses reveal well developed shear geometries and localization of displacement on Y surfaces in experiments that attained steady-state sliding. Experiments showing work hardening have few through-going fractures and relatively little grain-size reduction in the quartz. In gouge mixtures displacement is accommodated initially by the montmorillonite, but as the quartz grains rotate they come into contact initiating microfracturing. Concurrently strain hardening occurs. All experiments went through a change in fabric and work hardening before attaining steady-state sliding. Velocity history, gouge thickness and initial grain size are found to affect fabrics and the ability to attain steady-state sliding.

Chapter 2 Fabrics of Experimental Fault Zones: Their Development and Relationship to Mechanical Behavior

Abstract The fracture array of simulated fault zones is shown to evolve in a predictable and reproducible manner, from a stepwise fashion to a steady-state condition. At low confining pressures and increasing shear strain the sequence is: (1) Homogeneous shearing by grain-to-grain movements. (2) R 2 - and R 1 -fractures initiate at about the same time but propagate only a few grain diameters. They are at relatively high angles to the gouge-forcing block interface and widely spaced. These first two stages are one primarily of gouge compaction characterized by strain hardening. (3) Extension of R 1 S and coincident reorientation to lower angles closely paralleling the interface with the forcing blocks. P-fractures initiate. These occur from the ultimate strength through a strain softening stage. (4) Y-fractures form along which most of the displacement is accommodated, with the fracture array now close to steady state. Ys initially are close to one or both interfaces with the forcing blocks, but with increasing shear strain shift to the interior of the gouge. At this stage, sliding may change from stable slip to periodic oscillations, characteristic of stick-slip sliding. The development of the fracture array is interpreted to be the result of a reorientation of the stress field across and within the gouge zone. Riedel shears form in response to Coulomb failure, but Y-fractures appear as a result of the kinematic constraint produced by the more rigid bounding blocks. Modeling of the weak gouge zone within a stronger medium shows that the stress field may rotate to higher angles at the gouge boundaries. This is consistent with recent field observations. A significant implication is that without this recognition, laboratory values of frictional coefficients may be overestimated.

Effects of simulated clay gouges on the sliding behavior of Tennessee sandston

Abstract The effects of simulated fault gouge on the sliding behavior of Tennessee sandstone are studied experimentally with special reference to the stabilizing effect of clay minerals mixed into the gouge. About 30 specimens with gouge composed of pure clays, of homogeneously mixed clay and anhydrite, or of layered clay and anhydrite, along a 35° precut are deformed dry in a triaxial apparatus at a confining pressure of 100 MPa, with a shortening rate of about 5 · 10 −4 /sec, and at room temperature. Pure clay gouges exhibit only stable sliding, and the ultimate frictional strength is very low for bentonite (mont-morillonite), intermediate for chlorite and illite, and considerably higher for kaolinite. Anhydrite gouge shows violent stick-slip at 100 MPa confining pressure. When this mineral is mixed homogeneously with clays, the frictional coefficient of the mixed gouge, determined at its ultimate frictional strength, decreases monotonically with an increase in the clay content. The sliding mode changes from stick-slip to stable sliding when the frictional coefficient of the mixed clay-anhydrite gouge is lowered down below 90–95% of the coefficient of anhydrite gouge. The stabilizing effect of clay in mixed gouge is closely related to the ultimate frictional strength of pure clays; that is, the effect is conspicuous only for a mineral with low frictional strength. Only 15–20% of bentonite suppresses the violent stick-slip of anhydrite gouge. In contrast, violent stick-slip occurs even if the gouge contains as much as 75% of kaolinite. The behavior of illite and chlorite is intermediate between that of kaolinite and bentonite. Bentonite—anhydrite two-layer gouge exhibits stable sliding even when the bentonite content is only 5%. Thus, the presence of a thin, clay-rich layer in a fault zone stabilizes the behavior much more effectively than do the clay minerals mixed homogeneously with the gouge. This result brings out the mechanical significance of internal structures of a fault zone in understanding the effects of intrafault materials on the fault motion. Based on the present experimental results incorporated with some other experimental data, it is argued that although the stabilizing effect of montmorillonite and vermiculite is indeed remarkable at room temperature, the effect should be much less pronounced at elevated temperatures, due perhaps to the dewatering of the clays. In most geological environments where shallow earthquakes occur, the stabilizing effect of clays is probably not so conspicuous as to completely suppress the unstable motion of a fault.

The Sliding Characteristics of Sandstone on Quartz Fault-Gouge

SummaryThree types of triaxial compression experiments are used to characterize the frictional processes during sliding on quartz gouge. They are: 1) pre-cut Tennessee Sandstone sliding on an artificial layer of quartz gouge; 2) fractured Coconino Sandstone sliding along experimentally produced shear fractures; and 3) a fine-grained quartz aggregate deformed in compression. The specimens were deformed to 2.0 kb confining pressure at room temperature and displacement rates from 10−2 to 10−5 cm/sec dry and with water. There is a transition in sliding mode from stick-slip at confining pressures<0.7 kb to stable sliding at>0.7 kb. This transition is accompanied by a change from sliding at the sandstone-gouge contact (stick-slip) to riding on a layer of cataclastically flowing gouge (stable sliding). Quartz gouge between the pre-cut surfaces of Tennessee Sandstone lowers both the kinetic coefficient of friction and the magnitude of the stick-slip stress drops compared to those for a pre-cut surface alone. Stick-slip stress drops are preceded by stable sliding at displacements of 10−5 cm/sec. For a decrease in displacement rate between 10−3 and 10−5 cm/sec, stress-drops magnitudes increase from 25 to 50 bars. Tests on saturated quartz gouge show sufficient permeability to permit fluidpressure equilibrium within compacted gouge in 10 to 30 seconds; thus the principle of effective stress should hold for the fault zone with quartz gouge. Our results suggest that at effective confining pressures of less than 2.0 kb, if a fault zone contains quartz gouge, laboratory-type stick-slip can be an earthquake-source mechanism only if a planar sliding-surface develops, and then only when the effective confining pressure is less than 0.7 kb.

Effect of displacement rate on the real area of contact and temperatures generated during frictional sliding of Tennessee sandstone

SummaryThe real area of contact has been determined, and measurements of the maximum and average surface temperatures generated during frictional sliding along precut surfaces in Tennessee sand-stone have been made, through the use of thermodyes. Triaxial tests have been made at 50 MPa confining pressure and constant displacement rates of 10−2 to 10−6 cm/sec, and displacements up to 0.4 om. At 0.2 cm of stable sliding, the maximum temperature decreases with decreasing nominal displacement rate from between 1150° to 1175°C at 10−2 cm/sec to between 75° to 115°C at 10−3 cm/sec. The average temperature of the surface is between 75 and 115°C at 10−2 cm/sec, but shows no rise from room temperature at 10−3 cm/sec. At 0.4 cm displacement, and in the stick-slip mode, as the nominal displacement rate decreases from 10−3 to 10−6 cm/sec, the maximum temperature decreases from between 1120° to 1150°C to between 1040° to 1065°C. The average surface temperature is 115° to 135°C at displacement rates from 2.6×10−3 to 10−4 cm/sec.With a decrease in the displacement rate from 10−2 to 10−6 cm/sec, the real area of contact increases from about 5 to 14 percent of the apparent area; the avergge area of asperity contact increases from 2.5 to 7.5×10−4 cm2. Although fracture is the dominate mechanism during stick-up thermal softening and creep may also contribute to the unstable sliding process.

Influence of layering and boundary conditions on fault-bend and fault-propagation folding

The influence of heterogeneous layering and boundary conditions on the structural development of fault-bend and fault-propagation folds has been investigated through petrographic study of nonscaled rock models. The models are deformed in a triaxial rock-deformation apparatus at room temperature and a 50-MPa confining pressure. The models consist of a single layer of sandstone containing a saw-cut ramp that is inclined 20° to the layering, and an overlying, intact, thinly layered unit that is composed of limestone interlayered with lead or mica. Analysis of the fold-thrust structures generated in sequentially shortened models with different loading conditions and layer types suggests that the mode of fold-thrust interaction activated upon shortening will depend on fault zone drag, bending and shearing resistance of the hanging wall, shear strength of layer interfaces, and loading conditions. For the models, these parameters may be expressed as a strength ratio describing the resistance to foreland translation relative to the resistance to internal deformation of the thrust system. Low strength ratios favor fault-bend folding. High strength ratios favor internal shortening of the sheet; isotropic and thick (relative to ramp height) units above a propagating thrust tip will shorten primarily by faulting, whereas thinly layered, anisotropic units will shorten by fault-propagation folding. During both modes of fold-thrust interaction, the dips of the fold limbs increase, interlimb angles decrease, and imbricate faults form in the hanging wall or footwall with shortening. In one model suite, the imbrication is associated with a transition from fault-bend folding to fault-propagation folding and produces a highly asymmetric ramp anticline similar to a second-mode fault-bend fold or to a transported fault-propagation fold. The model data suggest that fault-propagation folding in heterogeneously layered rock occurs by the discontinuous formation, growth, and linkage of faults below the growing fold. Amplification of the fault-propagation fold is affected by the amount of slip transferred out of the deforming region, imbrication, and buckling. The changes in the mode of fold-thrust interaction and modifications in the local geometry and strain distribution that occur during shortening result from slip hardening on faults, or from rotation- or strain-induced variations in the strength of the layers.

Glass-Indurated Quartz Gouge in Sliding-Friction Experiments on Sandstone

Three types of glass-indurated quartzose gouge are recognized (optically and with SEM) along the sliding surfaces of 29° to 45° precut specimens of dry Tennessee Sandstone, deformed at 0.14- to 5.0-kb confining pressure, shortening rates of 10 to 10−4/sec, 24° to 410°C, and with displacements <1.0 cm. “Welded clumps” form in tests at 24° and ≤1.0-kb confining pressure. The welded portions of each clump are composed of quartz fragments indurated by an optically isotropic matrix (index of refraction of 1.500 to 1.520) that supports brittle fracture and is probably glass. As the temperature of the experiments is increased, “fibrous patches” become widespread. These patches are vesicular and conspicuously striated, they stand out in optical relief on top of quartz grains, they contain ordered microfractures that suggest extension along the sliding direction, and their “lee” edges are marked by very fine fibers that emanate from tapered bodies rooting in the patches. At high pressures and low temperatures, “welded plates” cover much of the sliding surfaces. These are fractured and striated, but nonfibrous. The glass indicates that local silica–fusion temperatures occurred during frictional sliding. The change in the nature of the gouge is accompanied by a change in sliding mode from stick-slip (25°C) to episodic (150° to 250°C) to stable sliding (410°C), and by a progressive increase in the coefficient of sliding friction from 0.58 at 25°C, to 0.72 at 410°C.

Experimental folding of rocks under confining pressure: Part III. Faulted drape folds in multilithologic layered specimens

Drape folds and reverse faults are produced experimentally at confining pressures to 2.0 kb and shortening rates of 10 −3 to 10 −6 sec −1 by displacing a block of brittle sandstone (2 by 3 by 12.6 cm) along a lubricated saw cut into one to five initially intact layers (0.2 to 1.0 cm thick and as much as 12.6 cm long) of limestone, sandstone, and rock salt. The saw cut is inclined at from 30° to 90° to the layer boundary. The deformation is characterized from studies of fault geometry, displacements and sequence, bedding-plane slip, layer-thickness changes, and the development of fault gouge, fold hinges, microfractures, calcite twin lamellae, and dimensional orientations of grains (in the rock salt). Stress trajectories are inferred from faults, microfractures, and calcite twin lamellae, and strains are calculated from layer-thickness changes and from calcite twin lamellae. Reverse faults curving concave downward propagate upward from the saw cut in the forcing block. With increasing displacement along the precut faults, the faults and associated gouge zones in the layer steepen and become progressively younger toward the upthrown block as displacement increases. The faults are preceded by swarms of extension microfractures that form throughout the deformation and that are the best clues to the stress trajectories. The downthrown layers are thickened by uniform flow and by repetition caused by the faulting. They are displaced away from the faults by bedding-plane slip. Trajectories of the greatest principal compressive stress (σ 1 ) are inclined at low angles to the layer boundaries near the faults and become perpendicular to these boundaries away from the fault. The maximum deformation of the downthrown block occurs when the saw cut is inclined at about 65° to the layering. The upthrown layers are all extended parallel to the layering and perpendicular to the fold axes, as indicated by extension fractures, thinned layers, and calcite twin lamellae and the development of graben zones and low-angle normal faults that are conjugate to the reverse faults. The layers are translated by bedding-plane slip away from the fault zone. Trajectories of σ 1 are inclined from 45° to 90° to the layering. The fabric data are internally consistent, and inferred stresses are in good agreement with those calculated from an elastic solution of the experimental boundary conditions. Principal strains calculated from calcite twin lamellae are within an average of 0.01 of those calculated from layer-thickness changes and permit clear resolution of individual events in domains of superposed deformations.