Diane E. Moore

Talc-bearing serpentinite and the creeping section of the San Andreas fault.

The section of the San Andreas fault located between Cholame Valley and San Juan Bautista in central California creeps at a rate as high as 28 mm yr-1 (ref. 1), and it is also the segment that yields the best evidence for being a weak fault embedded in a strong crust. Serpentinized ultramafic rocks have been associated with creeping faults in central and northern California, and serpentinite is commonly invoked as the cause of the creep and the low strength of this section of the San Andreas fault. However, the frictional strengths of serpentine minerals are too high to satisfy the limitations on fault strength, and these minerals also have the potential for unstable slip under some conditions. Here we report the discovery of talc in cuttings of serpentinite collected from the probable active trace of the San Andreas fault that was intersected during drilling of the San Andreas Fault Observatory at Depth (SAFOD) main hole in 2005. We infer that the talc is forming as a result of the reaction of serpentine minerals with silica-saturated hydrothermal fluids that migrate up the fault zone, and the talc commonly occurs in sheared serpentinite. This discovery is significant, as the frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on the shear strength of the fault, and its inherently stable sliding behaviour is consistent with fault creep. Talc may therefore provide the connection between serpentinite and creep in the San Andreas fault, if shear at depth can become localized along a talc-rich principal-slip surface within serpentinite entrained in the fault zone.

The effect of mineral bond strength and adsorbed water on fault gouge frictional strength

Recent studies suggest that the tendency of many fault gouge minerals to take on adsorbed or interlayer water may strongly influence their frictional strength. To test this hypothesis, triaxial sliding experiments were conducted on 1 5 different single-mineral gouges with various water-adsorbing affinities. Vacuum dried samples were sheared at 100 MPa, then saturated with water and sheared farther to compare dry and wet strengths. The coefficients of friction, µ, for the dry sheet- structure minerals (0.2-0.8), were related to mineral bond strength, and dropped 20-60% with the addition of water. For non-adsorbing minerals (µ =0.6-0.8), the strength remained unchanged after saturation. These results confirm that the ability of minerals to adsorb various amounts of water is related to their relative frictional strengths, and may explain the anomalously low strength of certain natural fault gouges.

The role of microcracking in shear-fracture propagation in granite

Microcracking related to the formation of a laboratory shear fracture in a cylinder of Westerly granite has been investigated using image-analysis computer techniques. Well away from the fracture (farfield), the deformed granite has about twice the crack density (crack length per unit area) of undeformed granite. The microcrack density increases dramatically in a process zone that surrounds the fracture tip, and the fracture tip itself has more than an order of magnitude increase in crack density over the undeformed rock. Microcrack densities are consistently higher on the dilational side of the shear than on the compressional side. Microcracks in the undeformed rock and in the far-field areas of the laboratory sample are concentrated within and along the margins of quartz crystals, but near the shear fracture they are somewhat more abundant within K-feldspar crystals. The energy release rate, gII, for mode II fracture progagation is estimated from the microcrack density data to be ≥ 1.7–8.6 kJ m−2. The microcracks that formed during the experiment are principally tensile cracks whose orientations reflect the local stress field: those formed prior to the nucleation of the fault are roughly parallel to the cylinder axis (loading direction), whereas those generated in the process zone make angles averaging 30 ° to the overall fault strike (and 20 ° to the cylinder axis). The preferred orientation and uneven distribution of microcracks in the process zone tends to pull the propagating fracture tip towards the dilational side, even though the trend is away from the overall fault strike. As a result, the propagating shear follows the microcrack trend for some distance and then changes direction in order to maintain an overall in-plane propagation path. This recurring process produces a zig-zag or sawtooth segmentation pattern similar to the sawtooth geometries of faults such as the San Andreas fault.

Low strength of deep San Andreas fault gouge from SAFOD core

The San Andreas fault accommodates 28–34 mm yr−1 of right lateral motion of the Pacific crustal plate northwestward past the North American plate. In California, the fault is composed of two distinct locked segments that have produced great earthquakes in historical times, separated by a 150-km-long creeping zone. The San Andreas Fault Observatory at Depth (SAFOD) is a scientific borehole located northwest of Parkfield, California, near the southern end of the creeping zone. Core was recovered from across the actively deforming San Andreas fault at a vertical depth of 2.7 km (ref. 1). Here we report laboratory strength measurements of these fault core materials at in situ conditions, demonstrating that at this locality and this depth the San Andreas fault is profoundly weak (coefficient of friction, 0.15) owing to the presence of the smectite clay mineral saponite, which is one of the weakest phyllosilicates known. This Mg-rich clay is the low-temperature product of metasomatic reactions between the quartzofeldspathic wall rocks and serpentinite blocks in the fault. These findings provide strong evidence that deformation of the mechanically unusual creeping portions of the San Andreas fault system is controlled by the presence of weak minerals rather than by high fluid pressure or other proposed mechanisms. The combination of these measurements of fault core strength with borehole observations yields a self-consistent picture of the stress state of the San Andreas fault at the SAFOD site, in which the fault is intrinsically weak in an otherwise strong crust.

Reduction of Permeability in Granite at Elevated Temperatures

The addition of hydrothermal fluids to heated, intact granite leads to permeability reductions in the temperature range of 300� to 500�C, with the rate of change generally increasing with increasing temperature. The addition of gouge enhances the rate of permeability reduction because of the greater reactivity of the fine material. Flow rate is initially high in a throughgoing fracture but eventually drops to the level of intact granite. These results support the fault-valve model for the development of mesothermal ore deposits, in which seals are formed at the base of the seismogenic zone of high-angle thrust faults. The lower temperature results yield varying estimates of mineral-sealing rates at shallower depths in fault zones, although they generally support the hypothesis that such seals develop in less time than the recurrence interval for moderate to large earthquakes on the San Andreas fault.

Permeability reduction in granite under hydrothermal conditions

The formation of impermeable fault seals between earthquake events is a feature of many models of earthquake generation, suggesting that earthquake recurrence may depend in part on the rate of permeability reduction of fault zone materials under hydrothermal conditions. In this study, permeability measurements were conducted on intact, fractured, and gouge-bearing Westerly granite at an effective pressure of 50 MPa and at temperatures from 150° to 500°C, simulating conditions in the earthquake-generating portions of fault zones. Pore fluids were cycled back and forth under a 2 MPa pressure differential for periods of up to 40 days. Permeability of the granite decreased with time t, following the exponential relation k=c(10−rt). For intact samples run between 250° and 500°C the time constant for permeability decrease r was proportional to temperature and ranged between 0.001 and 0.1 days−1 (i.e., between 0.4 and 40 decades year−1 loss of permeability). Values of r for the lower-temperature experiments differed little from the 250°C runs. In contrast, prefractured samples showed higher rates of permeability decrease at a given temperature. The surfaces of the fractured samples showed evidence of dissolution and mineral growth that increased in abundance with both temperature and time. The experimentally grown mineral assemblages varied with temperature and were consistent with a rock-dominated hydrothermal system. As such mineral deposits progressively seal the fractured samples, their rates of permeability decrease approach the rates for intact rocks at the same temperature. These results place constraints on models of precipitation sealing and suggest that fault rocks may seal at a rate consistent with earthquake recurrence intervals of typical fault zones.

Strength of chrysotile-serpentinite gouge under hydrothermal conditions: Can it explain a weak San Andreas fault?

Chrysotile-bearing serpentinite is a constituent of the San Andreas fault zone in central and northern California. At room temperature, chrysotile gouge has a very low coefficient of friction (μ ≈ 0.2), raising the possibility that under hydrothermal conditions μ might be reduced sufficiently (to ≤0.1) to explain the apparent weakness of the fault. To test this hypothesis, we measured the frictional strength of a pure chrysotile gouge at temperatures to 290 °C and axial-shortening velocities as low as 0.001 μm/s. As temperature increases to ≈ 100 °C, the strength of the chrysotile gouge decreases slightly at low velocities, but at temperatures ≥200 °C, it is substantially stronger and essentially independent of velocity at the lowest velocities tested. We estimate that pure chrysotile gouge at hydrostatic fluid pressure and appropriate temperatures would have shear strength averaged over a depth of 14 km of 50 MPa. Thus, on the sole basis of its strength, chrysotile cannot be the cause of a weak San Andreas fault. However, chrysotile may also contribute to low fault strength by forming mineral seals that promote the development of high fluid pressures.

Sliding behavior and deformation textures of heated illite gouge

Abstract The run products of a series of triaxial friction experiments on an illite-rich gouge have been examined petrographically to study the relationship between textural development and sliding mode. The samples show a complete range of textures, from ones in which the entire gouge layer is deformed to ones in which the deformation is concentrated along narrow subsidiary shears and the rest of the gouge layer is massive. The samples with a pervasively developed deformation fabric slide stably, whereas the samples containing shear bands show stick-slip motion if the intersection angles between boundary-parallel and cross-cutting (Riedel) shears are also relatively high. These textural differences suggest that localization of shear combined with higher-angle Riedel shears are somehow involved in stick-slip motion. The orientation of Riedel-type shears in natural fault zones may also have potential as a paleoseismological tool.

Comparative Deformation Behavior of Minerals in Serpentinized Ultramafic Rock: Application to the Slab-Mantle Interface in Subduction Zones

The layer-structure minerals serpentine, brucite, and talc are postulated to form in the mantle wedge above a subducting slab as a result of progressive hydration and silica metasomatism. Tectonic mixing at the slab-mantle interface generates serpentinite mélanges that contain blocks of high-pressure (HP) or ultrahigh-pressure (UHP) metamorphic rock derived from the subducting slab. Such serpentinite mélanges may provide a means of exhumation of HP/UHP metamorphic rocks, and may define the lower limit of locked regions on the subduction interface that fail in large earthquakes. We review recently obtained frictional strength data for brucite and talc over the temperature range 25-400°C at 100 MPa effective normal stress and compare them with new data for antigorite. These minerals respond to heating in different ways, causing their frictional strengths to diverge. Water-saturated antigorite strength increases toward the fixed dry value of μ ≈ 0.75-0.80 with heating: μ ≈ 0.50 at 25°C and μ > 0.60 at 400°C. The difference in μ between dry and watersaturated talc gouge also decreases with increasing temperature, but both the dry and watersaturated values of μ are lower at elevated temperatures. For dry talc, μ decreases from 0.35 to 0.25 between 25° and 300°C, whereas for water-saturated talc, μ is approximately 0.20 at 25°C and 0.10-0.15 at elevated temperatures. Weakening of the interlayer bond of talc with heating may be responsible for the overall reduction in its frictional strength. The strength of dry brucite also is fixed at μ = 0.45-0.50, but the water-saturated value of μ decreases from ≈0.30 at 25°C to 0.20-0.25 at 200°-400°C. The water-saturated brucite gouge has extensively recrystallized along the shear surfaces, and its weakening may be attributable to solution-transfer processes. Because the serpentine minerals become stronger at elevated temperatures, to achieve low frictional strength in a serpentinized mantle wedge or serpentine-rich mélange would require some other cause such as nearly lithostatic fluid pressures or the addition of lower-strength minerals. Increasing abundance of brucite and talc in serpentinite at constant physical conditions would progressively reduce its frictional strength. The concentration of talc along a metasomatic front at the edge of the mantle wedge or in reaction zones surrounding HP/UHP blocks in serpentinite mélange should tend to localize shear in this extremely weak material.

The Coefficient of Friction of Chrysotile Gouge at Seismogenic Depths

We report new strength data for the serpentine mineral chrysotile at effective normal stresses, σn between 40 and 200 MPa in the temperature range 25°-280°C. Overall, the coefficient of friction, μ (= shear stress/effective normal stress) of water-saturated chrysotile gouge increases both with increasing temperature and σn, but the rates vary and the temperature-related increases begin at ~100°C. As a result, a frictional strength minimum (μ = 0.1) occurs at low σn at about 100°C. Maximum strength (μ = 0.55) results from a combination of high normal stress and high temperature. The low-strength region is characterized by velocity strengthening and the high-strength region by velocity-weakening behavior. Thoroughly dried chrysotile has μ = 0.7 and is velocity-weakening. The frictional properties of chrysotile can be explained by its tendency to adsorb large amounts of water that acts as a lubricant during shear. The water is progressively driven off the fiber surfaces with increasing temperature and pressure, causing chrysotile to approach its dry strength. Depth profiles for a chrysotile-lined fault constructed from these data would pass through a strength minimum at ~3 km depth, where sliding should be stable. Below that depth, strength increases rapidly as does the tendency for unstable (seismic) slip. Such a trend would not have been predicted from the room-temperature data. These results therefore illustrate the potential hazards of extrapolating room-temperature friction data to predict fault zone behavior at depth. This depth profile for chrysotile is consistent with the pattern of slip on the Hayward fault, which creeps aseismically at shallow depths but which may be locked below 5 km depth.