Browsing by Subject "San Andreas Fault"
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Item Frictional Strength of the Creeping Segment of the San Andreas Fault(2012-02-14) Coble, Clayton GageThe San Andreas Fault (SAF) near Parkfield, CA moves by a combination of aseismic creep and micro-earthquake slip. Measurements of in situ stress orientation, stress magnitude, and heat flow are incompatible with an average shear stress on the SAF greater than approximately 20 MPa. To investigate the micro-mechanical processes responsible for the low strength and creeping behavior, gouge samples from the 3 km-deep scientific borehole near Parkfield (the San Andreas Fault Observatory at Depth, SAFOD) are sheared in a triaxial rock deformation apparatus at conditions simulating those in situ, specifically a temperature of 100?C, effective normal stress of 100 MPa, pore fluid pressure of 25 MPa, and a Na-Ca-K pore fluid chemistry. The 2 mm-thick gouge layers are sheared to 4.25 mm at shear rates of 6.0, 0.6, 0.06, and 0.006 mu m/s. The mechanical data are corrected for apparatus effects and the strength of the jacketing material that isolates the sample from the confining fluid. Experiments indicate that gouge is extremely weak with a coefficient of friction of 0.14, and displays velocity and temperature strengthening behavior. The frictional behavior is consistent with the inferred in situ stress and aseismic creep observed at SAFOD. The low frictional strength likely reflects the presence of a natural fabric characterized by microscale folia containing smectite and serpentinite.Item Grain-scale Comminution and Alteration of Arkosic Rocks in the Damage Zone of the San Andreas Fault at SAFOD(2012-02-14) Heron, BretaniSpot core from the San Andreas Fault Observatory at Depth (SAFOD) borehole provides the opportunity to characterize and quantify damage and mineral alteration of siliciclastics within an active, large-displacement plate-boundary fault zone. Deformed arkosic, coarse-grained, pebbly sandstone, and fine-grained sandstone and siltstone retrieved from 2.55 km depth represent the western damaged zone of the San Andreas Fault, approximately 130 m west of the Southwest Deforming Zone (SDZ). The sandstone is cut by numerous subsidiary faults that display extensive evidence of repeating episodes of compaction, shear, dilation, and cementation. The subsidiary faults are grouped into three size classes: 1) small faults, 1 to 2 mm thick, that record an early stage of fault development, 2) intermediate-size faults, 2 to 3 mm thick, that show cataclastic grain size reduction and flow, extensive cementation, and alteration of host particles, and 3) large subsidiary faults that have cemented cataclastic zones up to 10 mm thick. The cataclasites contain fractured host-rock particles of quartz, oligoclase, and orthoclase, in addition to albite and laumontite produced by syn-deformation alteration reactions. Five structural units are distinguished in the subsidiary fault zones: fractured sandstones, brecciated sandstones, microbreccias, microbreccias within distinct shear zones, and principal slip surfaces. We have quantified the particle size distributions and the particle shape of the host rock mineral phases and the volume fraction of the alteration products for these representative structural units. Shape characteristics vary as a function of shear strain and grain size, with smooth, more circular particles evolving as a result of increasing shear strain. Overall, the particle sizes are consistent with a power law distribution over the particle size range investigated (0.3 ?m < d < 400 ?m). The exponent (fractal dimension, D) is found to increase with shear strain and volume fraction of laumontite. This overall increase in D and evolution of shape with increasing shear strain reflects a general transition from constrained comminution, active at low shear strains to abrasion processes that dominate at high shear strains.Item Mesoscale fracture fabric and paleostress along the San Andreas fault at SAFOD(2009-05-15) Almeida, Rafael VladimirSpot cores from Phase 1 drilling of the main borehole at the San Andreas Fault Observatory at Depth (SAFOD) were mapped to characterize the mesoscale structure and infer paleostress at depth. Cores were oriented by comparing mapped structures with image logs of the borehole. The upper core (1476-1484 m measured depth, MD) is a medium-grained, weakly foliated, hornblende-biotite granodiorite containing leucocratic phenocrysts and lenses. Principal structures are sub-vertical veins, shallow dipping shears, and natural fractures of unknown kinematics. The features are compatible with horizontal extension and right-lateral, normal, oblique-slip on faults striking approximately parallel to the SAF. The lower core (3055.6-3067.2 m MD) has massivebedded, pebble conglomerates and coarse to fine grained arkosic sandstones grade into siltstones. Principal structure features are conjugate shears and two minor faults. The fracture fabric is consistent with strike-slip faulting and a maximum principal compressive paleostress at ~80? to the SAF plane. This paleostress is essentially parallel to the current in situ stress measured in the main borehole and to paleostresses inferred from fracture fabrics in exhumed faults of the San Andreas system to the south. The similarity between the current state of stress and paleostress states supports the suggestion that the maximum principal compressive stress direction is, on average, at high angles to the SAF and that the fault has been weak over geologic time.Item Microphysical Controls on the Strength and Transport Properties of Fault Zones(2014-10-07) French, Melodie Ellen LyndsTwo studies on the mechanical properties of smectite-rich fault gouge collected from the Central Deforming Zone (CDZ) of the San Andreas Fault (SAF) in the San Andreas Fault Observatory at Depth (SAFOD) are presented. Rotary shear experiments were conducted at co-seismic slip rates (0.1 to 1 m/s). Displacement and dynamic weakening result from slip along clay-foliation assisted by shear-heating pressurization of pore fluid in wet gouge and additional grain-size reduction and possible clay dehydration in dry gouge. The results of a stability analysis show that microseismic patches within the CDZ should arrest at in-situ deformation conditions despite the documented weakening of the gouge. In some cases, however, weakening may be sufficient to sustain propagation of a rupture that nucleates within the adjacent locked segment into the CDZ. Stress-relaxation tests were also conducted on the CDZ gouge, and achieved strain-rates within an order of magnitude of in-situ creep (~ 10^-10 s^-1). The gouge is frictionally weak (< 0:15) at all conditions tested, and exhibits a change in the rate-determining mechanism at strain-rates below 10^-8 s^-1. A microphysical model for deformation of the CDZ gouge is hypothesized; the strength of the CDZ gouge is consistent with intergranular sliding whereby geometric obstacles deform by fracture/delamination and dislocation glide operating as parallel-concurrent mechanisms. At strain-rates greater than 10^-8 s^-1, fracture/delamination are the rate-controlling processes, and at lower rates, dislocation glide is rate-controlling. To understand the effects of stress and deformation on fluid flow through faulted rock, the permeability of faulted, damaged, and intact sandstone, was measured and mapped with respect to effective mean stress (15 to 100 MPa), differential stress (0 to 140 MPa), and proximity to frictional failure. The permeability and porosity of intact and faulted Punchbowl Formation Sandstone, a low-porosity (7 %) low-permeability (10^-18 m^2) altered arkosic sandstone, are strongly correlated with mean stress and insensitive to differential stress and proximity to the failure envelope. The fault core is a conduit with enhanced permeability. Porosity-permeability relations indicate that the microphysical controls of the stress-dependence are the same for intact and faulted rock despite higher microfracture densities and a localized fluid conduit.Item The Fabric of Clasts, Veins and Foliations within the Actively Creeping Zones of the San Andreas Fault at SAFOD: Implications for Deformation Processes(2012-02-14) Sills, David WayneRecovered core samples from the San Andreas Fault Observatory at Depth (SAFOD), located near Parkfield, CA, offer a unique opportunity to study the products of faulting and to learn about the mechanisms of slip at 3 km depth. Casing deformation reflects active creep along two strands of the San Andreas Fault (SAF) at SAFOD. The two fault strands are referred to as the Southwest Deforming Zone (SDZ) at 3194 m measured depth (MD) and the Central Deforming Zone (CDZ) at 3301 m MD. The SDZ and CDZ contain remarkably similar gouge layers, both of which consist of a clay-bearing, ultrafine grain matrix containing survivor clasts of sandstone and serpentinite. The two gouges have sharp boundary contacts with the adjacent rocks. We have used X-ray Computed Tomography (XCT) imaging, at two different sampling resolutions, to investigate the mesoscale and microscale structure of the fault zone, specifically to characterize the shape, preferred orientation, and size distribution of the survivor clasts. Using various image processing techniques, survivor clast shape and size are characterized in 3D by best-fit ellipsoids. Renderings of survivor clasts illustrate that survivor clasts have fine tips reminiscent of sigma type tails of porphyroclasts observed in myolonites. The resolution of the XCT imaging permits characterization of survivor clasts with equivalent spherical diameters greater than 0.63 mm. The survivor clast population in both the SDZ and CDZ gouge layers have similar particle size distributions (PSD) which fit a power law with a slope of approximately -3; aspect ratio (major to minor axis ratios) distributions also are similar throughout ranging between 1.5 and 4, with the majority occurring between 2-2.5. The volume- and shape- distributions vary little with position across the gouge zones. A strong shape preferred orientation (SPO) exists in both creeping zones. In both the SDZ and CDZ the minor axes form a SPO approximately normal to the plane of the San Andreas Fault (SAF), and the major axes define a lineation in the plane of the SAF. The observation that the size-, shape- and orientation-distributions of mesoscale, matrix-supported clasts are similar in the SDZ and CDZ gouge layers, and vary little with position in each gouge layer, is consistent with the hypothesis that aseismic creep in the SDZ and CDZ is achieved by distributed, shearing. The consistency between the SPO and simple-shear, strike-slip kinematics, and the marked difference of PSD, fabric, cohesion and clast lithology of the gouge with that of the adjacent rock, is consistent with the hypothesis that the vast majority of the shear displacement on the SAF at SAFOD is accommodated within the gouge layers and the gouge displays a mature, nearly steady-state structure.