The Cajon Pass Scientific Drilling Experiment: Overview of phase 1

Zoback, Mark D.; Silver, Leon T.; Henyey, Thomas L.; Thatcher, Wayne

doi:10.1029/gl015i009p00933

Cited by 38 publications

(8 citation statements)

References 17 publications

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“…The discovery that there is no heat flux anomaly across the creeping segment [Brune et al, 1969;Lachenbruch and Sass, 1980] indicates that this fault slides with a much lower shear stress than that predicted by laboratory friction experiments [Byerlee, 1978]. This discovery, popularly known as the San Andreas stress-heat flow paradox [Hanks and Raleigh, 1980;Lachenbruch and Sass, 1992;Zoback et al, 1988], has raised at a very early stage the question of potential mechanisms that can produce creep, without heat production. Moreover, Mount and Suppe [1987] and Zoback et al [1987] provided evidence showing that the orientation of the maximum horizontal compressive stress is at a high angle to the fault.…”

Section: Geological Setting and General Featuresmentioning

confidence: 99%

Rock and mineral transformations in a fault zone leading to permanent creep: Interactions between brittle and viscous mechanisms in the San Andreas Fault

et al. 2014

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Creep processes may relax part of the tectonic stresses in active faults, either by continuous or episodic processes. The aim of this study is to obtain a better understanding of these creep mechanisms and the manner in which they change in time and space. Results are presented from microstructural studies of natural samples collected from San Andreas Fault Observatory at Depth borehole drilled through the San Andreas Fault, which reveal the chronology of the deformation within three domain types. (i) A relatively undeformed zone of the host rock reflects the first step of the deformation process with fracturing and grain indentations showing the coupling between fracturing and pressure solution. (ii) Shear deformation development that associates fracturing and solution cleavage processes leads to profound changes in rock composition and behavior with two types of development depending on the ratio between the amount of dissolution and deposition: abundant mineral precipitation strengthens some zones while pervasive dissolution weakens some others, (iii) zones with mainly dissolution trended toward the present-day creeping zones thanks to both the passive concentration of phyllosilicates and their metamorphic transformation into soft minerals such as saponite. This study shows how interactions between brittle and viscous mechanisms lead to widespread transformation of the rocks and how a shear zone may evolve from a zone prone to earthquakes and postseismic creep to a zone of steady state creep. In parallel, the authors discuss how the creeping mechanism, mainly controlled by the very low friction of the saponite in the first 3-4 km depth, may evolve with depth.

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Section: Geological Setting and General Featuresmentioning

confidence: 99%

Rock and mineral transformations in a fault zone leading to permanent creep: Interactions between brittle and viscous mechanisms in the San Andreas Fault

et al. 2014

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“…The drill hole is located within a structurally complex area with many of the structures clearly predating the San Andreas fault [Meisling and Weldon, 1989]. Although research at Cajon Pass was primarily designed to investigate present-day aspects of the San Andreas fault zone, in particular the "heat flow stress paradox" [Zoback et al, 1988], the project has also yielded much information on the pre-San Andreas fault geology of southern California [e.g., . Core material from the drill hole has been examined for brittle structures and microstructures in this study.…”

Section: Introductionmentioning

confidence: 99%

Aseismic fracturing and cataclasis involving reaction softening within core material from the Cajon Pass Drill Hole

Blenkinsop¹,

Sibson²

1992

J. Geophys. Res.

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Cataclastic deformation features in crystalline rocks from the Cajon Pass drill hole, located some 4 km NE of the San Andreas fault trace in southern California, appear to have developed at slow strain rates. There is no clear evidence of seismic deformation. Most of the observed structures and microstructures are inferred to have formed during pre‐Pliocene distributed deformation, before the San Andreas fault became active in this area. Extension fractures are filled by laumontite which increases in grain size from a fine amorphous habit to coarser prismatic crystals unidirectionally across fractures and does not generally display crack‐seal textures. Fragments within fractures were derived from adjacent grains without rotation or shear. Fracture opening and cementation were therefore synchronous and slow. Fluids were present at all times during fracture opening, but there was no repeated hydrofracturing after fracture formation. However pore fluid pressures probably stayed close to the least principal stress (σ3) during subsequent fracture growth. Plagioclase compositions in all fractured areas change from oligoclase to albite or anorthite as a consequence of albitization and laumontization. Particle size distributions in dilational areas, and in extension and in shear fracture fillings, show that alteration of the major phase, plagioclase, is the fundamental process of grain size reduction in a variety of rock types. Cataclastic stresses and strain rates were controlled by alteration reaction rates in a coupled process that is a form of transformation‐modified deformation. Fracturing leading to weakening was assisted by stresses due to the 60% volume increase accompanying in situ laumontization of plagioclase and by stress corrosion and subcritical crack growth facilitated by alteration. Deformation was also enhanced by replacement of plagioclase by weaker laumontite. Such dilatant cataclasis is consistent with fluid pressure levels having remained high during deformation with effective least principal compressive stress close to zero. This transformation‐modified deformation at low temperatures (90° < T < 250°C) may be a common process in feldspar‐rich rocks. The structures and microstructures described here could be used to distinguish the products of slow and fast Cataclastic deformation.

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“…of the overall scientific objectives of this project is given byZoback et al [1988] and Silver and James[1988]. The stress state at Cajon Pass is transitional between strike-slip and normal faulting[Healy and Zoback, 1988;Weldon and Springer, 1988;Zoback and Healy, 1992].…”

mentioning

confidence: 99%

Stress perturbations associated with active faults penetrated by boreholes: Possible evidence for near‐complete stress drop and a new technique for stress magnitude measurement

Barton¹,

Zoback²

1994

J. Geophys. Res.

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201

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Detailed studies of stress-induced wellbore breakouts in wells drilled through active faults reveal stress field discontinuities that are apparently associated with recent fault movements. These discontinuities are expressed as localized rotations in wellbore breakout orientation in the vicinity of the fault penetrated by the borehole. This phenomenon is observed in a variety of tectonic environments and rock types. Utilizing cases where relatively complete knowledge of the horizontal principal stresses is available from in situ measurements, we use three-dimensional dislocation modeling to demonstrate that these discontinuities can be explained as the superposition of a reference stress state and a perturbation caused by movement on preexisting faults. Case studies from normal, strike-slip and reverse faulting stress states indicate that nearly complete stress drop is required to match the observed breakout orientation anomalies. Hydraulic fracturing data independently confirm the occurrence of nearcomplete stress drop on some faults penetrated by drilling. Modeling of the observed interactions between breakouts and fractures can also be used to obtain information about the magnitude of in situ stress. ! ' I BARTON AND ZOBACK: STRESS PERTURBATIONS AT ACTIVE FAULTS 9375 AND ZOBACK: STRESS PERTURBATIONS AT ACTIVE FAULTS 9383 ods, lnt. J. Rock Mech. Min. $ci. Geomech. Abstr., 26(6), 461-469, 1989. Baumgfirtner, J., F. Rummel, and M.D. Zoback, Hydraulic fracturing in situ stress measurements to 3 km depth in the KTB pilot hole VB, KTB Rep. 90-6a, edited by K. Bram, J. K. Draxler, W. Kessels, and G. Zoth, Niedersfichsisches Landesamt far Bodenforschung, Hannover, 1990.

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The Cajon Pass Scientific Drilling Experiment: Overview of phase 1

Cited by 38 publications

References 17 publications

Rock and mineral transformations in a fault zone leading to permanent creep: Interactions between brittle and viscous mechanisms in the San Andreas Fault

Rock and mineral transformations in a fault zone leading to permanent creep: Interactions between brittle and viscous mechanisms in the San Andreas Fault

Aseismic fracturing and cataclasis involving reaction softening within core material from the Cajon Pass Drill Hole

Stress perturbations associated with active faults penetrated by boreholes: Possible evidence for near‐complete stress drop and a new technique for stress magnitude measurement

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