Low-temperature deformation in calcite veins of SAFOD core samples (San Andreas Fault) — Microstructural analysis and implications for fault rheology
Highlights
► Microstructures of SAFOD core samples indicate intense shearing and dissolution–precipitation as main deformation processes. ► Calcite veins show evidence for twinning and crystal plasticity. ► Lower stress bound estimated from microstructure analysis agrees with stress estimates from borehole breakout measurements. ► Assuming hydrostatic pore pressure, the inferred friction coefficient is quite low. ► Microstructural analysis supports the hypothesis of a weak San Andreas Fault.
Introduction
Numerous geological and geophysical studies investigate the rheological/mechanical behavior of faults with respect to earthquake nucleation and the role of fluids in fault weakening (e.g., Brodsky et al., 2010, Chester and Logan, 1986, Evans and Chester, 1995, Fagereng et al., 2010, Fulton et al., 2009, Schulz and Evans, 2000). In this context, fault-related veins play a key role in understanding faulting processes and the analysis of veins has emerged as a useful tool to study the behavior of faults. The composition of veins and their deformation mechanisms may provide information about fluid sources, fluid circulation, pressure and temperature-conditions, chemical alteration processes and fault rheology in general (Gratier et al., 2003, Herwegh et al., 2005, Herwegh and Kunze, 2002, Janssen et al., 1998). In addition, the formation of syntectonic veins may indicate elevated fluid pressure during vein formation because local high fluid pressures are often required to open fractures (Mittempergher et al., 2011, Pollard and Segall, 1987, Wiltschko et al., 2009). The state of stress of the San Andreas Fault (SAF) has long been a matter of debate. Some authors have suggested the fault to be mechanically weak (e.g., Brune et al., 1969, Lachenbruch and Sass, 1980, Lachenbruch and Sass, 1992, Lockner et al., 2011, Townend and Zoback, 2004, Zoback et al., 1987) whereas others advocated for a strong fault (e.g., Scholz, 2000, Scholz and Hanks, 2004). It is assumed that a weak fault with a low friction coefficient (≤ 0.2) may be due to the presence of high pore pressures and/or serpentinite, talc, or clay minerals. Reducing stresses in the upper crust to a few tens of MPa is also required to explain the lack of increased heat flow along the trace of the SAF (e.g., Carpenter et al., 2009, Chéry et al., 2004, Collettini et al., 2009, Lachenbruch and Sass, 1980, Moore and Rymer, 2007, Tembe et al., 2009). For rocks with a friction coefficient ≥ 0.6, in accordance with laboratory results (Byerlee, 1978), stresses at depth will exceed 100 MPa for an optimally oriented fault with respect to the direction of the maximum principal stress, requiring a reorientation of the maximum principal stress close to the fault towards a more acute angle with the SAF than what is measured in the far field. A combination of low friction minerals, local overpressure and/or local stress variations may also hold at the SAF (e.g., Faulkner et al., 2006, Hardebeck and Michael, 2004).
Hickman and Zoback (2004) estimated the stress orientation and magnitude in the SAFOD pilot hole near Parkfield, California, down to about 2 km depth. The authors infer low differential stresses of about 60–70 MPa operating in the fault zone at ≈ 2.2 km depth based on borehole breakout data, but considerable uncertainties exist.
Here, we present a detailed microstructure analysis of calcite veins within samples from the SAFOD main borehole. First, we describe microstructures examined with optical and transmission electron microscopes (TEM) with the aim of providing information on fault evolution. Second, we interpret dislocation and twin densities measured in the calcite veins to arrive at stress estimates based on paleo-piezometric relationships. Finally we compare microscopic observations with lattice strain measurements on the same samples with synchrotron microfocus Laue diffraction.
Section snippets
Geological setting of the San Andreas Fault
Central California is geologically separated by the San Andreas Fault (SAF), which is a transform fault at the boundary between the western Pacific plate and the eastern North American Plate. The SAFOD drill site is located at the transition between the creeping Parkfield segment in the North and the locked segment of the SAF to the South. Near the drill site arkosic sedimentary rocks predominate at the southwest of the fault and Great Valley sedimentary rocks northeast of the fault (Springer
Description of samples
We analyzed the microstructures of four samples (S1–S4) obtained from SAFOD phase III cores (for a detailed description of cores see also Photographic Atlas of the SAFOD Phase 3 Cores 2007, URL http://www.earthscope.org/data/safod_core_viewer). The samples, which are described in detail by Janssen et al., 2010, Janssen et al., 2011, were recovered from different core sections located close to or at small distance to the zones of active deformation. Sample S1 is from the arkosic sedimentary rock
Analytical techniques
In this study we focus on the microstructures of the calcite veins contained in the 4 samples investigated. We quantify density of calcite twins and the density of dislocations within the calcite grains to arrive at an estimate of the paleo-stresses governing deformation of the gouge during and after vein formation. In addition, stresses are estimated using residual strain analysis.
Microscopic description of calcite veins and twin densities
Veins within the four SAFOD core samples are composed of calcite. Density of calcite veins progressively increases toward the active fault trace suggesting that the veins formed during or after faulting. Using the cathodoluminescence (CL)-microscope reveals uniform yellow to orange CL-colors for all calcite veins (Fig. 3). The homogeneous CL pattern in the vein cements corroborate the lack of fluid pulses into the fault rocks, since with every fluid pulse (for example meteoric water) the
Discussion
Analysis of the four SAFOD gouge samples revealed a rather complex microstructure with evidence for intense cataclastic deformation, activity of solution–precipitation creep processes, high microporosity, and lubricating amorphous phases (Janssen et al., 2010, Janssen et al., 2011). Based on the analysis of dislocation and twin densities of calcite grains within the veins, we can estimate the flow stress in gouge samples using paleo-piezometric relationships. We are using the dislocation
Acknowledgments
We thank Stefan Gehrmann for thin section preparation, Anja Schreiber for TEM foil preparation, David Seydewitz for counting dislocation densities, and Manuel Kienast for discussions. Access to beamline 12.3.2. at ALS and help from Martin Kunz is gratefully acknowledged, as well as the SAFOD science team for providing samples. CJ was partly funded by DFG grant JA 573/4-1. HRW is appreciative for support through NSFEAR-0836402 and DOE. We are also thankful for the thoughtful reviews of Marco
References (87)
Calcite twins, their geometry, appearance and significance as stress–strain markers and indicators of tectonic regime: a review
J. Struct. Geol.
(1993)- et al.
Strength characteristics of the r, f, and c slip systems in calcite
Tectonophys.
(1997) - et al.
Cooling induced strain localization in carbonate mylonites within a large-scale shear zone (Glarus thrust, Switzerland)
J. Struct. Geol.
(2007) Critical re-evaluation of differential stress estimates from calcite twins in coarse-grained limestone
Tectonophys.
(1998)- et al.
A critical evaluation of crustal dehydration as the cause of an overpressured and weak San Andreas Fault
Earth Planet. Sci. Lett.
(2009) - et al.
The influence of nano-scale second-phase particles on deformation of fine-grained calcite mylonites
J. Struct. Geol.
(2002) - et al.
Combining natural microstructures with composite flow laws: an improved approach for the extrapolation of lab data to nature
J. Struct. Geol.
(2005) - et al.
Mechanisms of weak phase interconnection and the effects of phase strength contrast on fabric development
J. Struct. Geol.
(2006) - et al.
Fluid regime in faulting deformation of the Waratah Fault Zone, Australia, as inferred from major and minor element analyses and stable isotopic signatures
Tectonophys.
(1998) - et al.
Nanoscale porosity in SAFOD core samples (San Andreas Fault)
Earth Planet. Sci.
(2011)
Processes controlling the shrinkage of porphyroclasts in gabbroic shear zones
J. Struct. Geol.
Stress gradients around porphyroclasts palaeopiezometric estimates and numerical modeling
J. Struct. Geol.
A theory for granular flow accommodated by material transfer via intergranular fluid
Tectonophys.
Rock deformation tests to large shear strains in torsion
J. Struct. Geol.
Theoretical displacements and stresses near fractures in rock with application of faults, joints, veins dikes, and solution surfaces
Palaeostress estimation using calcite twinning: experimental calibration and application to nature
J. Struct. Geol.
High temperature flow and dynamic recrystallization in Carrara marble
Tectonophys.
Mesoscopic structure of the Punchbowl Fault, Southern California and the geological and geophysical structure of active strike-slip faults
J. Struct. Geol.
A superbend X-ray microdiffraction beamline at the advanced light source
Mat. Sci. Eng. A
Conditions during syntectonic vein formation in the footwall of Absaroka Thrust Fault, Idaho–Wyoming–Utah fold and thrust belt
J. Struct. Geol.
Focused Ion Beam (FIB) combined with SEM and TEM: advanced analytical tools for studies of chemical composition, microstructure and crystal structure in geomaterials on a nanometre scale
Chem. Geol.
The microstructure of experimentally deformed limestones
J. Mat. Sci.
Defects in deformed calcite and carbonate rocks
Deformation twinning in calcite, dolomite, and other rhombohedral carbonates
Phys. Chem. Min.
Ueber kuenstliche Kalkspath-Zwillinge nach − 1/2 R
Zeitschrift Krystallogr
Deformation microstructures and mechanisms in minerals and rocks
Microscale anatomy of the 1999 Chi-chi earthquake fault zone
Geochem. Geophys. Geosystems
Mineralogical and textural analyses of drill cuttings from the San Andreas Fault Observatory at Depth (SAFOD) boreholes: initial interpretations of fault zone composition and constraints on geological models
Geosphere
Drilling into faults quickly after earthquakes
EOS Trans. AGU
Heat flow, stress, and rate of slip along the San Andreas Fault, California.
J. Geophys. Res.
Friction of rocks
Pure Appl. Geophys.
Frictional behavior of materials in the 3D SAFOD volume
Geophys. Res. Lett.
Weakness of the San Andreas Fault revealed by samples from the active fault zone
Nature Geosci.
A mechanical model of the San Andreas Fault and SAFOD Pilot Hole stress measurements
Geophys. Res. Lett.
Elasticity of single-crystal calcite and rhodochrosite by Brillouin spectroscopy
Am. Mineral.
Deformation twinning and residual stress in calcite studied with synchrotron polychromatic X-ray microdiffraction
Phys. Chem. Minerals
Implication for mechanical properties of brittle faults from observations of Punchbowl fault zone, California
Pure Appl. Geophys
Fault zone fabric and fault weakness
Nature
Steady state dislocation densities in experimentally deformed calcite materials: single crystals versus polycrystals
J. Geophys. Res.
Fluid–rock interaction in faults of the San Andreas system: inferences from San Gabriel fault rock geochemistry
J. Geophys. Res.
Shear veins observed within anisotropic fabric at high angles to the maximum compressive stress
Nature Geosci.
Slip on ‘weak’ faults by the rotation of regional stress in the fracture damage zone
Nature
Crystallographic tables for the rhombohedral carbonates
Am. Mineral.
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2022, Journal of Structural GeologyCitation Excerpt :Vice versa, mean twin density, D (i.e. number of lamellae of a given twin set with respect to grain diameter) increases up-section (Fig. 4; Fig. 5d), ranging from 28 to 48 (data normalized to 1 mm). Mean twin density in calcite is mainly a function of the differential stress (Rowe and Rutter, 1990; Rybacki et al., 2011, 2013). For paleostress analysis, we excluded sample D21-10-81 from our dataset, as straight grain boundaries were interpreted as due to annealing or post-kinematic recrystallization and it was not really possible to infer if annealing occurred also after twinning.