Elsevier

Tectonophysics

Volume 509, Issues 1–2, 1 August 2011, Pages 107-119
Tectonophysics

Low-temperature deformation in calcite veins of SAFOD core samples (San Andreas Fault) — Microstructural analysis and implications for fault rheology

https://doi.org/10.1016/j.tecto.2011.05.014Get rights and content

Abstract

The microstructures of four core samples from the San Andreas Fault Observatory at Depth (SAFOD) were investigated with optical and transmission electron microscopy. These samples, consisting of sandstone, siltstone, and fault gouge from phase III of the drilling campaign (3141–3307 m MD), show a complex composition of quartz, feldspar, clays, and amorphous material. Microstructures indicate intense shearing and dissolution–precipitation as main deformation processes. The samples also contain abundant veins filled with calcite. Within the inspected veins the calcite grains exhibit different degrees of deformation with evidence for twinning and crystal plasticity. Dislocation densities (ranging from  3 · 1012 m 2 to ≈ 3 · 1013 m 2) and twin line densities (≈ 22 mm 1–165 mm 1) are used as paleo-piezometers. The corresponding estimates of differential stresses vary between 33 and 132 MPa, deduced from dislocation density and 92–251 MPa obtained from twin density, possibly reflecting chronologically different maximum stress states and/or grain scale stress perturbations. Mean values of stress estimates are 68 ± 46 MPa and 168 ± 60 MPa, respectively, where estimates from dislocation density may represent a lower bound and those from twin density an upper bound. The stress estimates are also compatible with residual lattice strains determined with microfocus Laue diffraction yielding equivalent stresses of 50–300 MPa in twinned calcite. The lower stress bound agrees with stress estimates from borehole breakout measurements performed in the pilot hole. From these data and assuming hydrostatic pore pressure and a low intermediate principal stress close to the overburden stress, frictional sliding of the San Andreas Fault at the SAFOD site is constrained to friction coefficients between 0.24 and 0.31. These low friction values may be related to the presence of clays, talc, and amorphous phases found in the fault cores and support the hypothesis of a weak San Andreas Fault.

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

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