Faulting processes in active faults – Evidences from TCDP and SAFOD drill core samples

doi:10.1016/j.jsg.2014.04.004

Journal of Structural Geology

Volume 65, August 2014, Pages 100-116

https://doi.org/10.1016/j.jsg.2014.04.004 Get rights and content

Highlights

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We characterize microstructures of SAFOD and TCDP samples.
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Substantial differences exist in the clay mineralogy of gouge samples.
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Amorphous material has been observed in SAFOD as well as TCDP samples.
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Clay-gouge fabrics are quite weak in SAFOD and TCDP samples.

Abstract

The microstructures, mineralogy and chemistry of representative samples collected from the cores of the San Andreas Fault drill hole (SAFOD) and the Taiwan Chelungpu-Fault Drilling project (TCDP) have been studied using optical microscopy, TEM, SEM, XRD and XRF analyses. SAFOD samples provide a transect across undeformed host rock, the fault damage zone and currently active deforming zones of the San Andreas Fault. TCDP samples are retrieved from the principal slip zone (PSZ) and from the surrounding damage zone of the Chelungpu Fault. Substantial differences exist in the clay mineralogy of SAFOD and TCDP fault gouge samples. Amorphous material has been observed in SAFOD as well as TCDP samples. In line with previous publications, we propose that melt, observed in TCDP black gouge samples, was produced by seismic slip (melt origin) whereas amorphous material in SAFOD samples was formed by comminution of grains (crush origin) rather than by melting. Dauphiné twins in quartz grains of SAFOD and TCDP samples may indicate high seismic stress. The differences in the crystallographic preferred orientation of calcite between SAFOD and TCDP samples are significant. Microstructures resulting from dissolution–precipitation processes were observed in both faults but are more frequently found in SAFOD samples than in TCDP fault rocks. As already described for many other fault zones clay-gouge fabrics are quite weak in SAFOD and TCDP samples. Clay-clast aggregates (CCAs), proposed to indicate frictional heating and thermal pressurization, occur in material taken from the PSZ of the Chelungpu Fault, as well as within and outside of the SAFOD deforming zones, indicating that these microstructures were formed over a wide range of slip rates.

Introduction

A key question of fault mechanics is related to the transition between seismic and aseismic slip. For example, many authors assume that the transition from aseismic creep at shallow depth to seismic slip occurs at depth between 5 and 15 km and has been attributed to the dehydration of smectite–illite (e.g. Hyndman et al., 1997, Saffer and Marone, 2003, Marone and Saffer, 2007). The lack of earthquakes at shallow depth can be related to the velocity-strengthening behavior of phyllosilicate-rich gouges (Imber et al., 2008, Ikari et al., 2011). However, the 2011 Tohoku megathrust earthquake has propagated through the shallow part of the subduction zone to the sea-floor surface (Ozawa et al., 2011, Kodaira et al., 2012).

Even though the analysis of fresh fault rocks has received increasing attention, physical mechanisms governing co-seismic slip and aseismic creep remain still elusive due to limited access of microstructures and their relation to co-seismic and aseismic slip. As a consequence of this gap was that several international fault zone drilling projects have started to address fundamental questions about physical and chemical processes controlling faulting and earthquake generation (e.g. Hickman et al., 2004, Song et al., 2007, Zoback et al., 2007, Zoback et al., 2010, Boullier, 2011).

This paper is structured around a combined analysis of core samples from two drilling projects (San Andreas Fault Observatory at Depth/SAFOD; Taiwan Chelungpu-fault Drilling project/TCDP). Both allow to identify key similarities and differences between characteristic microstructures of slip zones recently formed by co-seismic processes in earthquake faults (TCDP) or by active creep (SAFOD). Thus, by examining microstructures in SAFOD and TCDP core samples we may clarify which processes control and trigger creep processes and seismic slip events, respectively. Furthermore we expect that our comparative study of gouge/ultracataclastic material will provide more general insights into key structural features of slip zones controlling the physical and chemical conditions of fault zone deformation. In order to meet this objective, we represent a survey of available data combined with new investigations.

The focus of our study is the comparison of fault rock composition and dominant microstructures (e.g. amorphous material/melt, brittle fracturing, dissolution–precipitation processes, intracrystalline plasticity, clay fabric) in SAFOD and TCDP core samples. To characterize the microstructures we will predominantly use transmission electron microscopy (TEM) and scanning electron microscopy (SEM) combined with focused ion beam techniques (FIB) (Janssen et al., 2010, Janssen et al., 2011). Further analytical techniques include X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses, cathodoluminescence (CL) optical microscopy and electron back-scatter diffraction (EBSD).

Section snippets

Methods

XRD and XRF were used to analyze fault rock composition. All samples were dried and ground to a fine powder before analysis. X-ray diffraction analyses were conducted on air dried oriented clay slides before and after treatment with ethylene glycol and heating at 400 °C. X-ray patterns for TCDP and SAFOD samples were collected using a Siemens D5000 powder diffractometer and a diffractometer XRD 3000 TT (Seifert), respectively. The diffraction data for the Siemens diffractometer were recorded

Structural setting of SAFOD and TCDP samples

The San Andreas Fault (SAF) as well as the Chelungpu-Fault intersect similar types of country rocks, however the fault zone character differs considerably between both faults, notably the very different width of the SAFOD creeping zones and the TCDP principal slip zone, respectively (Table 1). The wider fault gouges of the SAF are likely caused by the cumulative displacement, which is much greater along the SAF than along the Chelungpu fault (Zoback et al., 2010).

Fault rock composition

The composition of SAFOD and TCDP samples is described extensively in several publications (SAFOD: e.g. Moore and Rymer, 2012, Holdsworth et al., 2011, Bradbury et al., 2011, Mittempergher et al., 2011; TCDP: Ye et al., 2007, Sone et al., 2007, Ishikawa et al., 2008, Boullier et al., 2009). Here we describe selected SAFOD and TCDP samples to illustrate similarities and differences in fault rock composition. The major element data of the TCDP samples were taken from Ishikawa et al. (2008).

Fault rock composition

The host rock composition of the San Andreas Fault zone is comparable with the Chelungpu fault. Both faults cut through a sedimentary sequence built up from alternating silts and shales. Differences in the clay-gouge composition of SAFOD and TCDP samples as revealed in XRD and XRF data may be attributed to different sample depths, and temperatures and, possibly, contrasting faulting regimes (Table 1). Moore and Rymer (2012) suggest that the Mg-bearing clays in the gouges of SAFOD samples from

Conclusions

New microstructural observations from SAFOD and TCDP core samples illustrate that in spite of differences in sample depth/temperature and faulting regime samples from both faults reveal amazing similarities. The use of TEM and SEM combined with focused ion beam sample preparation has been especially powerful for identification of microstructures down to the nm scale. The observations suggest processes such as pressure-solution creep, crystal plastic deformation, and diagenetic crystallization

Acknowledgments

Diane E. Moore and an anonymous reviewer provided very constructive comments and suggestions that helped to improve this paper. We also would like to thank Andreas Hendrich for help with drafting the figures, Stefan Gehrmann for sample preparation, and Anja Schreiber for TEM foil preparation using FIB technique. A special thanks is due to John Firth, Phil Rumford, Bradley Wymer and Molly Chamberlin for providing samples. This work was supported by Deutsche Forschungsgemeinschaft grant JA 573/4-1

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