<<

Journal of Structural 106 (2018) 70–85

Contents lists available at ScienceDirect

Journal of

journal homepage: www.elsevier.com/locate/jsg

Microstructures in landslides in northwest China – Implications for creeping T displacements?

∗ M. Schäbitza, C. Janssena, , H.-R. Wenkb, R. Wirtha, B. Schucka, H.-U. Wetzela, X. Mengc, G. Dresena a GFZ German Research Centre for Geosciences, Telegrafenberg, Potsdam, 14473, Germany b Department of Earth and Planetary Science, University of California, Berkeley, CA, 94720, USA c Research School of Arid Environment and Climate Change, Lanzhou University, 222 South Tianshui Road, Lanzhou, 730000, China

ARTICLE INFO ABSTRACT

Keywords: Microstructures, mineralogical composition and of selected landslide samples from three landslides in Landslide the southern part of the Gansu Province (China) were examined with optical microscopy, transmission electron Microstructure microscopy (TEM), x-ray diffraction (XRD) and synchrotron x-ray diffraction measurements. Common sheet Texture silicates are chlorite, illite, muscovite, kaolinite, pyrophyllite and dickite. Other are quartz, calcite, Pyrophyllite dolomite and albite. In one sample, graphite and amorphous carbon were detected by TEM-EDX analyses and Graphite TEM high-angle annular dark-field images. The occurrence of graphite and pyrophyllite with very low friction Grain size reduction coefficients in the gouge material of the Suoertou and Xieliupo landslides is particularly significant for reducing the frictional strength of the landslides. It is proposed that the landslides underwent comparable deformation processes as zones. The low friction coefficients provide strong evidence that slow-moving landsliding is controlled by the presence of weak minerals. In addition, TEM observations document that grain size reduction in clayey slip zone material was produced mainly by mechanical abrasion. For calcite and quartz, grain size reduction was attributed to both and cataclasis. Therefore, besides landslide composition, the occurrence of ultrafine-grained slip zone material may also contribute to weakening processes of landslides. TEM images of slip-zone samples show both locally aligned clay particles, as well as kinked and folded sheet silicates, which are widely disseminated in the whole matrix. Small, newly formed clay particles have random orienta- tions. Based on synchrotron x-ray diffraction measurements, the degree of preferred orientation of constituent sheet silicates in local zones of the Suoertou and Duang-He-Ba landslide is strong. This work is the first reported observation of well-oriented clay fabrics in landslides.

1. Introduction Some mechanisms involved in the formation of landslides may be similar to those that occur in fault zones. Particularly, the abundance of Slip zones of landslides are natural shear zones produced by dif- clay minerals in landslides may affect the mechanical properties of the ferent configurations and propagate through different slip zones similarly to clay-rich fault gouges (e.g. Warr and Cox, 2001; (Wen and Aydin, 2004). The composition and microstructures of Saffer et al., 2001; Wen and Aydin, 2003; Bhandary et al., 2005; Moore landslide slip zones are fundamental for understanding the mechanisms and Rymer, 2007, 2012; Moore and Lockner, 2008; Collettini et al., of landsliding and shear behavior of soils (Skempton and Petley, 1967; 2009; Tembe et al., 2010; Schleicher et al., 2010; Holdsworth et al., Skempton, 1985; Wen and Aydin, 2004). Slope failure often depends on 2011; Bradbury et al., 2011; Lockner et al., 2011; Janssen et al., 2014). the complex interaction of tectonic activity, site morphology, variations Also, the critical magnitude required to trigger landslides in humidity, and geological factors such as presence of a weak glide may be affected by conversion of a flocculated (stable) clay structure to plane, as well as texture and of both the host materials and a dispersed (unstable) structure due to changes in pH of the pore fluid the slip zone. To understand the underlying mechanisms of landslides, a or creep-induced textural changes (e.g. Rosenquist, 1966; Loeken, large number of studies have microstructurally examinated slip zones 1971; Reves et al., 2006). (e.g. Wen and Aydin, 2003; Bhandary et al., 2005; Chen et al., 2014; Jia Microstructures (e.g. amorphous material/melt, brittle fracturing, et al., 2014). dissolution-precipitation processes, intracrystalline plasticity, micro-

∗ Corresponding author. E-mail address: [email protected] (C. Janssen). https://doi.org/10.1016/j.jsg.2017.11.009 Received 10 May 2017; Received in revised form 9 November 2017; Accepted 17 November 2017 Available online 24 November 2017 0191-8141/ © 2017 Elsevier Ltd. All rights reserved. M. Schäbitz et al. Journal of Structural Geology 106 (2018) 70–85 pores, clay ) in the principal slip zone (PSZ) of landslides were related to Himalayan orogenic deformation and one of the four most formed by shear deformation due to dynamic slip and/or downslope active areas of landslides and debris flows in China (Wang et al., 2013). movement by creeping. For example, the geometric patterns of the It experienced two giant debris flows on August 8, 2010, as a result of a microstructures in the slip zone of the Shek Kip Mei landslide (Hong torrential rain. The debris flows destroyed half of the Zhouqu town and Kong, China) are similar to the S-C fabrics observed in tectonic shear killed 1756 people (Tang et al., 2011). zones (Wen and Aydin, 2003). Particle size reduction by crushing of The Xieliupo landslide (samples X1-X9) is located on the north bank grains and reorientation of grains by shearing generates an extremely of the Bailongjinag river (Figs. 1b and 2a). The geology of this area is fine-grained gouge in faults as well as landslides. Therefore, some de- mainly composed of Silurian slates and phyllites, Permian limestones, formation structures produced during faulting may help identify mi- Devonian limestones and slates, and Quaternary loess deposits (Tang crostructures resulting from slope failure and landsliding. et al., 2011; Yu et al., 2015). The formation of the landslide is con- Many quantitative studies of clay fabric intensity in natural fault trolled by the active Pingding-Huama fault with a mean slip rate of zones, using X-ray texture measurements, have shown that sheet silicate 2.6 mm/year (Meng, unpublished data, Fig. 1b). The active Xieliupo fabrics in fault gouges are predominantly weak (e.g. Warr and Cox, landslide is about 2600 m long and 550 m wide with a thickness of 50 m 2001; Shimamoto et al., 2001; Solum and van der Pluijm, 2009; Haines (Jiang et al., 2014). Movement has been observed in the lower part of a et al., 2009; Schleicher et al., 2010; Wenk et al., 2010; Buatier et al., slope whose toe was cut off for constructing the S313 provincial 2012; Janssen et al., 2012). The fabric strengths are generally much highway (Sun et al., 2015). The landslide moving/creep rate is several smaller than in shales, slates, and schists (e.g. Wenk et al., 2010; mm per year. In addition, an excavation pit provided a sample of clay Haerinck et al., 2015). gouge (slip zone) material from the neighboring Suoertou landslide So far, preferred orientation, also known as fabric or texture, of clay (sample S1), which is also affected by the same long-term creep de- particles in landslides has been examined only by microstructural ob- formation. The lithological structure of both landslides and their linking servations (e.g. Kawamura et al., 2007; Chen et al., 2014; Jia et al., to the active Pingding-Huama fault are the same (Figs. 1a–b and 2b), so 2014) and by a combination of microscopy and image analyses (Wen we included the Suoertou gouge material in our analysis. The Duang-He- and Aydin, 2003, 2004, 2005). Chen et al. (2014) suggested that Ba landslide (samples D1-D5) is located in the southeastern tip of macroscopic sliding in the slip zone was most likely dominated by Zhouqu County, 20 km north-west of Wudu (Figs. 1a and 2c). Here, a sliding of sheet silicate particles. Wen and Aydin (2003, 2004) re- river eroded a whole profile through a landslide down to its host cognized that porosity, particle-size distribution and particle orienta- and parallel to its movement direction. tion are key factors controlling the mechanical properties of landslide slip zones. During sliding induced by heavy rainfall, particle movement 3. Methods within the slip zone was governed by fluidized particulate flow, re- sulting in weak particle alignment or random particle orientation (Wen Microstructures of all samples were studied using optical and and Aydin, 2005). For example, microstructures of the Qinyu landslide transmission electron microscopy (TEM). Optical inspection of thin slip zone, observed by SEM, mainly show a flocculated structure (Jia sections allowed us to identify representative areas characterized by et al., 2014). fine-grained, clay-rich material potentially affected by fracturing and In this paper, composition, clay fabrics and microstructures from dissolution-precipitation processes. These areas were marked on high- samples of three landslides in the southern part of Gansu Province resolution optical scans and prepared for TEM using a focused ion beam (China) were examinated. Samples were collected from the main slip (FIB) device (FEI FIB200TEM) to avoid preparation-induced damage layer and the surrounding damage zone. We analyzed deformation (Wirth, 2004, 2009). TEM was performed with a FEI Tecnai G2 F20 X- microstructures and determined fabric intensities of the clay gouge of Twin TEM/AEM equipped with a Gatan Tridiem energy filter, a Fi- the slip zone samples. For the first time, lattice-preferred orientation of schione high-angle annular dark field detector (HAADF) and an energy constituent minerals in landslide samples was quantified using syn- dispersive X-ray analyzer (EDX). TEM diffraction patterns also provided chrotron X-ray diffraction measurements. The purpose of this in- a first indication for orientation and distribution of clay particles. vestigation is to determine to what extent microstructures and/or The crystallographic orientation of sheet silicates was determined composition influence the slip behavior of landslides. Finally, we from synchrotron X-ray diffraction images measured at the high-energy compare the results with observations from well-investigated fault beam line ID-11C of the Advanced Photon Source (APS) at Argonne zones (San Andreas Fault, Chelungpu Fault) to discuss how the shared National Laboratory. Because of the high costs and the considerable textural and compositional characteristics are indicative of common amount of time necessary for this measurement, we selected only the deformation processes. (e.g. Janssen et al., 2011, 2014). most promising slip zone samples S1 and D3. The method is described in detail in a tutorial (Lutterotti et al., 2013; Wenk et al., 2014). A 2. Geological setting monochromatic X-ray beam with a wavelength of 0.107863 Å and 1 × 1 mm in size was used to penetrate through a 1-mm thick slab of The geography of Central China is affected by active mountain sample. Diffraction images were recorded with a Perkin Elmer building that is associated with frequent occurrences of devastating 2024 × 2014 image plate detector. Seven images for each sample were and mass movements (Bai et al., 2012; Sun et al., 2015). measured at different sample tilt relative to the incident X-ray, from From more than 2000 medium and large landslides reported within −45° to 45° in 15° incremental steps. These sample tilts are necessary to Zhouqu and Wudu County, we selected three, Xieliupo, Suoertou and provide adequate pole-figure coverage. The X-ray beam was also Duang-He-Ba landslide, for our study. Xieliupo and Suoertou landslides translated parallel to the rotation axis over 2 mm to provide sufficient have experienced slow creeping deformation, threatening hundreds of grain statistics. Images were then deconvoluted for phase fractions, thousands of people's lives in Zhouqu and Wudu County (Sun et al., crystallite size, and preferred orientation with the Rietveld method 2015). The Duang-He-Ba landslide is a typical loess landslide, with loess (Rietveld, 1969) implemented in the MAUD (Materials Analysis Using moving on top of Silurian slates and phyllites bedrocks. These facts, Diffraction) software (Lutterotti et al., 1997, 2013). Orientation dis- together with excellent exposure conditions make them ideal candi- tributions for sheet silicates obtained by the Rietveld method were dates to study landsliding. exported from MAUD and further processed in BEARTEX (Wenk et al., The three landslides investigated in this study are situated along the 1998) to obtain pole figures, and to rotate and plot pole figures. Bailong River Corridor in the southern part of the Gansu province X-ray diffraction (XRD) was used to analyze fault rock composition (Fig. 1a; Jiang and Wen, 2014; Jiang et al., 2014; Yu et al., 2015). The of 11 samples (Table 1a–b). Samples were dried and ground to a fine Bailong River Corridor is a tectonically and seismically active region powder (2g) before analysis. X-ray diffraction analyses were conducted

71 M. Schäbitz et al. Journal of Structural Geology 106 (2018) 70–85

Fig. 1. Location of the study area. (a) Satellite image of the Bailong River Corridor in the southern part of the Gansu province of China showing the regional fault system and the position of landslides. The area of detailed investigation is indicated by rectangular area. (b) Geological map of the Zhouqu County showing the Suoertou and Xieliupo landslides in with the Pingding-Huama Fault (modified after Yu et al., 2015).

72 M. Schäbitz et al. Journal of Structural Geology 106 (2018) 70–85

Fig. 2. Detailed maps, field photographs and optical images of thin section of investigated (a) Xieliupo, (b) Suertou and (c) Duang-He-Ba landslides. The yellow circles with numbers indicate sample position and sample number. Sample D6 was taken outside of the profile. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) on air-dried sample slides before and after treatment with ethylene PANalytical Empyrean X-ray diffractometer operating with Bragg- glycol to identify smectite and heating to 550 °C to differentiate be- Brentano geometry at 40 mA and 40 kV with CuKα radiation, a step size tween chlorite and kaolinite. X-ray patterns were collected using a of 0.013 °2Θ from 4.6 to 85 °2Θ and 60 s per step. The Rietveld

73 M. Schäbitz et al. Journal of Structural Geology 106 (2018) 70–85

Table 1 . Quantitative phase analyses of selected landslide samples based on Rietveld refinements of XRD spectra and high energy x-ray diffraction images. Weight is followed by estimated standard deviation. (a) Xieliupo and Suoertou samples. (b) Duang-He-Ba samples.a

(a)

Composition Sample X4 [wt%] Sample X5 [wt%] Sample X7 [wt%] Sample X8 [wt%] Sample X9 [wt%] Sample S1 [wt%] Sample S1 MAUD [wt%]

Quartz 26.7 0.6 27.1 0.6 23.3 0.3 21.1 0.4 34.4 0.7 9.9 0.6 9.5. 0.2 Orthoclase 2.3 0.6 2.7 0.5 2.9 0.4 3.7 0.4 2.3 0.6 Albite 9.1 0.3 9.8 0.5 Calcite 18.4 0.5 18.5 0.4 40.6 0.5 31.4 0.5 4.6 0.4 Dolomite 4.7 0.5 8.6 0.7 1.8 0.3 2.0 0.2 6.2 0.6 8.7 0.6 < 1 Illite/mica 25.4 0.6 25.3 0.6 15.8 1.1 17.5 0.8 30.6 1.2 20.6 1.4 32.0 0.2 Kaolinite 7.7 0.8 6.6 0.7 2.9 0.6 3.9 0.5 1.2 0.5 6.2 1.2 2.1 0.1 Chlorite 6.9 0.7 4.2 0.6 2.2 0.5 4.5 0.6 5.1 0.8 1.5 0.6 < 0.1 Pyrophyllite 3.7 0.9 5.4 0.9 0.8 0.4 2.5 0.9 4.4 0.8 28.9 1.1 35.8 0.2 Hornblende 3.1 0.6 0.7 0.4 0.3 0.2 1.1 0.5 Dickite 14.2 1.2 8.4 0.1 Anatase 1.1 0.2 0.7 0.2 0.4 0.1 0.2 0.1 Siderite 8.5 0.6 11.7 0.1 Rutile 1.2 0.2 1.1 0.2

(b)

Composition Sample D1 [wt%] Sample D2 [wt%] Sample D3 [wt%] Sample D3 MAUD [wt%] Sample D5 [wt%] Sample D6 [wt%]

Quartz 42.2 0.8 34.8 0.6 33.0 0.5 8.7 0.4 13.3 0.5 26.5 0.5 Orthoclase 3.3 0.5 3.1 0.7 3.0 0.7 Albite 16.5 0.7 10.9 0.7 6.4 0.5 4.3 0.1 3.3 0.7 3.5 0.5 Microcline 4.9 0.6 4.1 0.7 4.5 0.8 Calcite 7.3 0.3 10.6 0.4 4.8 0.3 < 0.1 Dolomite 2.3 1.5 1.5 0.3 0.8 0.3 Illite/mica 14.2 0.2 28.4 43.5 1.1 68.3 0.4 54.2 0.4 45.5 0.5 Kaolinite 5.8 0.1 Chlorite 8.7 0.6 9.8 0.6 22.2 0.1 13.0 0.1 13.0 0.1 17.7 0.7 Anatase 0.6 0.1

a Samples S1 and D3 were used to prepare two subsamples for XRD and MAUD. algorithm BGMN was used for quantitative analysis (Bergmann et al., (mainly pyrophyllite, illite, kaolinite, dickite and chlorite; Table 1a). 1998; see supplementary material). Some black patches in the gouge matrix are organic material (see sec- tion 4.3). The amount of pyrophyllite in the Suoertou gouge sample (28.9%) is much greater than in the Xieliupo and Duang-He-Ba slip zone 4. Results samples (5.4% and 0%; Table 1a–b). The Duang-He-Ba outcrop provides a complete cross section through 4.1. Landslides and corresponding XRD analyses the whole landslide body. This allowed sampling of the complete profile from the surface of the slide down to the host rock. In this outcrop, the The sliding surface (slip zone) of the Xieliupo landslide is formed slip zone is up to 20 cm thick and located at the boundary between between grey-brown clayey soil with carbonate rock fragments in the Silurian phyllite and the overlaying Quaternary loess deposits with a upper part and black clayey soil with carbonaceous slate rock fragments thickness of up to 5 meters (Fig. 2c). The dark brown slip-zone sample of Middle Devonian in the lower part (Fig. 2a). The analyzed samples (D3) is rich in sheet silicates (chlorite, illite, muscovite) and quartz, were taken directly from the very fine-grained clayey slip zone mate- additional constituents are plagioclase and calcite/dolomite (Table 1b). rial, the surrounding damage zone and the overlying loess layers. The Similar to the Xieliupo landslide, the mineralogical composition of the foliated material from the slip zone differs from the adjacent damage damage zone (samples D2 and D5) surrounding the slip zone is very zone material due to its grey to brown color and smaller grain sizes. At heterogeneous. low magnification, slip-zone samples show a weak defined by aligned phyllosilicates that is remarkably similar to clay structures. 4.2. Optical microscopy XRD analyses and corresponding Rietveld fits illustrate a complex composition with many contributing phases (Table 1a–b, Fig. 3a–c). Inspection of the samples by optical microscopy revealed a distinct The poorly sorted slip-zone samples X4 and X5 are mainly composed of foliation, grain-scale fracturing and minor faults. The foliation is de- quartz and calcite/dolomite embedded in a clay-rich matrix of illite, fined by shape-preferred orientation of fragments, oriented fractures, kaolinite, chlorite and a minor amount of pyrophyllite (Table 1a). Da- pressure solution seams and aligned phyllosilicates, forming an inter- mage-zone samples (X7, X8, X9) are very heterogeneous. Quartz, cal- connected network. However, in contrast to the macroscopic de- cite/dolomite and clay contents vary highly between these formation patterns in the field and in specimens, microscopic ob- samples (Table 1a). servations revealed considerable differences in foliation intensity The Suoertou landslide represents a deeper section of the same between damage-zone and slip-zone samples and between different (fault-) slip zone (Fig. 1b). The slip zone (gouge material) of this landslides (Fig. 4a–c). For example, the foliation defined by sub-parallel landslide is a black seam of very soft and clayey material, containing alignment of sheet silicates in sample S1 (slip zone of Suoertou land- quartz clasts of different size (Fig. 2b). The black gouge is derived from slide) is more intensely developed as compared to the Xieliupo and neighbouring carbonaceous slates (Jiang et al., 2014). The black gouge Duang-He-Ba slip zone samples (samples X4, X5 and D3; Fig. 4a). For a sample S1 is composed of a very fine-grained phyllosilicate matrix single landslide, we found that the foliation in samples from the damage

74 M. Schäbitz et al. Journal of Structural Geology 106 (2018) 70–85

Fig. 3. XRD spectra for selected samples. (a) Sample X4, Xieliupo; (b) Sample S1, Suertou; and (c) Sample D3, Duang- He-Ba landslide. Some diffraction peaks of corresponding minerals are indexed (compare with Table 1 and diffraction patterns from synchrotron hard x-ray diffraction in Fig. 10).

zone is much less developed than in the slip zone (Fig. 4a–b). The host selected landslides, but slip zone and reworked material from the da- rock shows a weak foliation that is possibly due to deposition and likely mage zone show similar microfabrics (Figs. 5a–f and 6a-d). The slip- not a result of older landslide events (Fig. 4c). However, sheet silicates zone material of the Xieliupo and Duang-He-Ba landslides (samples X4 are difficult to identify accurately in optical thin sections due to their and D3) contains stacks of parallel-oriented detrital clay particles with small grain sizes. varying orientation (Figs. 5a and 6b). Calcite grains and to a lesser Contrary to foliation patterns, the resulting grain sizes displayed in extent, quartz grains are partly dissolved (Fig. 5a–b). At other places, Fig. 4a–c indicate only minor differences (with exception of sample S1) TEM observations of the same slip-zone samples reveal intensely com- between slip-zone material, host rock and damage zone samples. The minuted grains. The extremely fine-grained matrix particles (< 50 nm) slip zone and damaged zone samples of all three landslides consist of an are interpreted to be produced by mechanical abrasion (sample D3, extremely fine-grained, dark reddish-brown to black matrix with em- Fig. 6c). Quartz grains act as a planer, reducing the grain size of detrital bedded angular to subrounded quartz grains (Fig. 4a–b). But quartz sheet silicate crystals. Textures implying mechanical comminution of grains in slip-zone sample S1 are more rounded to subrounded and clay particles were also observed in several previous studies (e.g., much smaller than those from the sliding surface samples of the Xieliupo Chenu and Guerif, 1991; Baudett et al., 1999; Wen and Aydin, 2003). and Duang-He-Ba landslides. Likewise, dark pressure-solution seams Wen and Aydin (2003, 2004) concluded that landslide slip zones may indicating dissolution-precipitation processes are observed in all sam- have experienced grainscale cataclastic deformation. Some clay mi- ples. Other typical pressure-solution features like insoluble material nerals in slip zone and damage zone samples, protruding into open pore (pressure solution residues) were observed in virtually all thin sections, spaces, are interpreted as grains, newly formed by dissolution-pre- but more often in the slip-zone samples (Fig. 4a–c). Well-oriented mi- cipitation processes of grain boundaries (Fig. 5b). They are randomly neral fragments suggest local shearing (Fig. 4a). oriented. The gouge material of the Souertou landslide (sample S1) contains organic material and is characterized by a strong fabric de- fined by preferred orientation of aligned phyllosilicates (Fig. 6a). This 4.3. TEM observations strong fabric covers the entire sample. A significant amount of graphite and amorphous carbon were only observed in gouge material of sample The TEM observations are consistent with those from optical mi- S1 by TEM-EDX analyses and high-angle annular dark field images croscopy. TEM images display microfabrics that differ between the

75 M. Schäbitz et al. Journal of Structural Geology 106 (2018) 70–85

Fig. 4. Optical photomicrographs of thin sections. The sample number is in the lower left corner. (a) Thin sections from the weakly to strong foliated clayey slip-zone (gouge) matrix with embedded angular fragments and quartz grains. (b) Thin sections from the damage zone material with embedded angular fragments and quartz grains. (c) Thin sections from host rocks.

(Fig. 7a–c). (Fig. 5e). The pores were possibly filled with formation water and/or The poorly sorted loess and reworked damage zone material of the hydrothermal fluids possibly indicating elevated pore-fluid pressure Xieliupo and Duang-He-Ba landslides (samples X1, X2, X3, X7 and D2) preventing pore collapse. are characterized by detrital and newly formed clay particles. Similar to the slip zone samples, randomly oriented clay minerals alternate with 4.4. Preferred orientation data aggregates showing preferred orientation (samples X2 and X8; Fig. 5d–e). The latter only occur locally and form face-to-face particle A rough estimate of local fabric strength can be made by inter- contacts. These well-oriented stacks form small (< 1 μm) domains preting the TEM diffraction patterns in selected areas. Preferred or- (Fig. 5f) and do not produce a macroscopic fabric at the thin-section ientation corresponds to rings with distinct diffraction spots whereas a scale. They are widely disseminated in the whole matrix. In some areas, weak fabric is characterized by ring-shaped diffraction patterns. The TEM micrographs illustrate clay flakes arranged in a card house fabric preferred orientation of aligned phyllosilicates in the matrix of sample (Fig. 5e). Some quartz and/or feldspar grains in the matrix are wrapped X7 indicates a relatively strong fabric (Fig. 8a). In contrast, the random by clay minerals (Fig. 6d). Toward the sliding surface, stacked clay orientation of clay particles in sample X5 only produces diffraction particles are aligned parallel or kinked and folded (sample X3; Fig. 5c). rings with many spots, suggesting a weak fabric (Fig. 8b). Note that the Partly dissolved grains, newly formed clay particles and highly com- fault slip sample X5 reveals a weaker fabric compared to the damage minuted grains are observed in damage zone material and in slip zone zone sample X7. samples. Quantitative crystallographic preferred orientation (CPO) analyses Pores were found in all samples, but unequally distributed. In TEM were performed on two slip-zone subsamples (from sample S1 with images, we mostly observed sub-angular voids located between larger black clay layer, Suoertou landslide, and sample D3 with brown clay grains and fragments (Fig. 5d). Sub-rounded pores are formed by layer, Duang-He-Ba landslide), with synchrotron X-ray diffraction on a leaching of carbonates in the sliding surface (Fig. 5a). Inter-clay layer volume of about 4 mm3. CPO is immediately evident in 2D synchrotron pores occur between illite clay flakes (Fig. 6b). These voids show large diffraction images from the variations of intensity along diffraction aspect ratios with a long axis of several hundred nm mostly oriented rings of sheet silicate minerals (Fig. 9). Greater intensity (darker area) parallel to the clay layers. Illite clay flakes with edge-to-face contact along the rings indicates more lattice plane normals oriented in that form card house structures with pore spaces between the flakes particular direction.

76 M. Schäbitz et al. Journal of Structural Geology 106 (2018) 70–85

Fig. 5. TEM images from microstructures of the Xieliupo landslide. The sample number is in the lower left corner. (a) Embayed calcite grain that is interpreted to be partially dissolved. The grain is surrounded by stacks of parallel-oriented clay particles. (b) Eroded Quartz grain boundary and open pore spaces with clay minerals. (c) Kinked and bent clay particles. (d) Open pore spaces between quartz grains and preferred orientation of sheet silicates. (e) Clay flakes in edge-to-face contact form card house structures with small open pore spaces between the clay flakes. (f) Calcite grains embedded in a strong clay fabric.

2D diffraction images were decomposed into 36 diffraction patterns % in sample D3). The values are not identical because the actual by azimuthal averaging over 10 sectors and these 36 patterns for 7 samples are different and XRD powder analysis averages over much images at different sample tilts for each sample (total of 252 diffraction larger sample volumes. patterns) were then used as data for the Rietveld refinement. Fig. 10 The purpose of the synchrotron analysis was not to refine phase shows average diffraction patterns for S1 and D3 with measured data fractions, but rather to quantify CPO. Preferred orientation is displayed (dots) and corresponding Rietveld fit (solid line). The pattern can be in diffraction images (Fig. 9) and is even more obvious in stacks of compared with XRD results (Fig. 3a–c). Phase fractions from the MAUD diffraction patterns as function of azimuthal angle where sheet silicates Rietveld analysis are similar to the XRD analysis of powders display strong intensity variations in contrast to quartz (Fig. 11). The (Table 1a–b), with dominating pyrophyllite (35.8 wt% in Sample S1), good agreement not only indicates a reliable identification of compo- illite-mica (32 wt% in sample S1 and 68.3 wt% in sample D3), kaolinite nent minerals and their volume fractions but also of their orientation (2.1 wt% in sample S1 and 5.8 wtl% in sample D3) and chlorite (13 wt patterns.

77 M. Schäbitz et al. Journal of Structural Geology 106 (2018) 70–85

Fig. 6. TEM images from microstructures of the Suoertou and Duang-He-Ba landslides. The sample number is in the lower left corner. (a) Pyropyllite and organic material embedded in aligned phyl- losilicates. (b) Pore spaces between clay flakes. (c) Quartz grain acts as a planer along illite crystals. Notice that the planed illite particles have an extremely fine grain size. (d) Quartz grain is wrapped by clay and poorly sorted matrix particles.

Fig. 7. TEM microphotographs of sample S1. (a) High-angle annular dark-field (HAADF) image showing graphite embedded in sheet silicates. (b) High-resolution transmission electron micro- scopy (HRTM) image of graphite (c) HRTEM image of carbon.

78 M. Schäbitz et al. Journal of Structural Geology 106 (2018) 70–85

Fig. 8. Brightfield, and HRTEM TEM images. (a) The spotty rings indicate preferred orientation of clay crystals (sample X7) and (b) the diffuse scattering intensity shows a weak fabric (sample X5).

From orientation distribution patterns of sheet silicate minerals, There are strong maxima, up to 4 m.r.d. for pyrophyllite in sample S1 pole figures were calculated and used to display preferred orientation and up to 10 mrd for illite in sample D3. Kaolinite has a stronger texture patterns (Fig. 12). The coordinate system was rotated to display the in sample D3 (MaxPf = 4.5 m.r.d.) than in sample S1 characteristic (001) maxima of sheet silicates in the center. Pole figures (MaxPf = 1.6 m.r.d.). In sample S1, pyrophyllite has the strongest are plotted in equal-area projection and pole densities are represented texture (MaxPf = 4.0 m.r.d.), whereas the orientation distribution of in multiples of random distribution (m.r.d.). Note that all samples show dickite is relatively weak with 1.8 m.r.d.. The texture of chlorite was considerable heterogeneity and the methods used in this study, such as measured only in sample D3. With 3.3 m.r.d. it is quite strong. The TEM and synchrotron X-ray diffraction, only investigate small volumes. texture of quartz in both samples is random. Pole figures are displayed for kaolinite, illite/mica, pyrophyllite (only sample S1), dickite (only sample S1) and chlorite (only sample D3).

Fig. 9. 2D Diffraction images of samples (a) S1 and (b) D3 at 0° sample tilt. Some Debye rings are identified. Azimuthal intensity variations along Debye rings are indicative of crystallographic preferred orientation. Note that the intensity of heterogeneous ring development is more obvious in (b) than in (a).

79 M. Schäbitz et al. Journal of Structural Geology 106 (2018) 70–85

Fig. 10. Average diffraction patterns of the diffraction images in Fig. 9 for samples (a) S1 and (b) D3. Dots are experimental data and line is Rietveld fit. Some diffraction peaks are identified. Below the diffraction pattern are mineral phases and corresponding diffractions.

80 M. Schäbitz et al. Journal of Structural Geology 106 (2018) 70–85

Fig. 11. Stack of 36 diffraction spectra averaged over 10° azimuth of diffraction images in Fig. 9 for samples (a) S1 and (b) D3 represented as map plots with intensity (gray shade) as function of 2θ. Bottom are measured data, top the Rietveld fit. Some diffraction peaks are identified.

5. Discussion Fluid-related alteration reactions leading to mineral dissolution and precipitation are evident by the presence of authigenic phyllosilicates 5.1. Landslide composition, and the role of fluids in slip-zone mineralogy (Fig. 5b) and variations in the mineralogical composition such as the enrichment of kaolinite and pyrophyllite in slip zone samples Differences in the mineralogical composition of the landslides as (Table 1a–b). The lower calcite content of the Xieliupo slip-zone samples revealed by XRD data (Table 1a–b) may be attributed to different host compared to the host rock should also be seen in this context because rock compositions and provenances of the sample, i.e. if they originate fluid-assisted mass transfer by pressure solution potentially led to cal- from the damage or the slip zone. Quartz and chlorite are the only cite depletion in the slip zone and the relative enrichment of clay mi- minerals present in all samples. However quartz and chlorite contents nerals (compare Gratier et al., 2011). The interpretation of fluid-as- differ between landslides due to different host rocks (e.g. Devonian sisted alteration in slip zone samples is consistent with many fault zone slates vs. Silurian phyllites). The presence of illite/mica in slip zone studies that identified fluid-related alteration assemblages controlling samples of the Xieliupo, Suoertou and Duang-He-Ba landslides may be and fluid flow (e.g. Warr and Cox, 2001; Boullier et al., partly related to the illite content of the respective host rocks. A clear 2009, 2011; Schleicher et al., 2009; Moore and Rymer, 2012). increase in the abundance of sheet silicates toward the slip zone was not observed. The absence of smectite in all samples contrasts with the 5.2. Slip zone frictional strength compositions found in slip zones of other landslides (Shuzui, 2001). This result is consistent with investigations of Mancktelow et al. (2016). The presence of graphite and pyrophyllite in gouge material of the The authors suggest that a clay-rich low permeability slip zone may Suoertou landslide may indicate that frictional strength of the landslide prevent influx of meteoric water and thus inhibit smectite growth in was low. Both minerals have extremely low friction coefficients with fault gouge. 0.1 for graphite and < 0.2 for pyrophyllite (Al2SiO4). The TEM

81 M. Schäbitz et al. Journal of Structural Geology 106 (2018) 70–85

Fig. 12. (001) Pole figures of sheet silicates for gouge samples (a) S1 and (b) D3. Equal-area projection, contours in multiples of random distribution. Samples have been rotated from an arbitrary initial orientation to bring the (001) maximum into the center. evidence of graphite in the slip zone sample of the Suoertou landslide In general, the presence of weak clay minerals and graphite in slip- (sample S1, Table 1a) is consistent with fault zone studies. Within a zone gouge strongly suggests that landsliding may be controlled by the fault gouge, graphite possibly serves as a lubricant reducing frictional presence of these weak minerals in agreement with a plethora of strength (Oohashi et al., 2012; Kuo et al., 2014; Li et al., 2013; landslide and fault zone studies (e.g. Warr and Cox, 2001; Boullier Nakamura et al., 2015; Togo et al., 2011). This interpretation is sup- et al., 2009, 2011; Schleicher et al., 2009; Moore and Rymer, 2012). For ported by high-velocity friction experiments performed on samples example, the study of several landslides in Japan indicate that frictional containing amorphous carbon. Dramatic weakening was observed when resistance decreases as clay content (smectite) increases (Shuzui, 2001). carbon transformed to graphite (Oohashi et al., 2011, 2013, 2014). Within the creeping portion of the San Andreas Fault (SAF) at shallow Several processes of graphite enrichment have been proposed (Oohashi depth, the growth of Mg-rich phyllosilicates may play a key role in the et al., 2014): (1) graphite may derive from host rock via pressure so- mechanical behavior of the SAF and can be directly related to fault lution or solution transfer processes (Oohashi et al., 2012); (2) hydro- weakening (e.g. Moore and Rymer, 2007, 2012; Schleicher et al., 2010; thermal precipitation of graphite from a C-O-H-rich fluid (Zulauf et al., Holdsworth et al., 2011; Bradbury et al., 2011; Lockner et al., 2011). 1999); and (3) graphitization of amorphous carbon due to seismic fault Unlike Xieliupo and Suoertou landslides, Duang-He-Ba samples do not motion (Oohashi et al., 2011). The latter process was recognized in the contain extremely weak clay minerals. Here, illite (friction coeffi- principal slip zone of several fault zones, which underwent frictional cient > 0.2) is the most abundant mineral phase in the slip zone heating due to rapid sliding (e.g. Kuo et al., 2014; Nakamura et al., (sample D3) and acts as a lubricant reducing frictional strength. Illite 2015). For instance, XRD analyses of coseismic fault gouge samples may be formed by fragmentation of muscovite particles (Buatier et al., from the Longmenshan fault zone in Sichuan (China) indicate graphite 2012) that were initially present in the host rock (sample D5). The slip (Togo et al., 2011). Similar to the outcrop samples, drill core material zone sample is depleted in chlorite and muscovite while the quartz from the principal slip zone also contains graphite (Kuo et al., 2014). content increases compared to the host rock. These differences may The authors suggested that graphite may have formed due to seismic suggest that the slip zone material is not only derived from the host rock slip affecting in turn the mechanical properties of the slip layer. Strain- (phyllite) but also includes material from hanging wall sediments. induced amorphization of graphite with increasing frictional slip was also reported from fault zones of the Hidaka metamorphic belt 5.3. Microstructures (Nakamura et al., 2015). These fault zone studies suggest that the co- existance of graphite and amorphous carbon in gouge material of the In general, the observed microstructures are very similar to micro- Suoertou landslide (Fig. 7) is a strong indicator for graphitization of structures in gouge samples from major faults such as the San Andreas amorphous carbon during seismic slip. It is assumed that a former Fault and the Chelungpu Fault (e.g. Hadizadeh et al., 2012; Holdsworth earthquake (co-seismic event) preceeds the slow-moving (a-seismic) et al., 2011; Moore and Rymer, 2012; Janssen et al., 2014). For ex- landsliding. ample, the TEM observations indicate that both fracturing and pressure

82 M. Schäbitz et al. Journal of Structural Geology 106 (2018) 70–85 solution have led to grain size reduction. Phyllosilicates scraped by be mechanically emplaced into the slip zone. Another reason for this rigid clasts such as quartz (Fig. 6b) indicate that the extremely fine- finding might be that clay minerals are likely to creep relatively easily grained clay matrix particles (< 50 nm) in slip zone and damage zone because of their low friction coefficient (Gratier, 2011). samples (Fig. 6c–d) were produced by mechanical abrasion rather than The detection of a preferred clay fabric orientation in landslide by pressure solution. The opposite is true for calcite and quartz. The samples by synchrotron X-ray diffraction measurements is consistent dissolved grain boundaries indicate grain size reduction by pressure with microfabrics of the Jin'nosuke-dani landslide (northern central solution (Fig. 5a–b). Microstructural observations of fault gouges have Japan, Kawamura et al., 2007) but differs markedly from fault gouge shown that grain size reduction may occur over a wide range of slip samples, which have generally weak fabrics (e.g. Alpine Fault, Warr rates and different deformation mechanisms (Stünitz et al., 2010). and Cox, 2001; Moab Fault, Solum et al., 2005; Carbonera Fault, Solum Strong comminution may be related to dynamic rock fracturing during and van der Pluijm, 2009; Bogd fault, Buatier et al., 2012; San Andreas propagation of a single earthquake (Olgaard and Brace, 1983; Wilson Fault, Wenk et al., 2010; Janssen et al., 2014; Chelungpu Fault, Janssen et al., 2005; Brantut et al., 2008) and slow grinding (creeping) pro- et al., 2014). Solum and van der Pluijm (2009) described clay fabrics in cesses (Stünitz et al., 2010), involving pressure solution (Gratier and fault rocks of the Carboneras Fault (Spain) as isolated and extremely Gamond, 1990; Gratier et al., 2011). However, irrespective of whether localized structures in layers and domains, extending just a few tens of the ultrafine-grained particles in slip zone samples were produced by micrometers from the slip surfaces into the sample matrix structures. coseismic and/or aseismic slip, the presence of this fine-grained mate- The overall preferred orientation is poor above the scale of a few tens of rial implies that frictional strength of the slip zone material is sig- micrometers (Solum et al., 2010). Haines et al. (2009) suggested that nificantly reduced, which consequently promotes slip (e.g. Marone and the very weak fabric in fault gouges could result from growth of clays Scholz, 1989; Chester et al., 2005; Wilson et al., 2005; Ma et al., 2006; after fault slip ceased. In the slip zone samples, we have no clear evi- Collettini et al., 2009; Han et al., 2010, 2011; Viti, 2011). dence for clay growing after landsliding. Finally, the question arises about whether or not the composition of 5.4. Clay fabric in slip zones slip zone samples, and/or the orientation of clay particles determine the weakness of landslides. Our study yields no direct information about Microstructural analysis, using optical inspection, electron micro- the shear strength of the analyzed slip-zone samples. Results from scopy as well as X-ray diffraction techniques, reveal both randomly and frictional sliding experiments, however, suggest that thin localized preferentially oriented sheet silicates in the landslide slip-zone samples. zones of phyllosilicates along (and below) slip surfaces reduce the TEM images of sample S1 (Suoertou landslide) display well-oriented friction coefficient (e.g. Tembe et al., 2006; Collettini et al., 2009; phyllosilicate stacks for the entire TEM sample (Fig. 6a). These ob- Solum et al., 2010). Studies on clay-bearing soils, on the other hand, servations are consistent with synchrotron X-ray diffraction measure- document that clay with random orientations tend to be stronger and ments that reveal relatively strong clay fabrics with very regular or- more rigid (Mitchell, 1993). Considering these shear experiments, we ientation distributions and pole figures ranging from 1.6 m.r.d. in suggest that apart from slip-zone composition and extremely fine- kaolinite to 4 m.r.d. in pyrophyllite. (Fig. 12). Synchrotron X-ray dif- grained matrix particles also the identified strong clay fabrics are re- fraction measurements for sample D3 reveal an even stronger clay sponsible for a significant reduction of shear strength of the landslides. fabric than for sample S1. Maximum (001) pole densities range from 3.3 m.r.d. in chlorite, 4.5 m.r.d. in kaolinite to 10. m.r.d. in illite. In 6. Conclusions contrast, microscopic photographs and TEM images of damaged and slip zone samples X3, X4 and X5 (Xieliupo landslide) and sample D3 Microstructures and the presence of strong fabrics in samples de- (Duang-He-Ba landslide) show both locally aligned clay particles, as rived from three landslide slip-zones constrain the interpretation of slip well as kinked and folded sheet silicates, which are widely disseminated mechanisms that show some similarities with deformation processes in the whole matrix (Figs. 5a, c, 6b). Small, newly formed clay particles described from fault zone gouges. The presence of (weak) clay minerals also have a random orientation (Fig. 5b). These results suggest a clear and graphite favor weakening in narrow slip zones of landslides and distinction between a strong clay fabric in local slip zones (“slip-zone faults. The process of grain-size reduction depends on the minerals. fabric”) and a weak “matrix fabric”. The latter is considered primarily a Fine-grained clay matrix particles were produced by mechanical abra- result of the growth of new sheet silicates and fault-related deformation sion and calcite and quartz grain size reduction occurred by pressure as kinking and rotation on the particle scale (Janssen et al., 2012). solution. We suggest that both processes result from sliding events and Considering microstructures and pole figures, it is probable that the slow creep episodes. The degree of clay fabric intensity differs between strong fabrics in samples D3 and S1 are caused by different processes. the selected samples. The strong fabric in slip zone samples of the We suggest two possible explanations for the strong clay fabrics in Suoertou and Duang-He-Ba landslide is clearly different from fault gouge sample S1 (Suoertou landslide). (1) The original fabric of clay in a samples. To support this statement synchrotron X-ray diffraction mea- marine environment will be flocculated (card-house fabric; Bennett surements should be applied to a wider variety of samples from dif- et al., 1991). Later, when the saline water is leached out by fresh ferent landslides to improve our understanding of landsliding. , the stable clay fabric is changed to an unstable arrange- Speculatively, one of the worst natural disasters in China, the 2010 ment of a dispersed clay fabric subject to collapse when disturbed or Gansu mudslide, which killed 1756 people, could be caused by ground shaken, for example by earthquakes (Black et al., 1999). Second, fo- failure due to the loss of strength in sensitive clays as described for the liated domains with preferred orientation were formed by pressure Suoertou landslide. solution as described for the damaged zone of the San Andreas Fault (Richard, 2014). Accounting for the microscopic and TEM observations, Acknowledgements the microstructural record of the Suoertou sample S1 shows earthquake (co-seismic) related weakening mechanisms, which include amorphous Jean-Pierre Gratier and an anonymous reviewer together with the carbon and graphite. Hence, the first explanation is probably the most editorial guidance of William M. Dunne provided very constructive likely scenario for the Suoertou landslide. comments and suggestions that helped to improve this paper. This work Deformation patterns and the strong fabric in sample D3 (Duang-He- is part of the collaborative research program TIPTIMON funded by the Ba landslide) are difficult to interpret. We found neither indication that Federal Ministry of Education and Research (FKZ 03G0809A). The an earthquake acted as an triggering mechanism for ground failure, nor Lanzhou University provided substantial financial and logistical sup- strong evidence for pressure-solution creep. One possible explanation is port. The authors wish to thank to R. Naumann for XRD and XRF that patches of oriented clay particles from the host rock (phyllite) may analysis and A. Hendrich and M. Dziggel for help with drafting. HRW

83 M. Schäbitz et al. Journal of Structural Geology 106 (2018) 70–85 acknowledges support from NSF (EAR-0836402) and DOE (DE-FG02- Janssen, C., Wirth, R., Reinicke, A., Rybacki, E., Naumann, R., Kemmnitz, H., Wenk, H.- 05ER15637) and access to beamline 11-ID-C at the Advanced Photon R., Dresen, G., 2011. Nanoscale porosity in SAFOD core samples (San Andreas Fault). Earth Planet. Sci. Lett. 301, 179–189. Source (APS) of Argonne National Laboratory and help from Y. Ren. Janssen, C., Kanitpanyacharoen, W., Wenk, H.-R., Wirth, R., Morales, L., Rybacki, E., Kienast, M., Dresen, G., 2012. Clay fabrics in SAFOD core samples. J. Struct. Geol. 43, Appendix A. Supplementary data 118–127. Janssen, C., Wirth, R., Wenk, H.-R., Morales, L., Naumann, R., Kienast, M., Song, S.-R., Dresen, G., 2014. Faulting processes in active faults – evidences from TCDP and Supplementary data related to this article can be found at http://dx. SAFOD drill core samples. J. Struct. Geol. 65, 100–116. doi.org/10.1016/j.jsg.2017.11.009. Jia, X., Liang, S., Fan, C., 2014. Microstructural Characteristics of Qinyu landslide slip soil, NW China. In: In: Sassa, K. (Ed.), Landeslide Science for a Safer Geoenvironment, vol. 2 Springer International Publishing, Switzerland. http://dx.doi.org/10.1007/ References 978-3-319-05050-8_29. Jiang, S., Wen, B.P., Zhao, C., Li, R., 2014. Factors controlling kinematic behaviour of a huge slow-moving landslide in China. In: In: Sassa, K. (Ed.), Landeslide Science for a Bai, S., Wang, J., Zhang, Z., Cheng, C., 2012. Combined landslide susceptibility mapping Safer Geoenvironment, vol. 2 Springer International Publishing, Switzerland. http:// after Wenchuan earthquake at the Zhouqu segment in the Bailongjiang Basin, China. dx.doi.org/10.1007/978-3-319-05050-8_42. Catena 99, 18–25. http://dx.doi.org/10.1016/j.catena.2012.06.12. Jiang, X., Wen, B.P., 2014. Creep behavior of slip zone of a giant slow-moving landslide in Baudett, G., Perrotel, V., Seron, a., Stelletelli, M., 1999. Two dimensional communition of Northwest China: the Suoertou landslide as an example. In: In: Sassa, K. (Ed.), kaolinite clay particles. Powder Technol. 105, 125–134. Landeslide Science for a Safer Geoenvironment, vol. 2 Springer International Bennett, R.H., O'Brien, N.R., Hulbert, M.H., 1991. Determination of clay and shale mi- Publishing, Switzerland. http://dx.doi.org/10.1007/978-3-319-05050-8_23. crofabric signatures: processes and mechanisms. In: Bennett, R.H., Bryant, W.R., Kawamura, K., Ogawa, Y., Oyagi, N., Kitahara, T., Anma, R., 2007. Structural and fabric Hulbert, M.H. (Eds.), Microstructure of Fine-Grained Sediments. Springer-Verlag, analyses of basal slip zone of the Jin’nosuke-dani landslide, northern central Japan: New York, pp. 5–32. its application to the slip mechanism of decollement. Landslides 4, 371–380. http:// Bergmann, J., Friedel, P., Kleeberg, R., 1998. BGMN- a new fundamental parameters dx.doi.org/10.1007/s10346-007-0094-z. based Rietveld program for laboratory X-ray sources, it's use in quantitative analysis Kuo, L.W., Li, H., Smith, S.A.F., Di Toro, G., Suppe, J., Song, S.-R., Nielsen, S., Sheu, H.-S., and structure investigations. ?CPD? Newsletter 20, 5–8. Si, J., 2014. Gouge graphitization and dynamic fault weakening during the 2008 Mw Bhandary, N., Yatabe, R., Takata, S., 2005. Clay minerals contributing to creeping dis- 7.9 Wenchuan earthquake. Geology 42, 47–50. http://dx.doi.org/10.1130/ placement of zone landslides in Japan. In: Sassa, K., Fukuoka, H., Wang, G34862.1. F.W., Wang, G. (Eds.), Landslides – Risk Analysis and Sustainable Disaster Li, H., et al., 2013. Characteristic of the fault-related rocks, fault zones, and the principal Management. Springer International Publishing, Switzerland, pp. 219–223. slip zone in the Wenchuan earthquake fault scientific drilling project Hole-1 (WFSD- Black, B.D., Solomon, B.J., Harty, K.M., 1999. Geology and Geological Hazards of Tooele 1). 584, 23–42. Valley and the West Desert Hazardous Industry Area, Tooele County, Utah, Special Lockner, D.A., Morrow, C., Moore, D., Hickman, S., 2011. Low strength of deep San Study 96. Utah Geological Survey. Andreas fault gouge from SAFOD core. 472. http://dx.doi.org/10.1038/ Boullier, A.-M., Yeh, E.-C., Boutareaud, S., Song, S.-R., Tsai, C.-H., 2009. Microscale nature09927. anatomy of the 1999 Chi-Chi earthquake fault zone. Geochem. Geophys. Geosys. 10. Loeken, T., 1971. Recent research at the Norwegian geotechnical institute concerning the http://dx.doi.org/10.1029/2008GC002252. influence of chemical additions on quick clay. Norges Geotek. Inst. Publ. 87, Boullier, A.M., 2011. Fault-zone Geology: Lessons from Drilling through the Nojima and 133–147. Chelungpu Faults, vol. 359. Geological Society, London, pp. 17–37 Special Lutterotti, L., Matthies, S., Wenk, H.-R., Shultz, A.J., Richardson, J.W., 1997. Combined Publication. texture and structure analysis of deformed limestone from time-of-flight neutron Bradbury, K.K., Evans, J.P., Chester, J.S., Chester, F.M., Kirschner, D.L., 2011. diffraction spectra. J. Appl. Phys. 81, 594–600. and internal structure of the San Andreas fault at depth based on characterization of Lutterotti, L., Vasin, R., Wenk, H.-R., 2013. Rietveld texture analysis from synchrotron Phase 3 whole-rock core in the San Andreas Fault Observatory at Depth (SAFOD) diffraction images: I. Basic analysis. Powder Diffr. 29, 76–84. borehole. Earth Planet. Sci. Lett. 304. http://dx.doi.org/10.1016/j.epsl.2011.07.020. Ma, K., et al., 2006. Slip zone and energetics of large earthquake from the Taiwan Brantut, N., Schubnel, A., Rouzaud, J.-N., Brunet, F., Shimamoto, T., 2008. High-velocity Chelungpu-fault Drilling project. Nature 444, 473–476. frictional properties of a clay-bearing fault gouge and implications for earthquake Mancktelow, N., Zwingmann, H., Mulch, A., 2016. Timing and conditions of clay fault mechanics. J. Geophys. Res. 113. http://dx.doi.org/10.1029/2007JB005551. gouge formation on the Naxos detachment (Cyclades, Greece). 35, Buatier, M.D., Chauvet, A., Kanitpanyacharoen, W., Wenk, R., Ritz, J.F., Jolivet, M., 2012. 2334–2344. http://dx.doi.org/10.1002/2016TC004251. Origin and behavior of clay minerals in the Bogd fault gouge, Mongolia. J. Struct. Marone, C., Scholz, C.H., 1989. Particle-size distribution and microstructures within si- Geol. 34, 77–90. mulated fault gouge. J. Struct. Geol. 11, 799–814. Chen, J., Dai, F., Xu, L., Chen, S., Wang, P., Long, W., Shen, N., 2014. Properties and Mitchell, J.K., 1993. Fundamentals of Soil Behavior, 2nd. ed. Wiley, New York 437 pp. microstructure of a natural slip zone in loose deposits of red beds, southwestern Moore, D.E., Lockner, D.A., 2008. The friction in the temperature range 25°-400°C: re- China. Eng. Geol. 183, 53–64. http://dx.doi.org/10.1016/j.enggeo.2014.10004. levance for Fault-one Weakening. Tectonophysics 449, 120–132. Chenu, C., Guerif, J., 1991. Mechanical strength of clay minerals as influenced by an Moore, D.E., Rymer, M.J., 2007. Talc-bearing serpentinite and the creeping section of the absorbed polysaccharide. Soil Sci. Soc. Am. J. 55 (4), 1076–1080. San Andreas fault. Nature 448 doi:10.1038. Chester, J.S., Chester, F.M., Kronenberg, A.K., 2005. Fracture surface energy of the Moore, D.E., Rymer, M.J., 2012. Correlation of clayey gouge in a surface exposure of Punchbowl fault, San Andreas system. Nature 437, 133–136. serpentinite in the San Andreas Fault with gouge from the San Andreas Fault ob- Collettini, C., Niemeijer, A., Viti, C., Marone, C., 2009. Fault zone fabric and fault servatory at depth (SAFOD). J. Struct. Geol. 38, 51–60. weakness. Nature 462. http://dx.doi.org/10.1038/nature08585. Nakamura, Y., Oohashi, K., Toyoshima, T., Satish-Kumar, M., Akai, J., 2015. Strain-in- Gratier, J.P., Gamond, J.F., 1990. Transition between seismic and aseismic deformation in duced amorphization of graphite in fault zones of the Hidika metamorphic belt, the upper crust. Geol. Soc. Spec. Publ. 54, 461–473. Hokkaido, Japan. J. Struct. Geol. 72, 142–161. http://dx.doi.org/10.1016/jsg.2014. Gratier, J.P., 2011. Fault permeability and strength evolution related to fracturing and 10012. healing episodic processes (years to millennia): the role of pressure solution. Oil Gas Olgaard, D.L., Brace, W.F., 1983. The microstructure of gouge from a -induced Sci. Technol. Rev. IFP Energies Nouvelles 66, 491–506. seismic . Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 20, 11–19. Gratier, J.-P., Richard, J., Renard, F., Mittempergher, S., Doan, M.-L., Di Toro, G., Oohashi, K., Hirose, T., Shimamoto, T., 2011. Shear induced graphitization of carbo- Hadizadeh, J., Boullier, A.-M., 2011. Aseismic sliding of active faults by pressure naceous material during seismic fault motion: experiments and possible implications solution creep: evidence from the San Andreas Fault observatory at depth. Geology for fault mechanics. J. Struct. Geol. 33, 1122–1134. http://dx.doi.org/10.1016/j.jsg. 12, 1131–1134. 2011.01.007. Hadizadeh, J., Mittempergher, S., Gratier, J.P., Renard, F., Di Toro, G., Richard, J., Babie, Oohashi, K., Hirose, T., Kabayashi, K., Shimamoto, T., 2012. The occurrence of graphite- H.A., 2012. A microstructural study of fault rocks from the SAFOD: implications for bearing fault rocks in the Atotsugawa fault system, Japan: origin and implications for deformation mechanisms and strength of creeping segment of the San Andreas Fault. fault creep. J. Struct. Geol. 38, 39–50. http://dx.doi.org/10.1016/j.jsg.2011.10.011. J. Struct. Geol. 42, 246–260. Oohashi, K., Hirose, T., Shimamoto, T., 2013. Graphite as lubricating agent in fault zones: Haerinck, T., Wenk, H.-R., Debacker, T.N., Sintubin, M., 2015. Preferred mineral or- an insight from low- to high-velocity friction experiments on a mixed graphite-quartz ientation of a chloritoid-bearing slate in relation to its magnetic fabric. J. Struct. Geol. gouge. J. Geophys. Res. 118, 2067–2084. http://dx.doi.org/10.1002/jgrb.50175. 71, 125–135. http://dx.doi.org/10.1016/j.jsg.2014.09.013. Oohashi, K., Han, R., Hirose, T., Shimamoto, T., Omura, K., Matsuda, T., 2014. Carbon- Haines, S.H., van der Pluijm, B.A., Ikari, M.J., Saffer, D.M., Marone, C., 2009. Clay fabric forming reactions under a reducing atmosphere during seismic slip. Geology. http:// intensity in natural and artificial fault gouges: implications for brittle fault zone dx.doi.org/10.1130/G35703.1. processes and sedimentary basin clay fabric evolution. J. Geophys. Res. 114. http:// Reves, G.M., Sims, I., Cripps, J.C. (Eds.), 2006. Clay materials Used in Construction, vol. dx.doi.org/10.1029/2008JB005866. 21. Geological Society, London, pp. 1–51 , Special Publication. Han, R., Hirose, T., Shimamoto, T., 2010. Strong velocity weakening and powder lu- Richard, J., 2014. Large-scale mass transfer related to pressure solution creep-faulting brication of simulated carbonate faults at seismic slip rates. J. Geophys. Res. interactions in mudstones: driving processes and impact of lithification degree. 115http://dx.doi.org/10.1029/2008JB006136. B03412. Tectonophysics 612–613, 40–55. Han, R., Hirose, T., Shimamato, T., Lee, Y., Ando, J., 2011. Granular nanoparticles lu- Rietveld, H.M., 1969. A profile refinement method for nuclear and magnetic structures. J. bricate faults during seismic slip. Geology 39, 599–602. Appl. Crystallogr. 2, 65–71. Holdsworth, R.E., van Diggeln, E., Spiers, C., de Bresser, H.J., Walker, R.J., Bowen, L., Rosenquist, I.T.H., 1966. Norwegian Research into the properties of quick clay – a review. 2011. Fault rocks from SAFOD core samples. J. Struct. Geol. 33, 132–144. Eng. Geol. 1 (6), 445–450.

84 M. Schäbitz et al. Journal of Structural Geology 106 (2018) 70–85

Saffer, D.M., Frye, K., Marone, C., Mair, K., 2001. Laboratory results indicating weak and 2009JB006383. potentially unstable frictional behavior of smectite clay. Geophys. Res. Lett. 28. Togo, T., Shimamoto, T., Ma, S., Wen, X., He, H., 2011. Internal structure of http://dx.doi.org/10.1029/2001GL012869. Longmenshan fault zone at Hongkou outcrop, Sichuan, China, that caused the 2008 Schleicher, A.M., Warr, L.N., van der Pluijm, B.A., 2009. On the origin of mixed-layered Wenchuan earthquake. Earth Sci. 24, 249–265. http://dx.doi.org/10.1007/s11589- clay minerals from the San Andreas Fault at 2.5-3 km vertical depth (SAFOD drillhole 011-0789-z. at Parkfield, California). Contrib. Mineral. Petrol. 157, 173–187. Viti, C., 2011. Exploring fault rocks at the nanoscale. J. Struct. Geol. 33, 1715–1727. Schleicher, A.M., van der Pluijm, B.A., Warr, L.N., 2010. Nanocoatings of clay and creep Wang, L., Wu, Z., Wang, P., Chen, T., 2013. Characteristic causation, and rehabilitation of of the San Andreas fault at Parkfield. Geology 38, 667–670. Zhouqu extraordinarily serious debris flow in 2010, China. J. Cent. South. Univ. 20, Shimamoto, T., Takemira, K., Fujimoto, K., Tanaka, H., Wibberley, C.A.J., 2001. Nojima 2342–2348. http://dx.doi.org/10.1007/s11771-013-1742-1. Fault Zone probing by core analyses. Isl. Arc. 357–359. Warr, L.N., Cox, S., 2001. Clay mineral transformation and weakening mechanisms along Shuzui, H., 2001. Process of slip-surface development and formation of slip-surface clay in the Alpine Fault, New Zealand. In: In: Holdsworth, R.E., Strachan, R.A., Magloughlin, landslides in Tertiary volcanic rocks, Japan. Eng. Geol. 61, 199–219. J.F., Knipe, R.J. (Eds.), The Nature and Tectonic Significance of Fault Weakening, vol. Skempton, A.W., Petley, D.J., 1967. The strength along structural discontinuities in stiff 186. Geological Society, London, pp. 85–1001 Special Publication. clays. Proc. Geotec. Conf. Oslo vo 2, 29–46. Wen, B.P., Aydin, A., 2003. Microstructural study of a natural slip zone: quantification Skempton, A.W., 1985. Residual strength of clays in landslides, folded strata and the and deformation history. Eng. Geol. 68, 289–317. laboratory. Geotechnique 35 (1), 3–18. Wen, B.P., Aydin, A., 2004. Deformation history of a landslide slip zone in light of soil Solum, J.G., van der Pluijm, B.A., Peacor, D.R., 2005. Neocrystallization, fabrics and age microstructure. Environ. Eng. Geosci. 2, 123–149. of clay minerals from the exposure the Moab Fault, Utah. J. Struct. Geol. 27, Wen, B.P., Aydin, A., 2005. Mechanism of a rain-induced slide-debris flow: constraints 1563–1576. from microstructure of its slip zone. Eng. Geol. 78, 69–88. http://dx.doi.org/10. Solum, J.G., van der Pluijm, B.A., 2009. Quantification of fabrics in clay gouge from the 1016/j.enggeo.2004.10.007. Carboneras fault, Spain and implications for fault behavior. Tectonophysics 475, Wenk, H.-R., Matthies, S., Donovan, J., Chateigner, D., 1998. BEARTEX: a Windows-based 554–562. program system for quantitative texture analysis. J. Appl. Crystallogr. 31, 262–269. Solum, J.G., Davatzes, N.C., Lockner, D.A., 2010. Fault-related clay authigenesis along the Wenk, H.-R., Kanitpanyacharoen, W., Voltolini, M., 2010. Preferred orientation of phyl- Moab Fault: implications for calculations of fault rock composition and mechanical losilicates: comparison of fault gouge, shale and shist. J. Struct. Geol. 32, 478–489. and hydrologic fault zone properties. J. Struct. Geol. 32, 1899–1911. Wenk, H.-R., Lutterotti, L., Kaercher, P., Kanitpanyacharoen, W., Miyagi, L., Vasin, R., Stünitz, H., Keulen, N., Hirose, T., Heilbronner, R., 2010. Grain size distribution and 2014. Rietveld texture analysis from synchrotron diffraction images: II. Complex microstructures of experimentally sheared granitoid gouge at coseismic slip rates - multiphase materials and diamond anvil cell experiments. Powder Diffr. 29, criteria to distinguish seismic and aseismic faults? J. Struct. Geol. 32, 59–69. 220–232. Sun, Q., Zhang, L., Ding, X.L., Hu, J., Li, Z.W., Zhu, J.J., 2015. Slope deformation prior to Wilson, B., Dewers, t., Reches, Z., Brune, J., 2005. Particle size and energetics of gouge Zhouqu, China landslide from INSAR time series analysis. Remote Sens. Environ. 156, from earthquake rupture zones. Nature 434, 749–752. 45–57. Wirth, R., 2004. A novel technology for advanced application of micro- and nanoanalysis Tang, C., Rengers, N., van Asch, Th.W.J., Yang, Y.H., Wang, G.F., 2011. Triggering in geosciences and applied mineralogy. Eur. J. Mineral. 16, 863–876. conditions and depositional characteristics of a disastrous debris flow event in Wirth, R., 2009. Focused Ion Beam (FIB) combined with SEM and TEM: advanced ana- Zhouqu city, Gansu Province, northwestern China. Nat. Hazards Earth Syst. Sci. 11, lytical tools for studies of chemical composition, microstructure and crystal structure 2903–2912. in geomaterials on a nanometre scale. Chem. Geol. 261, 217–229. Tembe, S., Lockner, D.A., Solum, J.G., Morrow, C.A., Wong, T.-F., Moore, D.E., 2006. Yu, G., Zhang, M., Cong, K., Pei, L., 2015. Critical rainfall thresholds for debris flows in Frictional strength of cuttings and core from SAFOD drillhole. Geophys. Res. Lett. 33. Sanyanyu, Zhouqu county, Gansu province, China. Q. J. Eng. Geol. Hydrogeol. http://dx.doi.org/10.1029/2006GL02726. http://dx.doi.org/10.1144/qjegh2014-078. Tembe, S., Lockner, D.A., Wong, T.-F., 2010. Effects of clay content and mineralogy on Zulauf, G., Palm, S., Petschick, R., Spies, O., 1999. Element mobility and volumetric strain frictional sliding behavior of simulated gouges: binary and ternary mixtures of in brittle and brittle-viscous shear zones of the super deep well KTB. Ger. Chem. Geol. quartz, illite, and montmorillonite. J. Geophys. Res. 115. http://dx.doi.org/10.29/ 156, 135–149. http://dx.doi.org/10.1016/S0009-2541(98)00189-2.

85