Mineral changes quantify frictional heating during a large low-friction landslide

Data on thermal decomposition constrain events around the transition on 5 June 2009 from creep deformation (over many decades) with sporadic rockfalls to catastrophic collapse of Jiweishan Mountain, China, which poured millions of tons of limestone rubble from a cliff top into the valley below, causing 79 deaths. We quantified frictional resistance (μ, 0.1–0.2) and rapid frictional heating using thermogravimetry, mineral alteration close to the sliding surface, and high-speed rotary experiments. The differential thermogravimetry (DTG) trough temperature varied from 794 ± 4 °C at the sliding interface to ~720 °C below 10 mm. We inferred that DTG trough changes reflected the substrate thermal history due to landslide frictional heating. X-ray diffraction (XRD) analysis showed thermal decomposition of talc (a major mineral in the shale at Jiweishan Mountain) to magnesium metasilicate (formation temperature of ~600 °C) and enstatite (formation temperature of 800–905 °C). The intensity of the enstatite XRD peak decreased with increasing depth below the sliding surface to disappear below 6.5 mm. Magnesium metasilicate appeared between 6.5 mm and 10.5 mm depth. We replicated the temperatures and mineral changes in high-speed rotary shear experiments. Heating above 800 °C at the sliding surface was confirmed by temperature measurements and thermal decomposition of dolomite to magnesium and calcium oxides in the shallowest samples (which could not have survived for 7 yr at our field site and thus were not detected in the field). We inferred that the slide surface was heated to at least 800 °C by frictional sliding and that the temperature below 10.5 mm depth did not rise above 600 °C. Evidence of dynamic localized recrystallization was found on both field and experimental sliding surfaces, which may be a further explanation for the ultra-low friction during the rapid sliding, besides the presence of a significant basal pore-gas pressure from CO2 and steam. INTODUCTION It is generally believed that increase in temperature during slip can dramatically decrease the friction at the base of a large landslide and thus greatly affect its dynamic behavior (Voight and Faust, 1982). Much previous work on landslide and fault frictional heating has focused on the presence of melt rock, which is direct evidence of melting of rock at a sliding interface (Weidinger and Korup, 2009; Tsutsumi and Shimamoto, 1997; Di Toro et al., 2006). However, in most rapid landslides, melt rock is not found, although thermodynamics theory requires that frictional heating at the sliding surface must occur to some degree (Thompson, 1798). Herein we used X-ray diffraction (XRD) and thermogravimetry (TG) to quantify the recent thermal history of substrate bedrock below a landslide sliding interface. We confirmed our findings experimentally using samples of shale from Jiweishan Mountain, China, in a high-speed rotary shear device. Our objectives were to quantify the frictional resistance and temperatures experienced during the sliding in A.D. 2009 on the bedrock interface beneath the large rapid Jiweishan landslide (Yin et al., 2011). GEOLOGICAL SETTING OF THE LANDSLIDE At approximately 3:00 p.m. on 5 June 2009, after years of inferred creep deformation (Yin et al., 2011) (Fig. DR4 in the GSA Data Repository1), a mass of ~13.3 × 106 tons of limestone and shale unexpectedly accelerated and slid rapidly along a weak shale interlayer in the coal-measure rocks on Jiweishan Mountain, Wulong County, China (Yin et al., 2011) (Fig. DR1). Although nearly 1000 townsfolk 1 GSA Data Repository item 2018053, details of the consequence of the movement of Jiweishan landslides, experimental methods, details of sliding surface striae, and details of temperature measurement, is available online at http://www.geosociety . org /datarepository /2018/ or on request from editing@geosociety.org. from the immediate area below the mountainside had long been evacuated to safety, some 79 people were killed in the unanticipated giant rock avalanche, which devastated the wider valley floor of Tiejiang Creek, destroying farms and an iron mine (see the Data Repository). Images of the landslide and the sampling area are shown in Figure 1A and in Figure DR2. Figure 1D is the profile along I-I′ in Figure 1A (Yin et al., 2011). Jiweishan Mountain is on a northwestern limb of the Zhaojiaba anticline in which lower Permian and Silurian coal-measure strata dip at 20°–32° toward N15°W, almost parallel to the adjacent valley. Most of the landslide mass was made up of rocks from the lower Permian *E-mail: 706618681@qq.com GEOLOGY, March 2018; v. 46; no. 3; p. 223–226 | GSA Data Repository item 2018053 | https://doi.org/10.1130/G39662.1 | Published online 4 January 2018 © 2018 The Authors. Gold Open Access: This paper is published under the terms of the CC‐BY license. Figure 1. Location of the Jiweishan Mountain (China) landslide sliding surface where frictionally heated samples were obtained (at 29.2385°N, 107.4337°E) and geological cross section of landslide before failure. A: Image of sliding source area. Yellow circle shows location of the sampling area (shown in B). B: Polished and scratched pavement formed on dolomitic black shale. C: Hand specimen of pavement shale from which samples were taken for X-ray diffraction and thermogravimetric analyses at various shallow depths to 13 mm below sliding surface. D: Geological cross section I-I′ of landslide. Green color highlights sliding rock mass of landslide. Red arrows indicate the interaction between sliding mass and the bedrock. Section modified from Yin et al. (2011). Downloaded from https://pubs.geoscienceworld. org/gsa/geology/article-pdf/46/3/223/4072567/223.pdf by Chengdu University of Technology user on 16 July 2019 224 www.gsapubs. org | Volume 46 | Number 3 | GEOLOGY Maokou Group consisting of mainly grayish-white thick-layered dolomitic limestone, which forms the crest of the mountain above steep cliffs facing the valley of Tiejiang Creek. The lower part of the source mass is the lower Permian Qixia Group, composed dominantly of dark gray and gray medium-layered black dolomitic limestone interbedded with micritic dolomitic limestone (Fig. 1D; Yin et al., 2011). The steep cliffs below the landslide source mass expose gray limestone with a thickness of 90–95 m. This massive limestone is overlain by a 40-m-thick, dark-gray, largely limestone stratum. Near the top of the 40-m-thick layer, there is a 30-cm-thick, particularly weak, black calcareous shale interlayer on which the rupture surface was formed, as shown in Figure 1D (Yin et al., 2011). MATERIALS AND METHODS We examined exposures of the sliding surface above the cliff in May 2016. The surface is preserved on a stratum of well-indurated, dark-gray, dolomitic shale and displays well-developed scratches in the direction of landslide motion (Figs. 1B and 1C; more details are provided in the Data Repository, including Fig. DR5). These clearly indicate that the landslide moved in close frictional contact with its base. The surface was highly reminiscent of a striated pavement from beneath a glacier. In addition to examining the obvious evidence of erosion and physical damage to the exposed bedrock (Fig. 1B; Fig. DR5), we also sought evidence of damage from the frictional heat that must have been generated as the landslide slid over the pavement. We took samples of the exposed stratum back to the laboratory where we determined mineral and temperature changes using TG, XRD, scanning electron microscopy (SEM), and electron probe (see details of the experimental method in the Data Repository). We took ten samples at different depths below the sliding surface at site A (Fig. 1B) (sample [1], 0–2 mm depth; [2], 2–4 mm; [3], 4–5 mm; [4], 5–6 mm; [5], 6–7 mm; [6], 7–9 mm; [7], 9–10 mm; [8], 10–11 mm; [9], 11–12 mm; [10], 12–13 mm). Sample [11] was taken across the 10 layers (0–13 mm). Samples were cut from the shale parallel to the striated surface using a sharp knife. Depth increments were measured with a digital micrometer. Samples were ground to fine powder using a mortar and pestle, before conventional TG–differential thermal analysis measurements (see the Data Repository) were carried out.