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2. Characterization of Fault Gouge from Deep Drilling and Implications for Fault Maturity minerals Article Carbonaceous Materials in the Fault Zone of the Longmenshan Fault Belt: 2. Characterization of Fault Gouge from Deep Drilling and Implications for Fault Maturity Li-Wei Kuo 1,* , Jyh-Rou Huang 2, Jiann-Neng Fang 3, Jialiang Si 4 , Sheng-Rong Song 5, Haibing Li 4 and En-Chao Yeh 2 1 Department of Earth Sciences, National Central University, Taoyuan 320, Taiwan 2 Department of Earth Sciences, National Taiwan Normal University, Taipei 106, Taiwan; [email protected] (J.-R.H.); [email protected] (E.-C.Y.) 3 National Taiwan Museum, Taipei 100, Taiwan; [email protected] 4 Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China; [email protected] (J.S.); [email protected] (H.L.) 5 Department of Geosciences, National Taiwan University, Taipei 106, Taiwan; [email protected] * Correspondence: [email protected]; Tel.: +886-3-4227151 (ext. 65628) Received: 22 June 2018; Accepted: 5 September 2018; Published: 7 September 2018 Abstract: In recent works on the determination of graphitization of carbonaceous materials (CM) within the principal slip zone (PSZ) of the Longmenshan fault (China), we demonstrated that the formation of graphite, resulted from strain and frictional heating, could be evidence of past seismic slip. Here we utilize Raman Spectroscopy of CM (RSCM) on the CM-bearing gouges in the fault zone of the Longmenshan fault belt, at the borehole depth of 760 m (FZ760) from the Wenchuan earthquake Fault Scientific Drilling project-1 (WFSD-1), to quantitatively characterize CM and further retrieve ancient fault deformation information in the active fault. RSCM shows that graphitization of CM is intense in the fault core with respect to the damage zone, with the graphitized carbon resembling those observed on experimentally formed graphite that was frictionally generated. Importantly, compared to the recognized active fault zone of the Longmenshan fault, the RSCM of measured CM-rich gouge shows a higher degree of graphitization, likely derived from high-temperature-perturbation faulting events. It implies that FZ760 accommodated numerous single-event displacement and/or at higher normal stresses and/or in the absence of pore fluid and/or along a more localized slip surface(s). Because graphite is a well-known lubricant, we surmise that the presence of the higher degree graphitized CM within FZ760 will reduce the fault strength and inefficiently accumulate tectonic stress during the seismic cycle at the current depth, and further infer a plausible mechanism for fault propagation at the borehole depth of 590 m during the Mw 7.9 Wenchuan earthquake. Keywords: graphitization; carbonaceous material; Longmenshan fault; Wenchuan earthquake; WFSD-1 1. Introduction During an individual seismic event, fault-zone rocks presumably accommodate most of the deformation [1] and likely preserve the evidence of slip events. Deformation events occurring at typical seismic rates of ~1 m/s, considered as localized frictional sliding, may drive thermally activated physical and chemical processes such as flash heating of asperities [2], frictional melting [3], gelification [4], and decarbonation [5]. The products derived from the aforementioned processes have Minerals 2018, 8, 393; doi:10.3390/min8090393 www.mdpi.com/journal/minerals Minerals 2018, 8, 393 2 of 12 been experimentally and theoretically demonstrated for fault lubrication during earthquakes [6,7]. In particular, fault pesudotachylytes, derived from rapid solidification of frictional melts, have been naturally and experimentally documented and is widely recognized as the evidence of seismic slip which allows the retrieval of earthquake source parameters [3,8–10]. However, because of severe alteration on fault rocks during long term fluid-rock interaction [11], few other geological evidence of fault slip have been reported [12–16], and it remains challenging to bridge rock record and fault deformation. The NE–SW-trending active Longmenshan thrust belt, bordering the western margin of the Sichuan basin (China), resulted in the Mw 7.9 Wenchuan earthquake in SE-Tibet and produced a ~240 km-long Yingxiu-Beichuan surface rupture zone (Figure1a) [ 17]. Importantly, the Wenchuan earthquake Fault Scientific Drilling project-1 (WFSD-1) was initiated in 2006 to investigate the physical and chemical processes associated with the large slip earthquake, a maximum slip of ~9.5 m and the estimated co-seismic slip velocity up to 3 m/s in Wenchuan [18]. The borehole was drilled to a depth of 1201 m and passed from the Pengguan complex (the hanging wall) into the Xujiahe Formation (the foot wall), approximately remarking the location of the Longmenshan fault zone between 570 m and 595 m at depths (Figure1b) [ 19]. The detailed observation of recovered borehole materials indicated that the Longmenshan fault is composed of fault breccias, cataclasites and black fault gouges enriched with poorly crystalline anthracite mainly derived from the coal bed within the Xujiahe Formation [19]. Kuo et al. [20] reported for the first time the co-seismic graphitization occurred within the CM-rich gouge of WFSD-1. As this 54-cm-thick fault gouge hosts the ~200-µm-thick principal slip zone (PSZ) of the 2008 Mw 7.9 Wenchuan earthquake at the depth of 589.22 m, Kuo et al. [20] proposed that graphitization was likely driven by frictional heating of CM. The discovery of co-eismic graphitization is a potential seismic indicator of fault slip, as graphite is relatively stable at the scale of geological time. Kuo et al. [21] soon experimentally demonstrated that co-seismic graphitization shows distinguishable characteristics of Raman spectra with respect to the surrounding fault breccia. The rock friction experiments on CM-rich gouge show that CM graphitization results in the variation of peak parameters, presumably due to strain [22] rather than strain rate. In addition, the transformation of poor crystallinity CM into graphite only took place with the presence of high temperature (~300 ◦C) which was deformed at a seismic rate of 3 m/s under room dry condition. Importantly, Kuo et al. [21] utilized the discovery of graphitization to determine seismic Longmenshan fault movement, which is in accordance with the observation from the WFSD-1 [23,24]. Here we report the first detailed characterization of both CM-bearing gouge and surrounding fault breccia within the Longmenshan fault system recovered from the WFSD-1. The CM-bearing gouge was collected from the borehole depth of ~760 m (Figure1c). By means of X-ray powder diffraction analysis (XRD) and micro-Raman spectroscopy, we establish a reference minerals and CM data set from the seismically active fault, address the geological evidence of on-fault graphitization, compare the characteristics of graphitization with the recognized active fault zone at 590 m depth (FZ590 hereafter) [19–21], and, by extension, present an application of these data for studies of exhumed CM-bearing fault segments, e.g., the trench at Jiulong where ruptured during the 2008 Mw 7.9 Wenchuan earthquake, as shown in the companion paper. Minerals 2018, 8, 393 3 of 12 Minerals 2018, 8, x FOR PEER REVIEW 3 of 12 FigureFigure 1. Shaded-relief 1. Shaded-relief map map of of the the eastern eastern TibetTibet showing the the major major Longmenshan Longmenshan fault fault belt, belt,the 2008 the 2008 Mw 7.9Mw Wenchuan 7.9 Wenchuan earthquake, earthquake, China, China, and and thethe locationlocation of of the the Wenchuan Wenchuan eart earthquakehquake Fault Fault Scientific Scientific DrillingDrilling project-1 project-1 (WFSD-1) (WFSD-1) drilling drilling site site [[19].19]. (a) The 240 240 km-long km-long Yingxiu-Beichuan Yingxiu-Beichuan surface surface rupture rupture and theand 80the km-long 80 km-long Guanxian-Anxian Guanxian-Anxian surfacesurface rupturerupture associated associated with with the the Mw Mw 7.9 7.9earthquake, earthquake, and and the locationthe location of WFSD-1of WFSD-1 site. site. Red Red starstar is is the the epicenter epicenter of the of themain main shock. shock. Inset is Inset a schematic is a schematic plot of the plot of Tibetan Plateau. (b) An N135°E cross-section of the WFSD-1 showing the Longmenshan fault system the Tibetan Plateau. (b) An N135◦E cross-section of the WFSD-1 showing the Longmenshan fault and surrounding formations encountered in the borehole. The upper arrow showing the 2008 system and surrounding formations encountered in the borehole. The upper arrow showing the 2008 earthquake principal slip zone located at 589.22 m depth. The lower arrow showing the bottom of the earthquakeLongmenshan principal fault slip system zone located located at at759 589.22 m depth m (the depth. target The fault lower zone arrow in thisshowing study), and the the bottom images of the Longmenshanof core samples fault of system the WFSD-1 located was at 759enlarged m depth in the (the right target panel fault as (c zone). (c) inThe this core study), image andexhibiting the images of coremajor samples portions of of the the WFSD-1 FZ760 along was the enlarged borehole in of the WFSD-1 right including panel as (brecciac). (c) and The gouge. core image exhibiting major portions of the FZ760 along the borehole of WFSD-1 including breccia and gouge. 2. Materials and Methods 2. Materials and Methods On the basis of the core description of the WFSD-1, the Longmenshan fault system, made of ten majorOn the fault basis zones, of the was core sandwiched description with of the Neop WFSD-1,roterozoic the Longmenshan Pengguan complex fault (mainly system, volcanic made of ten majorrocks fault and zones, pyroclastics) was sandwiched and the Triassic with Xujiahe the Neoproterozoic Formation (sandstone, Pengguan siltstone, complex and shale (mainly with some volcanic rockslamination and pyroclastics) of coal beds) and [19]. the Among Triassic these, Xujiahe the ac Formationtive Longmenshan (sandstone, fault siltstone, zone, corresponding and shale withto the some lamination2008 Mw of 7.9 coal Wenchuan beds) [19 earthquake,].
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