Geochemical features of the pseudotachylytes in the Longmen Shan thrust belt, eastern Tibet

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Geochemical features of the pseudotachylytes in the Longmen Shan thrust belt, eastern Tibet Huan Wanga,b, Haibing Lia,b,∗, Jialiang Sia,b, Lei Zhanga,b, Zhiming Sunc a

Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources, Beijing, 100037, China Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037, China c Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing, 100081, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Pseudotachylyte Geochemical analyses μXRF WFSD Longmen Shan

Pseudotachylytes, produced by frictional heating during seismic slip, convey information that is critical to understanding the physics of earthquakes, their geochemical features triggered by frictional melting shed light on the seismic processes and frictional strength as well. However, whether their geochemical properties changed during exhumed process is still uncertain. To provide insight to this topic, we conducted a case study of pseudotachylytes in the Longmen Shan thrust belt, eastern Tibet. Pseudotachylyte samples drilled from the WFSD boreholes at deep depth and collected at surface outcrops were analyzed by μXRF core scanner to estimate their geochemical variations. Macro- and micro-structural observations suggest that the pseudotachylytes are from melt-origin. Comparative analyses of the geochemical characteristics between surface and deep pseudotachylytes show that the former are K- and Ti-depleted in an oxidation environment, while the latter have low Si, high K, Fe, Ti and Mn in a reductive environment. Besides the selective melting process, another reasonable explanation for these differences is that the geochemical properties of the surface pseudotachylytes have changed due to the prolonged fluid influence during the long-term exhumation processes, which cannot exactly reflect the nature of the initial frictional melt. Therefore, we conclude that primary pseudotachylytes are characterized by enrichment of K, Fe, Ti, Mn and depletion of Si. The frictional melt was generated in high-temperature reductive environment, and fluids have influenced their chemical composition during the exhumation process. These results are of great significance for correctly understanding earthquake mechanics and fault behavior.

1. Introduction Fault related pseudotachylytes, commonly associated with either cataclastic rocks (e.g. Sibson, 1975; Magloughlin, 1992; Di Toro and Pennacchioni, 2004) or mylonitic rocks (e.g. Swanson, 1992; Camacho and Vernon, 1995; Lin et al., 2003) develop over a wide range of crustal depths. Considered as one of the few indicators of seismic slip in the geologic record (Rowe and Griffith, 2005), pseudotachylytes potentially hold valuable information about the physical-chemical conditions during the earthquake processes (O'Hara, 1992; O'Hara and Huggins, 2005), as well as the textures and structures resulting in the deformation processes (e.g. Sibson, 1975; Magloughlin, 1989). These information provide insight into the fault frictional strength, earthquake energy budget, and fault behavior, which are of great importance for earthquake forecasting and early warnings. The mineralogical and geochemical features gathered from the melt-origin pseudotachylytes can help to constrain the coseismic temperatures generated by fault slip

∗

(Hirono et al., 2006) and possibly fluid-rock interaction during seismic faulting. However, the reported pseudotachylytes were usually generated at deep depths and exposed at the surface following exhumation or uplift, thus whether their chemical features are altered or not is still unclear due to the scarcity of direct evidence. Pseudotachylytes associated with cataclasites are well-exposed in the Longmen Shan thrust belt at the eastern margin of the Tibetan Plateau. Previous research have reported their macro- and microstructures (Wang et al., 2015, 2018), rock magnetism measurement (Zhang et al., 2017), and isotopic dating (Zheng et al., 2016), but geochemical analyses are still lacking in order to properly constrain the thermodynamic conditions in the fault zone as well as the earthquake source mechanics. The micro-X-Ray Fluorescence (μXRF) core scanner, known as a rapid, precise and non-destructive technique to obtain elemental variations of rocks, has been widely used in sedimentology to interpret past environmental and climatic changes (e.g. Ramsey et al., 1995; De Vries and Vrebos, 2002), but are rarely used in fault rock

Corresponding author. Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037, China. E-mail address: [email protected] (H. Li).

https://doi.org/10.1016/j.quaint.2018.12.030 Received 31 March 2018; Received in revised form 7 December 2018; Accepted 31 December 2018 1040-6182/ © 2019 Elsevier Ltd and INQUA. All rights reserved.

Please cite this article as: Wang, H., Quaternary International, https://doi.org/10.1016/j.quaint.2018.12.030

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and the Yangtze Platform. With a long history of geological evolution and complex structures (Burchfiel et al., 1995, 2008; Xu et al., 1992, 2007), the seismogenic Longmen Shan thrust belt is composed of three subparallel imbricated faults (Fig. 1b) (Deng et al., 1994; Densmore et al., 2007): the Wenchuan-Maoxian fault to the west, the central Yingxiu-Beichuan fault (YBF), and the Guanxian-Anxian fault to the east. Field investigation reveals that these faults have been active throughout the late Quaternary, with slip rates up to 1.0–1.5 mm/yr (Li et al., 2006; Densmore et al., 2007). The 2008 Wenchuan earthquake occurred in the Longmen Shan area (Fig. 1b), and generated surface ruptures of ∼270 km-long along the YBF and ∼80 km-long along Guanxian-Anxian fault (Li et al., 2008; Fu et al., 2011). The WFSD project was rapidly implemented, only 178 days after the main shock (Li et al., 2013). The first (WFSD-1) and second (WFSD-2) boreholes were drilled in the southern part of the YBF at Bajiaomiao village in the Hongkou area (Fig. 1c), where Neoproterozoic Pengguan complex and Triassic Xujiahe Formation are located (Li et al., 2013; Wang et al., 2014, 2015). Drilling research and field investigation reveal that fault rocks prevail in the YBF area, indicating that the YBF has been active repeatedly as a seismogenic fault produced large earthquakes (Wang et al., 2015, 2018). 3. Material and methods 3.1. General description and sample collection Pseudotachylytes intimately associated with cohesive cataclastic rocks are found in the Pengguan complex in the southern part of the YBF at the Bajiaomiao village in Dujiangyan, Sichuan, near where WFSD-1 and WFSD-2 boreholes were drilled (Fig. 1c). Pseudotachylytes, brownish, gray, dark gray and black in color, occur as fault veins along the generation surface and injection veins into the fractured grayish cataclastic rocks (Wang et al., 2015, 2018) (Fig. 2a–e). Different colored fault veins, each with thicknesses ranging from several millimeters to decimeters, are subparallel with sharp boundaries between the adjacent ones (Fig. 2b and c) and locally show overprinting relationships (Fig. 2d). The margin of the black veins are relatively more aphanitic and compact in their centers (Fig. 2c–e), and clasts can clearly be seen in the pseudotachylyte veins (Fig. 2c). Pseudotachylytes are also observed in the WFSD-1 and WFSD-2 drilling cores at ∼570-600 m-depth (Fig. 2f–g). Pseudotachylytes, darkgray and black in appearance, are found at ∼585 m-depth in WFSD-1 (Fig. 2f), where grayish clasts with different sizes lie in the dark matrix (Fig. 2h). In WFSD-2, pseudotachylytes are located at ∼579-600 mdepth, in cataclastic rocks (Fig. 2g) with much complex and irregular morphologies and gray, dark-gray, brown and black colors (Fig. 2i). The veins, ranging from several millimeters to decimeters, commonly contain small sub-angular to sub-rounded clasts in the aphanitic matrix. Two sample blocks (Fig. 2c and e) collected by a drilling machine with a diameter of 10 cm in the Bajiaomiao outcrop, were cut and polished for μXRF measurement and to prepare thin sections. In addition, two cylinders from the WFSD-2 drilling cores, one at depths of 585.14–585.65 m, the other at 585.93–586.73 m-depth (Fig. 2i), were chosen for this study. The cylinders were cut in half and polished carefully, analyses were carried out in split halves and the other halves were archived.

Fig. 1. (a) Tectonic structures of the Tibetan Plateau. (b) Active faults in the Longmen Shan area and its adjacent areas, which are located at the eastern margin of the Tibetan plateau (red box in Fig. 1a). F1: Wenchuan-Maoxian fault, F2: Yingxiu-Beichuan fault, F3: Guanxian-Anxian fault. (c) Google Earth image shows the location of the WFSD drilling sites on the hanging wall along the Yingxiu-Beichuan fault, as well as that of the surface outcrop where pseudotachylytes exposed (yellow star). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

studies. This research is an attempt at examining the geochemical alteration ranges and processes that take place in natural pseudotachylytes collected from the Bajiaomiao outcrop and from the Wenchuan earthquake Fault Scientific Drilling (WFSD) cores (Fig. 1), to compare the geochemical features of the pseudotachylytes from surface and at deep depth. Chemical compositions are measured using an Itrax μXRF core scanner to estimate the subtle variations in fault rocks. These data can be used to illustrate more accurate elemental variations in fault rocks, which provides new insights into the element migration in the pseudotachylytes due to frictional melting and fluid-rock interaction afterwards. This research will supply important information that will improve our understanding of seismic faulting mechanisms and fault behavior in seismogenic fault zones.

2. Geological setting

3.2. Methods

The eastern margin of the Tibetan Plateau is an active tectonic boundary composed of three major faults (Fig. 1a): the NWW-trending East Kunlun fault to the north, the NW-trending Xianshuihe fault to the south, and the NE-trending Longmen Shan thrust belt at the eastern margin located in between the former two. The Longmen Shan thrust belt extends from Guangyuan City in the north to Tianquan County in the south, located at the boundary between the Songpan-Ganzi fold belt

For this study, the microscopic structures were observed using optical microscope and scanning electron microscope (SEM). The chemical composition were measured by μXRF core scanner, which provides continuous high resolution elemental profiles to distinguish the subtle chemical differences. Two split archived core from the WFSD-2 drilling cores and two sample blocks from the Bajiaomiao outcrop were processed using an Itrax μXRF core scanner with a Mo-tube at 40 kV and 2

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Fig. 2. Features of the pseudotachylytes in the YBF. (a–e) Pseudotachylytes from the Bajiaomiao outcrop. (f) Pseudotachylyte distribution in the WFSD-1 drilling cores. (g) Pseudotachylyte distribution in the WFSD-2 drilling cores. (h) Pseudotachylytes at 584.73–585.00 m-depth in WFSD-1. (i) Pseudotachylytes from WFSD-2 at 585.93–586.73 m-depths. Pst: pseudotachylyte, fv: fault vein, iv: injection vein, Cata: cataclasite. Fig. 2a–e are modified from Wang et al. (2015).

30 mA. Counts of elements were obtained every 200 μm for the surface samples and 500 μm for the drilling core halves measured at different periods. Based on our previous experience, the elemental profiles with intervals of 200 μm and 500 μm are much similar for the fault rocks, which do not affect the analyzed results. The above measurements were carried out in the Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources in Beijing. The whole-rock major elements measurements were conducted using a 3080E X-Ray Fluorescence (XRF) Spectrometer, at the National Research Centre of Geoanalysis in Beijing. Elemental mapping was carried out on the pseudotachhylytes thin sections, using a M4 Torando of Bruker Corporation in Shanghai.

observed under the SEM. Typical melt textures are observed as strings stream and melt patches (Fig. 3i). 4.2. Whole-rock major elements Thirteen samples, six from the surface and seven from the core halves, were selected for whole-rock major elemental analyses. The results are shown in Table 1 and plotted in Fig. 4. The SiO2 contents range from 61.92% to 72.01% in most samples, except for the two gray pseudotachylyes (46.56% for C2 and 55.08% for C5) collected from the core halves. Al2O3 contents are 13.42–13.94% for the surface cataclasites and 11.61–14.09% for the cataclasites from the WFSD-2 drilling cores. For the dark pseudotachylytes, SiO2 contents are 61.92–71.26%, and Al2O3 contents are 11.82–16.66%. In the gray pseudotachylytes, SiO2 contents are 46.56–63.65%, and 7.72–18.23% for Al2O3. The other chemical components, such as CaO, Fe2O3, FeO, K2O, TiO2 and CO2, show slight variations in most samples except the gray pseudotachylytes (S2, C2 and C5). Na2O and P2O5 display much changes between different fault rocks. The total alkalis versus silica diagram shows that most of the fault rocks are granitic-granodioritic rocks (Fig. 5), except a few pseudotachylytes (C2, C5 and C6) which migrate to grabbro-diorite areas.

4. Results 4.1. Microstructural observations At the thin-section scale, pseudotachylytes within the felsic cataclastic rocks occur as μm-to cm-thick single or foliation-parallel veins, which show sharp contacts with the adjacent rocks (Fig. 3a–d). The diagnostic characteristics of the pseudotachylytes is clast-in-matrix texture wherein clasts, or survived grains, as sub-angular to rounded and embayed monomineralic clasts (quartz and feldspar) and polymineralic fragments are typically matrix-supported. The clasts in the matrix show finer grain sizes compared to the host cataclastic rocks. As noted optically, the matrix is generally rich in phyllosilicates where flow bandings are visible (Fig. 3a–c). Microlites, appearing as fan-like (Fig. 3e), plume-like (Fig. 3f), stumpy, granulous and sperulitic (Fig. 3g and h), are present in the melt matrix. The stumpy microlites and sperulites are nano-scaled to a few microns in length (Fig. 3g and h) as

4.3. μXRF chemical profiles There are significant variations in the scanned μXRF element profiles of all studied samples. The results for the samples from the Bajiaomiao outcrop are shown in Fig. 6. K, Ca, Fe, Ti and Mn show clear variations across each lamination (Fig. 6a): in the thick layered 3

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Fig. 3. Microstructures of pseudotachylytes in the YBF. (a) Photo of a thin section showing multiple generations of pseudotachylytes in cataclasite. (b–c) Pseudotachylyte with flow structure and clasts in dark matrix. (d) Pseudotachylyte vein injected into cataclasite. (e) Fan-like microlites. (f) Plume-like microlites. (g) Spherulites and stumpy microlites. (h) Spherulites. (i) Viscous melt patches. a–f: plane-polarized light images. g–i: SEM images. Cata: cataclasite. Pst-1, Pst-2 and Pst-3 indicate the different generations of pseudotachylytes.

The μXRF mapping of the pseudotachylyte and cataclasite shows that elements in cataclasite are diffused, while in the pseudotachylytes, the elements are enriched in distinct bands (Fig. 8): the gray pseudotachylyte (Pst-1) is enriched in Si and S while depleted in K, Ba and Ti. The dark pseudotachylyte (Pst-3) is enriched in K, Ba and Ti, and depeleted in Si. The elemental enrichment of the Pst-2 is between Pst-1 and Pst-3.

pseudotachylytes, the gray one (Pst-1) is higher in Ca, Fe and Mn, lower in K and Ti, and the ratios of Ca/Fe, Mn/Fe and Fe/Ti are higher. In the black layer (Pst-3), the elements show no clear variation compared to that of cataclasite (Fig. 6a). The other sample displays a similar element variation trend in the lower part of the pseudotachylyte vein (Fig. 6b), enriched in Si, Ca, Fe and Mn, depleted in K, Ti and with higher Ca/Fe, Mn/Fe and Fe/Ti ratios. For the core halves from the WFSD-2 drilling cores, the elements show highly variable responses to the different rocks as well (Fig. 7). The pseudotachylytes have much higher magnetic susceptibility than cataclasites. The gray to dark-gray pseudotachylyte veins at depths of 585.14–585.65 m show clear enrichment of K, Ca, Fe, Ti and Mn, along with depletion of Si compared to the cataclasites (Fig. 7a). At 585.93–586.73 m-depth, the pseudotachylyte veins are smaller with more complex morphologies, and each vein shows similar increase of K, Ca, Fe, Ti and Mn and decrease of Si. Fe, Ti and the magnetic susceptibility are in good correlation.

5. Discussion 5.1. Chemical properties of the seismic frictional melting Based on the macro- and micro-structural observations, the distinct textures of flow structures, microlites and melt patches suggest that the pseudotachylytes are melt-origin (Wang et al., 2015, 2018). Physicalchemical properties of the seismic frictional melt is of great significance for understanding the earthquake mechanisms and seismogenic

Table 1 Results of whole-rock major elements analyses (in wt%) of the representative pseudotachylytes and cataclasites in the Yingxiu-Beichuan fault zone. Sample locations are shown in Figs. 6 and 7. Pst: pseudotachylyte, Cata: cataclasite. Sample

lithology

SiO2

Al2O3

CaO

Fe2O3

FeO

K2 O

MgO

MnO

Na2O

P2O5

TiO2

CO2

H2O+

S1 S2 S3 S4 S5 S6 C1 C2 C3 C4 C5 C6 C7

Cata Gray Pst Dark Pst Cata Dark Pst Cata Cata Gray Pst Cata+pst vein Dark Pst Gray Pst Dark Pst Cata

66.93 63.65 66.79 72.14 70.76 67.38 68.83 46.56 67.37 71.26 55.08 61.92 72.51

13.94 7.72 16.09 13.42 11.82 13.87 11.61 17.23 14.09 14.17 18.23 16.66 13.45

1.45 5.38 1.65 1.89 3.25 3.18 5.73 9.86 3.08 1.44 4.41 0.74 2.65

0.93 0.73 0.41 0.46 0.25 0.52 0.34 0.92 0.90 0.94 1.33 1.02 0.33

3.92 6.86 2.43 1.59 2.00 2.13 1.10 3.06 1.50 1.66 2.00 6.01 1.05

3.31 2.13 4.01 2.84 3.09 2.36 1.92 5.16 4.22 3.99 5.64 3.26 2.79

1.00 1.92 1.01 0.87 1.08 1.23 0.39 1.20 0.72 0.73 1.00 1.45 0.47

0.17 0.36 0.09 0.08 0.10 0.12 0.11 0.17 0.06 0.04 0.07 0.03 0.06

2.33 0.67 2.19 2.99 1.11 3.93 3.49 0.19 0.18 0.54 0.17 2.03 2.98

0.44 0.22 0.42 0.09 0.37 0.08 0.04 0.47 0.25 0.10 0.52 0.17 0.05

0.55 0.17 0.40 0.26 0.28 0.24 0.21 2.85 1.29 0.41 2.92 0.73 0.28

3.88 9.65 2.60 2.93 4.22 4.52 5.09 9.07 2.36 1.28 3.48 0.69 2.45

2.10 1.36 1.92 1.36 1.78 1.30 1.32 3.46 2.20 2.32 3.02 3.46 1.52

4

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Fig. 4. Plot of the major elements in the cataclasites and pseudotachylytes. Sample locations are shown in Figs. 6 and 7.

properties of the primary frictional melts and the variations caused afterwards during their uplift.

environment (Zoback et al., 2007; Okamoto et al., 2007). Fault-related pseudotachylyte usually generated at deep depth of > 4 km (Sibson, 1975; Swanson, 1992), conveys important information about seismic faulting at depth. In general, it is difficult to retrieve the fault rocks at deep depths, especially at/near the seismogenic depth (∼15 km, Sibson, 1975). Pseudotachylytes exposed at the outcrop or at shallow depth are likely altered or devitrified during their prolonged exhumation or uplift history, the initial melt components are difficult to be preserved (Petrík et al., 2003), thus constraining our understanding of the seismic faulting mechanisms and seismogenic environment of large earthquakes. Comparative study of pseudotachylytes from the outcrop and the deep drilling cores can help to recognize the chemical features that are preserved in the frictional melt and alterations afterwards. Recent research shows that the surface pseudotachylytes in the Longmen Shan thrust fault were formed 231–238 Ma ago near the seismogenic zone at ∼10-15 km-depth (Zheng et al., 2016; Wang et al., 2018). The different geochemical features preserved in pseudotachylytes at deep depths and at the surface may shed light on the chemical

5.1.1. Chemical differences between the pseudotachylytes from the surface and the drilling cores Chemical properties of the surface pseudotachylytes are different from that of the drilling cores. From the major elemental data, the content of Fe2O3, TiO2 and H2O+ in the drilling cores are higher than that in the surface samples, while Na2O, MnO and CO2 are lower (Table 1 and Fig. 4). The SiO2 content in the deep pseudotachylytes (C2, C5, C6) is much higher than in the surface ones. The protolith of these pseudotachylytes are granitic-granodioritic cataclasites, the SiO2 contents should be ≥ 65% (gray area in Fig. 5). However, for the core samples of C2, C5 and C6, SiO2 is clearly depleted, and the surface sample S2 also shows that the SiO2 content decreased (Fig. 5). The μXRF scanning data show a similar trend with the major elemental data: Si is slightly enriched in the surface pseudotachylytes (Fig. 6) and clearly depleted in the deep pseudotachylytes (Fig. 7). Previous

Fig. 5. Total alkalis versus silica diagram (after Middlemost, 1994) of cataclasites and pseudotachylytes in the Longmen Shan area. 5

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Fig. 6. Elemental intensity profiles for the pseudotachyltes at the Bajiaomiao outcrop determined by Itrax μXRF core scanner (200 μm step size). Sample locations are shown in Fig. 2c and e. The yellow spots indicate the sample locations taken for whole-rock major elements analyses. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

the core samples (Fig. 4, Table 1), and gray pseudotachylytes are higher in Si, Ca, Fe, Mn and lower in K, Ba, Ti, compared to the dark pseudotachylytes (Figs. 6a, 7 and 8). Therefore, these pseudotachylytes are considered to be formed by the selective melting of constituent materials during frictional melting rather than total or partial melting, their composition are controlled by the proportions of minerals that preferentially enter the melt. Chemical features of these pseudotachylytes were changed during their uplift process from ∼10 to 15 km-depth (Zheng et al., 2016) to 600 m-depth or to the surface. These results demonstrate that frictional melting occurred in a chemical non-equilibrium state, their composition is constrained by the proportion of minerals that preferentially melted. Chemical properties of pseudotachylytes have changed during the uplift and exhumation processes from their formation depth to shallow depth (< 600 m) or to the surface. Ankerite and calcite prevailing in the pseudotachylytes suggest that CO2-rich fluids passed through the pseudotachylytes, causing localized mineral alteration. Thus the chemical differences between these pseudotachylytes can reflect the chemical variations due to their later evolution.

research shows that the major minerals in the deep pseudotachylytes (C5 and C6) are quartz (39-30%), plagioclase (9–17%), muscovite (37–41%) and calcite (12%), and for the surface paeudotachylytes, they are quartz (50–72%), plagioclase (7–29%), muscovite (1–7%), ankerite (7–20%), calcite (1%) and apatite (7%) (Table 2, Fig. 9, data from Wang et al., 2018). Heating experiments have been conducted on cataclasites from the WFSD-2 drilling cores (Zhang et al., 2018). The results show that calcite disappeared due to the thermal decomposition at 900 °C when the melting process has not occurred. When heated to 1100 °C, melting occurred and muscovite disappeared, when heated to 1300 °C, quartz and plagioclase were partially melted and the new mineral cristobalite appeared. When heated to 1500 °C, all the quartz and plagioclase melted and cristobalite remains as the only mineral in the amorphous material (Fig. 9, Table 2). Apparently, mineral phase varies with temperature at different melting stages, by preferential melting of hydrous minerals, then plagioclase and quartz (Spray, 1992; Petrík et al., 2003). The μXRF scanning results show that K is significantly lower in the surface pseudotachylytes (Pst-1) than in their surrounding rocks (Fig. 6). In the drilling core samples, K, Fe and Ti are higher in the pseudotachylyte veins (Fig. 7). Major elemental analyses show that the K2O content in the gray pseudotachylyte is apparently lower than the dark pseudotachylyte and cataclasites, while in the drilling core samples, the K2O content in the pseudotachylytes is much higher than their host cataclasite (Fig. 4). The concentrations of CaO, K2O, Fe2O3, MnO, P2O5, TiO2, CO2 and H2O+ are higher in the gray pseudotachylytes in

5.1.2. Geochemical features of the primary frictional melt Pseudotachylyte veins usually share a similar bulk chemical composition with their host rocks (e.g. Sibson, 1975; Magloughlin, 1989, 1992; Lin, 1994a,b; Lin and Shimamoto, 1998; Di Toro and Pennacchioni, 2004). However, the similarity in bulk composition does not mean a complete melting of host rocks (Lin, 2008). The 6

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Fig. 7. Elemental intensity profiles for pseudotachyltes in WFSD-2 drilling core determined by Itrax μXRF core scanner (500 μm step size). Sample locations are shown in Fig. 2g. Image b is one of the split halves of Fig. 2i. The yellow spots indicate the sample locations taken for whole-rock major elements analyses. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

2011). Therefore, the enrichment of K and depletion of Si in deep pseudotachylytes suggest that, due to the frictional heating caused by seismic faulting, minerals (such as quartz and feldspar) were partially melted, thus their grain sizes were smaller. Pseudotachylyte veins are usually higher in Fe, Ti and Mn contents with higher magnetic susceptibility compared to their host rocks (Fig. 7). Previous research show that both natural and artificially generated pseudotachylytes are more highly magnetic than their protoliths, due to the fine-grained ferromagnetic minerals and neo-formation of magnetite (Nakamura et al., 2002; Ferré et al., 2005, 2012; Pei et al., 2014; Zhang et al., 2017) or zero-valent iron (Zhang et al., 2018) under high temperature. However, in the outcrop samples, pseudotachylytes are depleted in K and Ti (Fig. 6), while in Fig. 6b, the pseudotachylyte is slightly riched in Si, which is in contrast to those found in the WFSD-2 drilling cores. K is considered to be the most reliable indicator of moisture fluctuations (Foerster et al., 2012), when covered by extensive vegetation, K would be preferentially leached away through chemical weathering and result in lower K values (Davies et al., 2015). Ti is a conservative element which commonly co-varies with Fe, which is widely used to record terrigenous sediment delivery, particularly through runoff, and hence assess hinterland climate, particularly rainfall (Kissel et al., 2010; Peterson et al., 2000). The enrichment in Ti usually refers to the

pseudotachylyte composition is controlled by the proportion of minerals that preferentially enter the melt. The major minerals in the studied cataclasites and pseudotachylytes are highly similar and mainly consist of quartz, plagioclase, mica, ankerite and calcite while chlorite, siderite, apatite and Kaolinite occur as secondary phases (Wang et al., 2015, 2018; Zhang et al., 2017). Compared to the cataclasites, gray pseudoatchylytes show a shift towards low SiO2 and high CaO, CO2 (Fig. 4), while dark pseudotachylytes show similar trends as cataclasites. The data in the elemental profiles (Figs. 6 and 7) display a large spread due to compositional heterogeneity caused by unavoidable mineral fragments. However, the overall trends show that the composition of the deep pseudotachylytes differ from that of cataclasites with low Si and high K, Ca, Fe, Ti and Mn contents (Fig. 7), which is similar to the chemical properties reported in other areas (e.g. O'Hara, 1992; Jiang et al., 2015). The depletion of Si in the pseudotachylyte matrix is likely because of the preferential melting of minerals with lower melting temperatures such as phyllosilicates, mica and albite, whereas those with high melting temperatures such as quartz survived as fragments. Si profile shapes may give information on textural characteristics of grain sizes, with high Si value referring to coarse grains and low Si referring to fine ones (Cuven et al., 2010). High K values indicate the fine-grains of the substance (Kylander et al., 7

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Fig. 8. Single element maps measured on thin section by M4 Torando. (a) Microstructure of pseudotachylyte, location shown in Fig. 3a. (b–h) Single elemental maps of Si, K, Ca, Ba, Fe, Ti and Mn. Color bars represent XRF intensity. The red areas correspond to high abundance. Laminations are clearly visible in Si, K, Ca, Ba and Ti. K, Ba and Ti are in good correlation, which share an inverse correlation with Si. Laminations in Fe and Mn are not distinct. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Table 2 Mineral characteristics of the fault rocks in the Yingxiu-Beichuan fault. Pst: pseudotachylyte, Cata: cataclasite. Sample

Lithology

Mineralogy

S1 S2 S5 C3 C5 C6 Ex1 Ex2 Ex3 Ex4 Ex5

Cata Gray Pst Dark Pst Cata+Pst vein Gray Pst Dark Pst Cata Cata heated to Cata heated to Cata heated to Cata heated to

Quartz 49%, Plagioclase 4%, Ankerite 40%, Mica 2%, Calcite 2%, Chlorite 2% Quartz 56%, Plagioclase 27%, Ankerite 8%, Mica 6%, Siderite 2% Quartz50%, Plagioclase 29%, Ankerite 7%, Mica 7%, Apatite 7% Quartz 46%, Plagioclase 30%, Mica 12%, Calcite 12% Quartz 39%, Plagioclase 9%, Mica 37%, Calcite 12%, Kaolinite 4% Quartz 30%, Plagioclase 17%, Mica 41%, Calcite 12% Quartz 47%, Plagioclase 31%, Mica 13%, Calcite 9% Quartz 60%, Plagioclase 25%, Mica 14% Quartz 71%, Plagioclase 29% Quartz 58%, Plagioclase 25%, Cristobalite 17% Cristobalite 100%

900 °C 1100 °C 1300 °C 1500 °C

Data for Ex1-5 are from Zhang et al. (2018); the rest is from Wang et al. (2018).

5.1.3. Redox conditions evaluation of pseudotachylytes Element ratios, as process and environmental proxies, are used for evaluating the redox environment (Rothwell and Croudace, 2015). As the most common element on Earth, Fe usually occurs in the form of iron oxide (hematite, magnetite). High Fe values not only indicate rock compactness (Van Daele et al., 2014) but also reflect the redox conditions (e.g. O'Hara and Huggins, 2005; Sluijs et al., 2009). Mn is a highly redox-sensitive indicator (Rothwell and Croudace, 2015). The Mn/Fe ratio is used to assess redox condition, with high Mn/Fe value indicating oxidation environment, while low values refer to a reductive environment (Marsh et al., 2007; Burn and Palmer, 2014). The low Fe/ Ti also suggests a reductive environment (Rothwell and Croudace, 2015). The pseudotachylytes from the WFSD-2 drilling cores at 585.93–586.73 m-depth are higher in Fe, corresponding to lower Mn/ Fe and Fe/Ti values (Fig. 7), which probably implies a reductive environment during frictional melting. While for the surface pseudotachylytes, the gray one (Pst-1) with high Mn/Fe and Fe/Ti values (Fig. 6a) likely point to an oxidation environment after having been exhumed. These indicate that the surface pseudotachylytes have experienced supergenetic oxidation processes.

presence of compact material formed in high temperature environments, such as tephra (Kylander et al., 2012). Generally, Ti is not easily leached away, but in warm and humid conditions where chemical weathering is significant, Ti (mainly in the form of TiO2·nH2O) can migrate due to excessive leaching (Nesbitt and Markovics, 1997; Li et al., 2010). The Longmen Shan area is covered by lush vegetation, lots of plant residues are involved in the weathering process, organic acids such as oxalic acid produced by the decomposition of plant residues may lead to the decomposition of Ti-bearing minerals and removal of Ti (Mclennan, 1993; Fedo et al., 1995). These are probably responsible for the low Ti content in surface pseudotachylytes. Further research is still needed to explain the higher Si content in surface pseudotachylytes. Taken together, we propose that the stable elemental abundances in all pseudotachylytes (high Fe and Mn) and the variable element enriched (K, Ti) in deep pseudotachylytes, likely represent the initial chemical characteristics of the melts. In other words, the initial chemical properties of pseudotachylytes in the Longmen Shan fault zone are characterized by enrichment of Fe, Ti, K, Mn, and depletion of Si.

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Fig. 9. X-ray diffraction spectra of the fault rocks in the Yingxiu-Beichuan fault zone. Qz: quartz, Pl: plagioclase, Ank: ankerite, Cal: calcite, Ms: muscovite, Sd: siderite, Ap: apatite, Kln:kaolinite, Crs: cristobalite. Samples are listed in Table 2.

(Fig. 11d). Surface pseudotachylytes are ankerite rich while drilling core pseudotachylytes are calcite rich, probably duo to the surface microbial effect (Liu et al., 2016; Bei et al., 2018). Pseudotachylytes formed by heating experiments show that their microstructures varied with increasing temperature (Zhang et al., 2018): pseudotachylyte formed with few vesicles and microlites when the cataclasite was heated to 1100 °C; lots of vesicles and stellate aggregate microlites formed at 1300 °C, microlites grew at the edge of the embayed quartz grains (Fig. 11e and f); when heated to 1500 °C, no microlites existed in the melt due to the high freezing rate (Zhang et al., 2018). Based on the microstructural characteristics displayed above, we infer that the surface pseudotachylytes have undergone a heavier alteration than the pseudotachylytes from the drilling cores.

5.2. Microstructural differences between the pseudotachylytes from the surface and the drilling cores Pseudotachylytes from both the surface and drilling cores are of melt-origin, as confirmed by their microstructural characteristics, meanwhile, microstructural features can also reflect their chemical differences. Microstructures of the surface pseudotachylytes show features of matrix-supported fabric with varying sizes of clasts (Fig. 10a), three different colored bands (marked as Pst-1, Pst-2 and Pst-3) display clear differences in textures and compositions. Viewed from Pst-1 to Pst-2 and Pst-3, their matrix are getting coarser, clast sizes are increasing, the foliations vary from strong to weak, the clasts in the matrix have dark rims composed of ankerite. Ankerite content is decreasing from Pst-1 to Pst-3 (Fig. 10), which is consistent with the Ca intensity (Fig. 6a). This probably suggests three generations of pseudotachylytes, which have suffered different degrees of alteration due to CO2-rich fluid, especially for the gray pseudotachylyte (Pst-1), whose initial melting characteristics have been erased. Viewed from the microstructural characteristics of the pseudotachylytes from the WFSD drilling cores, we can see less clasts and higher calcite content in the matrix of gray pseudotachylytes (Fig. 11a and b, sample C4) compared to that of the dark ones (Fig. 11c and d, sample C6). In the gray pseudotachylytes, quartz clasts are mainly surrounded by calcite, some have been replaced by calcite (Fig. 11b), while in the dark pseudotachylytes, calcite is much less present

5.3. Mechanisms of chemical variation in the pseudotachylytes Faults are the dominant fluid migration pathways. Fluid activity in fault zone plays an extremely important role in earthquake nucleation, propagation and fault weakening (Sibson, 1992; Hickman et al., 1995; Caine et al., 1996; Evans et al., 1997; Faulkner et al., 2010). Indeed, frictional melting is a disequilibrium process as the individual minerals appear to melt depending on their respective melting temperature (e.g. Spray, 1992), which may lead to the differences in chemical composition of pseudotachylytes. However, pseudotachylytes formed at deep depths commonly underwent different degrees of fluid-rock 9

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Fig. 10. Microstructural characteristics of pseudotachylytes at the Bajiaomiao outcrop. (a) Microstructures of pseudotachylytes from Fig. 6a. (b) Microstructures of Pst-1, showing that the matrix is heavily altered, and clasts are surrounded by ankerite (c). (d) Microstructures of Pst-2, showing that the matrix are less altered than Pst-1. (e) Close up of (d). (f) Microstructures of Pst-3, showing that the matrix is less altered than Pst-2. (g) Close up of (f). (b), (d) and (f) are plane-polarized light images, (c), (e) and (g) are cross-polarized light images. Qz: quartz, Pl: plagioclase, Ank: ankerite.

(Fig. 7a) reflect an oxidative environment, implying that oxidizing fluids likely penetrated deep into the fault. Besides, Ca/Fe ratio positively correlates with texture characteristics (grain size) (Rothwell et al., 2006), and Fe/Ti ratios are associated with grain sizes (Cuven et al., 2010). The high Ca/Fe and Fe/Ti ratios probably reveal that the gray pseudotachylytes at ∼585.60 m-depth have devitrified due to later hydrothermal fluids, and newly-formed minerals shown a tendency of grain size increase. The low K, Ti, high Ca, Mn, Mn/Fe, Ca/Fe and Fe/Ti is a prominent feature of the surface pseudotachylytes (Fig. 6a). Fe/Ti is used as a proxy for diagenetic Fe enrichment (Blanchet et al., 2009; Van der Land et al., 2011), excess Fe over basaltic lithogenic values suggesting additional sources of Fe-rich material (Marsh et al., 2007). Therefore, low Ti together with high values of Mn/Fe and Fe/Ti probably suggest strong hypergene effects, high temperature and humidity, and enrichment of Fe, thus newly formed ankerite prevail in the devitrified pseudotachylytes (Fig. 10). Elemental mapping carried out on a thin section of the surface pseudotachylytes shows that elements are

interactions in the process of exhumation or uplift to the surface or to shallow depths. The most important feature is that elements accompanied by fluids are taken in and out of the fault rocks during hydrothermal activity (Campbell et al., 2004), whereas active elements are more sensitive. As a relatively active element, K easily migrate with fluids (Kuhlmann et al., 2004). Ca is commonly an effective climatic indicator, with low Ca during glacials and high values during interglacials (e.g. Arz et al., 2001; Gebhardt et al., 2008; Van Rooij et al., 2007). High Ca/Fe ratios also refer to warm environments (Rothwell and Croudace, 2015). As shown in Fig. 6a, the gray pseudotachylyte (Pst-1) characterized by depletion of K and Ti, and enrichment of Ca with high Ca/Fe ratios, probably indicate that they experienced hydrothermal processes, in agreement with the major elemental profiles (Fig. 4) where CO2 is much higher than in the host rocks. This is in accordance with the abundant ankerite in the surface pseudotachylytes (Wang et al., 2015) and of calcite in the drilling core pseudotachylytes. The high Mn/Fe ratios in the gray pseudotachylyte at 585.6 m-depth 10

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Fig. 11. Microstructures of the pseudotachylytes in WFSD drilling cores and from heating experiments. (a) Microstructures of the gray pseudotachylyte C4. (b) A close up show that calcite prevail in the matrix. (c) Microstructures of the dark pseudotachylyte C6. (d) A close up shows that the matrix is slightly altered by calcite. (e, f) Microstructures of the melt produced by cataclasite heated to 1300 °C, rounded vesicles and microlites are present. (a), (c) and (e) are plane-polarized light images, (b), (d) and (f) are cross-polarized light images. Pst: pseudotachylyte, Cata: cataclasite, Qz: quartz, Pl: plagioclase, Cal: calcite, Ms: muscovite.

drilling cores suggest a reductive environment, in contrast to an oxidation environment for the surface pseudotachyltes (Pst-1). This likely indicates that the surface pseudotachyltes are strongly influenced by supergene fluids and are devitrified, their chemical properties cannot exactly reflect that of the initial frictional melt when formed. (3) Besides the selective melting, fluid influence, highly visible in the Longmen Shan area, is likely responsible for the chemical variations in the pseudotachylytes. Multiple generations of pseudotachylytes, as the products of large earthquakes, have been subjected to different degrees of fluid influence.

distributed in distinct laminations (Fig. 8). K, Ba and Ti are in good correlation, which share an inverse correlation with Si. Frictional melting is a chemical non-equilibrium process (Lin and Shimamoto, 1998), which may result in the differences of elemental distribution, and the residual fragments in pseudotachylytes also influenced the element distribution. However, the melt phase is usually well mixed due to advection (Spera, 1980), thus it is hard to produce the distinct elemental layers in a single event. Therefore, these distinct elemental laminations may refer to multiple generations of pseudotachylytes formed by seismic events. 6. Conclusions

Acknowledgements

This research presents new insights into chemical changes in seismic generated pseudotachylytes based on μXRF data obtained by an Itrax core scanner. According to our comparative study carried out in pseudotachylytes at the surface outcrop and at deep depths from the WFSD drilling cores, we arrive at the following conclusions:

We would like to express our thanks to Junling Pei, Yue Zhao, Dongliang Liu, Jianguo Wu, Chenglong Li, Kun Yun, Jiajia Zhang and Xiangli He for their help in sample collection and measurement. We thank Jinchuan Wei for preparing the thin sections and Bin Shi for his help in acquiring the SEM images. We are grateful to Aiming Lin and two anonymous reviewers for their reviews, which substantially improved the manuscript. This work is supported by the National Natural Science Foundation of China (grants 41620104006, 41602226, 41830217 and 41802223) and the Basic Outlay of Scientific Research from the Chinese Academy of Geological Sciences (grants J1619 and YYWF201601).

(1) The frictional melting processes which occur at/near the seismogenic zone happened under a reductive environment during large earthquakes along the Longmen Shan thrust belt. The primary frictional melts are characterized by enrichment of K, Fe, Ti, Mn, and depletion of Si, together with high magnetic susceptibility values. (2) Geochemical features of the pseudotachyltes from the WFSD 11

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Appendix A. Supplementary data

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