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Tectonophysics

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Paleo-earthquakes revealed by rock magnetic evidence from the Anxian-Guanxian Fault, Sichuan Province, China

Yuhang Caia, Junling Peia,⁎, Huan Wangb, Mei Shenga, Jialiang Sib

a Key Laboratory of Paleomagnetism and Tectonic Reconstruction of Ministry of Land and Resources, Institute of Geomechanics, Chinese Academy of Geological Sciences,Beijing 100081, Chinab Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China

A R T I C L E I N F O

Keywords:Wenchuan EarthquakeRock magnetismSlip ZoneMagnetic susceptibility

A B S T R A C T

To determine the rock magnetic characteristics of this co-seismic rupture generated by the 2008 Wenchuanearthquake in Sichuan Province, China, we excavated a trench on the Anxian-Guanxian fault. High-resolutionmagnetic susceptibility and other rock magnetic measurements were conducted on the field section and onsamples from the trench. We divided the fault rocks into 8 units according to lithology and measurements ofmagnetic susceptibility were made on fault rocks within different units, both in the field and in the laboratory.The results demonstrate that the highest magnetic susceptibility occurs in the red gouge (64.08× 10−6 SI forsurface magnetic susceptibility and 30.61× 10−8 m3/kg for mass magnetic susceptibility), while relatively highmagnetic susceptibility occurs in the black gouge (14.31× 10−6 SI for surface magnetic susceptibility and7.15×10−8 m3/kg for mass magnetic susceptibility) compared to the protolith. More detailed magnetic mea-surements were used to determine the magnetic mineralogy of each unit. The principal mechanism responsiblefor the high magnetic susceptibility values of the red gouge is the neoformation of magnetite from siderite orpyrrhotite caused by thermal pressurization during the latest earthquakes, which reveals the location of thelatest seismic slip zone. By contrast, the magnetic susceptibility values of the black gouge have been reduced bythe formation of goethite during weathering. We conclude that although the occurrence of goethite revealspaleo-earthquake, factors such as the oxidation or dissolution of fluids that effect magnetic minerals need furtherinvestigation.

1. Introduction

Rock magnetic methods are a useful tool for studying fault gougeand they provide information on thermal history (Yang et al., 2012a;Liu et al., 2014), seismic slip parameters (Yang et al., 2012b, 2016; Heet al., 2018) and other geological information related to fault move-ment (Hailwood et al., 1992). For example, stepwise thermal de-magnetization of NRM (natural remanent magntization) intensity(Hailwood et al., 1992; Yang et al., 2012a, 2012b; Chou et al., 2012; Liuet al., 2014) and κ-T curves (thermomagnetic curves) (Yang et al.,2012a, 2012b; Chou et al., 2012; Liu et al., 2014) are often used todetect Curie-Neel temperatures. Previous research (Liu et al., 2014) hassuggested that fault motion can induce frictional heating at a tem-perature of< 300 °C, which preserves Fe-sulfides as a major magneticcarrier in fault zones. In the case of the Chelungpu fault, Chou et al.(2012) observed abnormally high S-ratios (based on isothermal

remanent magnetization and its demagnetization) of the primary slipzone (PSZ) in fault gouge and they used the magnetic record cycle todemonstrate the acquisition of magnetic direction during co-seismicand inter-seismic processes.

Previous research (Yang et al., 2012a, b; Pei et al., 2014a, b) showedthat the Yingxiu-Beichuan Fault of the Longmen Shan thrust belt (LSTB)in southwestern China is characterized by fault gouge which is almost20 cm thick; however, no anomalous rock magnetic properties wereobserved. Preliminary work (Liu et al., 2014) on the Anxian-GuanxianFault in the LSTB revealed that the average magnetic susceptibility ofthe fault gouge was slightly less than that of the potential protolith. Thiswas attributed to the transformation of magnetite to Fe-sulfides at lowtemperatures in a reducing environment. However, the mechanism offormation of the Fe-sulfides was unclear, and in addition, stepwisethermal demagnetization of the NRM intensity did not reveal thermaldemagnetization behavior typical of Fe-sulfides. In summary, it remains

https://doi.org/10.1016/j.tecto.2018.12.027Received 22 May 2018; Received in revised form 21 November 2018; Accepted 30 December 2018

⁎ Corresponding author at: Key Laboratory of Paleomagnetism and Tectonic Reconstruction of Ministry of Land and Resources, Institute of Geomechanics, CAGS,11#Minzu Daxue Nanlu, Beijing 100081, China.

E-mail address: jlpei@qq.com (J. Pei).

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uncertain whether or not the fault gouge generated in the Anxian-Guanxian fault possesses consistent rock magnetic characteristics andespecially the high magnetic susceptibility zone typical of neo-earth-quakes. Moreover, a previous rock magnetic study (Liu et al., 2014)lacked a thorough exploration of the systematic rock magnetic char-acteristics of the protolith or fault gouge of the Anxian-Guanxian Fault.

In order to identify the principal magnetic minerals of fault rocks inthe Anxian-Guanxian fault, as well as their formation by specific geo-logical process, we conducted a detailed, high-stratigraphic-resolutionrock magnetic investigation of the LSTB. We wished to resolve theuncertainties evident in previous research (Liu et al., 2014) and todetermine whether Fe-sulfides were presented in the fault gouge. Forthis purpose, a trench at Jiulong on the Anxian-Guanxian fault, thesame location as that studied by Liu et al. (2014), was excavated andsampled to determine the characteristics of the co-seismic ruptureproduced by the 2008 Wenchuan earthquake. Continuous high-resolu-tion measurements were used to produce a detailed magnetic suscept-ibility stratigraphy. In addition, more detailed rock magnetic mea-surements were used to determine the magnetic mineralogy of thesection. We then use the results to try to interpret the mechanisms in-volved in the production of the Anxian-Guanxian fault.

2. Geological setting

The Longmen Shan is the principal mountain range and one of thesteepest margins along the eastern edge of the Tibetan Plateau inSichuan Province, China. The significant deformation in WesternSichuan is governed by interactions among three crustal blocks

(Songpan, Chuandian, and South China) (Fig. 1) (Burchfiel et al., 1995,2008; Densmore et al., 2007). The Longmen Shan thrust belt at theeastern margin of the Tibetan Plateau consists of the Wenchuan-Maoxian, Yingxiu-Beichuan and Anxian-Guanxian faults (e.g. Burchfielet al., 2008; Densmore et al., 2007). These faults are characterized bylong-term activity and active faulting has developed along previousfaults since the late Triassic (Deng et al., 1994; Burchfiel et al., 1995; Liet al., 2006; Densmore et al., 2007; Xu et al., 2008). The Wenchuanearthquake on May 12, 2008 (Mw 7.9) was the largest inland earth-quake to affect southwest China during the present century. Two sur-face ruptures extend along the NE-striking Yingxiu-Beichuan and An-xian-Guanxian faults. Most previous studies have determined the lengthof the co-seismic surface rupture to be 200–240 km along the NE-striking Yingxiu-Beichuan faults (e.g. Liu-Zeng et al., 2009; Xu et al.,2009; Zhang et al., 2010). However, the 2008 Wenchuan earthquakeruptured the Qingchuan Fault, with a total length of the rupture zone of~275 km (Xu et al., 2008; Li et al., 2008; Fu et al., 2011) and even up to285–300 km (Lin et al., 2012). The Yingxiu-Beichuan surface rupturezone is characterized by thrusting with dextral slip. Another surfacerupture along the NE-striking Anxian-Guanxian faults (AGF) is ~80 kmin length. The Anxian-Guanxian rupture, striking N40°–70°E, is mainlycomposed of folding structures. In some areas, the rupture fault planedips 30–50° to the NW, with a significantly lower dip angle compared tothe Yingxiu-Beichuan rupture. The rupture is characterized by an al-most pure thrust motion that has uplifted the surfaces of roads, schoolbuildings, rivers, terraces and cultivated fields. The vertical displace-ment averages 2–3m, with a maximum of ~4m (Fu et al., 2008, 2011;Li et al., 2008; Lin et al., 2009; Liu-Zeng et al., 2009; Xu et al., 2009;

Fig. 1. Geological map of the Longmen Shan and western Sichuan basin area, showing the site of the Wenchuan-earthquake Fault Scientific Drilling, and highlightingthe Wenchuan-earthquake surface rupture (thick red lines) on two faults of the LongmenShan thrust fault system (revised after Li et al., 2013). Cross-sections acrossthe central LMS are modified after Tian et al. (2013). F1, F2, F3 are representative of the location of Wenchuan-Maoxian fault, Yingxiu-Beichuan fault and Anxian-Guanxian fault, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Zhang et al., 2010; Pan et al., 2014).The Triassic Xujiahe Formation, which consists of gray sandstone,

siltstone, and dark gray mudstone with coal beds, is the hanging host ofthe AGF. The footwall of the AGF contains Jurassic grayish-green anddark-purple or red sandstones.

The Jiulong trench, striking NEE85°, is located along the AGF, to thesouthwest of Hanwang Town, in Mianzhu County, Sichuan Province,China (Figs. 1 and 2a) (Liu et al., 2014). At this site, the surface ruptureassociated with the Wenchuan earthquake is NNE10° striking, and itcrosscuts the riverbed and the first terrace. The riverbed has been up-lifted, forming a fault scarp with ~2.2 m vertical displacement (Panet al., 2014). Fault rocks are produced by the thrust motion of the

hanging wall and cover the alluvial deposits on the surface (Fig. 2b). Anexcavation of the Jiulong trench was designed to determine the char-acteristics of the co-seismic rupture produced by the Wenchuan earth-quake. Based on field observations, the strata revealed by the trenchconsist of the following lithological units:

Unit 1 is loose sediments comprising large well-rounded pebbleswith the interstices filled with coarse sand and fine gravel (Fig. 2a andb). Unit 2 is gray-black fault breccia derived from the Triassic XujiaheFormation, consisting of gray sandstone, siltstone, and dark graymudstone with coal beds (Fig. 2a and c). Unit 3 is gray-black cataclasiteor ultracataclasite that is foliated in places, and forms a gradationalcontact with Unit 2 and the underlying black gouge (Fig. 2a, c, and e).

Fig. 2. Photographs of the Jiulong trench section (the parts of the units shown are those referenced in the text). (a) Panorama of Jiulong trench; (b) units 1, 4 and 5and sampling locations; (c) units 2, 3, 4 and 5; (d) units 4, 5 and 6 and sampling locations; (e) unit 3 and sampling locations; (f) units 7 and 8 and sampling locations.Units 4 and 5 (black gouges) are associated with the Wenchuan earthquake in 2008.

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Unit 3 contains fault plane-parallel layers of finely comminuted in-cohesive cataclasite. Unit 4 is continuous black, fine grained fault gouge(Fig. 2a–d); it varies in thickness from 15 to 25 cm and contains ≥90%matrix grains. A distinguishing characteristic of this unit is the presenceof coal or graphite-rich layers that were not identified in the sur-rounding rocks. Unit 5 is continuous black, red-brown and gray-bluefine grained, non-cohesive fault gouge along the fault plane (Fig. 2a–d);it varies in thickness from 2 to 3 cm, and contains ≥90% matrix grainsthat overlie and form a sharp contact with the footwall rocks. Thecontact strikes NNE10° and dips 55° to the west (Fig. 2a–d). Unit 6 isJurassic gray-blue sandstone, which lacks the Jurassic brown sand-stone; unit 1 contacts with unit 5 (Fig. 2a and d). Unit 7 is discontinuousgray-red fine grained, incohesive cataclasite or fault gouge (Fig. 2a andf). Unit 8 is gray-red and blue sandstone and mudstone (Fig. 2a and f).Units 4 and 5 occur continuously along the fault plane and are the faultcore lithology sampled for magnetic properties in this study. Unit 7 hasmaximum surface magnetic susceptibility values and therefore the faultrocks from the hanging wall lithology were sampled for magneticmeasurements.

3. Methods

After removing surfacial material from the trench, we systematicallysampled different fault rocks and lithologies by inserting cubic plasticboxes (8 cm3); in addition, we collected hand specimens from units 4and 5, forming the fault core, as well as from unit 7. In total, 73 sampleswere collected whose locations are shown in Fig. 3; they range from no.381 on the left to no. 453 on the right. The samples were stored in arefrigerator to minimize physical and chemical changes. The surfacemagnetic susceptibility was measured on the north wall at 1-cm inter-vals using a Bartington MS2E surface scanning sensor (Bartington In-struments, Witney, UK). The active region of the MS2E sensor is at theend of a 25-mm diameter ceramic cylinder mounted in line with theelectronics unit. The active region of the sensing surface is a rectangleof dimensions 10.5×3.8mm.

Mass magnetic susceptibility was measured using a Kappabridge(AGICO, MFK1-FA) at frequencies of 976 Hz and 15616 Hz at theIntegrated Geophysical Laboratory of the Institute of Geophysics, ChinaEarthquake Administration.

Thermomagnetic curves (κ-T) were measured using a Kappabridge(AGICO, KLY-4, Czech Republic) with a CS-3 high temperature furnace.Thermomagnetic measurements were conducted in air or in argon, in a

steady field of 300 A/m. Samples were stepwise heated from 350 °C to700 °C and then cooled to room temperature at a rate of 10 °C/min.Various temperatures (350 °C, 400 °C, 450 °C, 530 °C, 580 °C and700 °C) were applied to reveal any transformations of the magneticminerals. Details of the heating procedure are presented in Fig. 4.Thermal demagnetization of orthogonal IRMs (Lowrie, 1990) was alsoconducted; fields of 2.4 T, 0.4 T and 0.12 T were applied successivelyalong the Z, Y and X axes of the oriented specimens, respectively, usingan IM-10 pulse magnetizer. Subsequently, stepwise thermal demagne-tization up to 680 °C was performed on the samples and measured usingan AGICO JR-6 spinner magnetometer. The orientation of the samples isshown in Fig. 2d. The above experiments were performed at the KeyLaboratory of Paleomagnetism and Tectonic Reconstruction of theMinistry of Land and Resources, China.

Magnetic hysteresis loops were measured using an alternating gra-dient magnetometer (Princeton Micromag 3900, Westerville, OH, USA)at the Integrated Geophysical Laboratory of the Institute of Geophysics,China Earthquake Administration. The magnetic field was cycledbetween± 1.0 T. Saturation magnetization (Ms), saturation remanence(Mrs) and coercivity (Hc) was determined after correction for theparamagnetic contribution (χpara, the high-field magnetization slope).Coercivity of remanence (Hcr), the reverse field required to reduce thesaturation isothermal remanence (SIRM) to zero, was determined bybackfield measurements after magnetization at 1.0 T. First-order re-versal curves (FORCs) (Roberts et al., 2000) were measured using anaveraging time of 0.5 s and the data were processed using FORCinelsoftware (Harrison and Feinberg, 2008).

Low temperature measurements were performed using a QuantumDesign magnetic property measurement system (MPMS XL-5, sensitivityof 5×10−10 Am2) at the Institute de Physique du Globe de Paris(IPGP), France. Zero-field-cooled (ZFC) and field-cooled (FC) curveswere obtained by cooling samples from 300 to 20 K in zero field and ina 2.0 T field, respectively, followed by imparting an SIRM in a 2.0 Tfield and then measuring the remanence in zero field during warming to300 K.

4. Results

No measurements of either surface magnetic susceptibility or massmagnetic susceptibility were made on unit 1; this was because the widerange of grain sizes, from pebbles to coarse sand and fine gravel, whichwould give rise to numerous measurement errors, given that the

Fig. 3. Sketch map of the Jiulong trench section showing sample locations and surface magnetic susceptibility values.

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measurement interval of the Bartington MS2E surface scanning sensoris only 1 cm. The surface magnetic susceptibility values of the otherunits range from 0 to 111× 10−6 SI. The minimum value(10.31×10−6 SI) was obtained from a coal bed, and the notablemaximum value (64.08× 10−6 SI) from unit 7 (Fig. 3). The averagemagnetic susceptibility values of units 2–8 are 14.04, 10.31, 14.31,23.2, 31.85, 64.08 and 18.26×10−6 SI, respectively (Table 1). Thelow-frequency mass magnetic susceptibility values range from

2.37–42.76×10−8 m3/kg (Fig. 3) and the average values of units 2–8are 7.01, 7.57, 8.53, 10.59, 15.85, 34.11 and 9.02×10−8 m3/kg, re-spectively (Table 1). The high frequency mass magnetic susceptibilityvalues range from 2.20–39.23×10−8 m3/kg (Fig. 3); the average va-lues of unit 2–8 are 5.93, 5.64, 7.15, 9.35, 13.31, 30.61 and5.89×10−8 m3/kg, respectively (Table 1).

The surface magnetic susceptibility of unit 4 (black gouge) isslightly higher than that for unit 2 (fault breccia or protolith), whereas

Fig. 4. Temperature-dependent magnetic susceptibility curves of selected samples. QQTG22 is from unit 4, 427 from unit 5, QQTG18 is from unit 6, 451 from unit 7,and 444 is from unit 9. Sample names are followed by the heating procedure (shown as temperature), separated by a short dash. ‘O’ after the heating proceduresindicates the heating procedure was conducted in air, while ‘Ar’ indicates that is was conducted in argon. For example, ‘QQTG22-350-O (Unit 4)’ indicates thatsample QQTG22 from unit 4 was heated from room temperature to 350 °C and then cooled down to room temperature in air. ‘QQTG22-530-700-Ar’ indicates thatsample QQTG22 was first heated and then cooled down to room temperature, and then heated from 530 °C to 700 °C and cooled down to room temperature in argon.‘QQTG18-O (Unit 6)’ indicates that sample QQTG18 from unit 6 was heated from room temperature to 700 °C and then cooled back to room temperature in air.

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the mass magnetic susceptibility values for units 4 and 2 are slightlyhigher. The minimum surface magnetic susceptibility occurs in unit 3,but the low-frequency mass magnetic susceptibility of unit 3 is slightlyhigher than for unit 2, while the high-frequency susceptibility is slightlylower than for unit 2 (Table 1). Unit 3 consists of finely comminutednon-cohesive cataclasite, containing fault plane-parallel layers and coallayers whose magnetic susceptibility is usually close to zero (Figs. 2eand 3, Table 1). The surface and high-frequency susceptibilities of unit5 are higher than those of unit 4 and lower than for unit 6. The mag-netic susceptibility of unit 7 is the highest in the section and it exhibits aprominent peak. Unit 7 also has the maximum surface magnetic sus-ceptibility, and therefore the fault rocks from the hanging wall lithologywere selected for additional magnetic measurements. The surface andlow-frequency magnetic susceptibilities of unit 8 are intermediate be-tween those of unit 4 and unit 5, but the high-frequency mass magneticsusceptibility is lower than that of unit 2 and slightly higher than that ofunit 3 (Figs. 2e and 3, Table 1).

κ-T curves can be used to identify different types of magnetic ma-terials (Dunlop and Ŏzdemir, 1997). The heating curve of sampleQQTG22 from unit 4 is asymptotic, following the Curie law up to 350 °C(Fig. 4a), which indicates that the magnetization is dominated byparamagnetic phases. In addition, the heating and cooling curves ofsample QQTG22 are reversible, and there is no evidence of the trans-formation of magnetic minerals below 350 °C. The heating curve fol-lows the Curie-Weiss law above 400 °C. During heating, the magneticsusceptibility begins to increase at 400 °C (Fig. 4b and d) and thendisplays a rapid increase in slope after 420 °C (Fig. 4d), with a pro-nounced peak at ~530 °C (Fig. 4f, h, and j), and decreases to near zeroat about 585 °C (Fig. 4h and Fig. 4j). The irreversible cooling curve isrelatively smooth with the main peak at around 400 °C, which is clearlyhigher than the peak in the heating curve above 400 °C (Fig. 4b, d, f, h,and j). In summary, the irreversible curves above 400 °C are indicativeof the transformation of magnetic minerals during heating.

For QQTG22 from unit 4, the cooling curves are also irreversiblebelow 530 °C (Fig. 4c, d, e and f). Above 530 °C, the cooling curves arereversible with those of the heating curves, except for the peaks at~450 °C in the cooling curves which are higher than those of theheating curves (Fig. 4g, i, and k). The heating curve in an argon at-mosphere (Fig. 4j) is similar to that in air (Fig. 4l). In addition, thecooling curve in argon is completely reversible with the heating curve(Fig. 4m). Notably, whether conducted in air or in argon, the reversibleheating and cooling curves above 530 °C of sample QQTG22 indicatethat the absence of any transformation of magnetic minerals above thistemperature. In summary, magnetic mineral transformations occurprincipally within the temperature range of 400 °C to 530 °C.

The κ-T curve of sample 427 from unit 5 shows a relatively slowincrease after ~350 °C, with a marked peak at ~480 °C in air and at500 °C in argon in the heating curves. The magnetic susceptibility de-creases to near zero at about 680 °C. The cooling curves show a clear‘hump’ between 500 °C and 300 °C with a peak at ~360 °C in air and

~380 °C in argon, the magnitude of which is substantially higher thanthat of the peak in the heating curves (Fig. 4n and Fig. 4o). The heatingcurve of sample QQTG18 from unit 6 exhibits a relatively low decreasein slope until ~580 °C, and then a drop at ~585 °C. The cooling curve isreversible above 580 °C and the cooling curve is reversible above580 °C, but below 560 °C it is higher than the heating curve (Fig. 4p).Sample 451 from unit 7 has a reversible curve with a relatively gradualincrease in slope up to a peak at ~600 °C, followed by a decrease to zeroat ~660 °C (Fig. 4q). The curve of sample 444 from unit 9 is simple,with a Curie temperature at ~680 °C (Fig. 4r).

The room temperature hysteresis loops of sample QQTG13A arelinear (unit 4) and exhibit significant drift after paramagnetic correc-tion. Samples QQTG11D (unit 5) and QQTG9 (unit 8) have similarloops, which are wide and not fully closed at fields of 1.0 T afterparamagnetic correction (Fig. 5). The loop of sample QQTG7B fromunit 7 has a substantial high-coercivity component and the loop is fullysaturated in an applied field of 1.0 T (Fig. 5).

FORC diagrams can provide substantially more information thanstandard hysteresis measurements (Roberts et al., 2000). The FORCdiagrams of the studied samples were noisy due to the low concentra-tions of ferrimagnetic grains relative to the paramagnetic contribution,and therefore to minimize the noise we used smoothing factors (SF) of 9and 10. Samples 429 (unit 5), 432 (unit 5) and QQTG8 (units 6) havesimilar characteristics with high values on the Hu=0 axis and withelongated contours with little vertical spread (Fig. 6). The peak in theFORC distribution for sample 444 (unit 7) is centered near 30mT andhas a high coercivity interval up to 60 mT; the contours that define thepeak have little vertical spread and are centered on the Hu=0 axis.Based on these characteristics, we infer that unit 7 contains non-inter-acting SD particles. The peak of the FORC distribution for sample 435(unit 6) is centered near 20mT, and the contours that define the peakhave a greater vertical spread and are centered on the Hu=0 axis;from this we infer that the sample contains interacting SD particles.

A Day plot (Day et al., 1977; Dunlop, 2002) and Squareness-coer-civity (SC) plot (Tauxe et al., 2002) were constructed to estimate thegrain sizes of the magnetic minerals (Fig. 7). The hysteresis parameters,Mr/Ms and Hcr/Hc, are plotted on a Day plot (Day et al., 1977) to-gether with theoretical mixing curves (Dunlop, 2002). Most of thespecimens show a clear ferromagnetic component after paramagneticslope correction with the hysteresis parameters typically below the SD-SP trend within the pseudo-single domain (PSD) region. Most samplesfrom unit 6 are in the PSD (pseudo-single domain) area, except for unit5, where the samples are plotted in the SD (single domain) area. Thesamples from unit 7 are in the SD-PSD area, except for sample QQSC9.The CSD-USD trend (cubic single domain-unaxial single domain trend)in the SC plot (Tauxe et al., 2002), which reflects the diverse grain sizesof samples from unit 7 (red gouge) to unit 4 (black gouge), indicatesthat the magnetic grain sizes in unit 7 may be coarser than those of unit4. The saturation fields of the samples from units 4, 5 and 8 are thehighest, and that for the sample from unit 7 is significantly higher than

Table 1Magnetic parameters of Jiulong trench.

Lithological units χlf χhf χsurface χpara Ms Mrs Hc Hcr

10−8 m3/kg 10−8 m3/kg 10−6 SI 10−8 m3/kg 10−3 Am2/kg 10−3 Am2/kg mT mT

2 7.01 5.93 14.04 2.55 0.35 0.07 22.9 66.43 7.57 5.64 10.31 5.54 0.26 0.11 33.8 41.04 8.53 7.15 14.31 6.54 0.86 0.14 16.4 43.95 10.59 9.35 23.2 7.98 12.2 5.75 215 5396 15.85 13.31 31.85 3.30 10.6 1.37 10.2 26.07 34.11 30.61 64.08 5.12 36.3 12.0 24.6 44.68 9.02 5.89 18.26 3.38 5.41 2.83 142 408

Notes: χlf – low field magnetic susceptibility; χpara – paramagnetic susceptibility determined by hysteresis measurements; χferri – ferrimagnetic susceptibility cal-culated from the difference between χlf and χpara; Ms. – saturation magnetization; Mrs. – saturation remanence; Hc – coercivity; Hcr – coercivity of remanence.

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those from units 2, 3 and 6.IRM acquisition and reverse field demagnetization was conducted

on several samples. The IRM of samples 386 (Fig. 8a), 401 (Fig. 8b), and435 (Fig. 8e) reach saturation below 300 mT, but those for samples 420(Fig. 8c), 427 (Fig. 8d) and 443 (Fig. 8f) continues to increase above300mT and the samples do not reach saturation until 2.5 T, with quasi-saturation (> 80%) behavior at about 1.1 T (Fig. 8a–f). Remanentcoercivities (Hcr) calculated from the backfield demagnetization curvesare< 70mT for samples 386, 401, and 435;> 500mT for samples 420

and 443; and> 200mT for sample 427.The thermal demagnetization of three-axis IRM curves reveals dif-

ferent types of behavior. The hard IRM component of samples 386 and401 is reduced to zero at around 300 °C, while the magnetization of theintermediate and soft IRM components is reduced slowly to zero by580 °C (Fig. 8g and h). Samples 420 and 427 have similar character-istics: a decrease in intensity from 80 to 150 °C for the high- andmedium-coercivity components, with an unblocking temperature of~585 °C of the soft component and a maximum unblocking

Fig. 5. Magnetic hysteresis loops before (left) and after (right) paramagnetic correction of selected samples. Samples QQTG13A, QQTG11D, QQTG7B and QQTG9 arefrom units 4, 5, 7 and 8, respectively.

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temperature of 670–690 °C of the intermediate and hard components(Fig. 8i and j). For sample 435, all three components gradually decreaseto zero up to ~580 °C (Fig. 8k). In sample 443, there is an indistinctchange in the slopes of the soft and intermediate components at~250–300 °C and all three components decrease to zero at ~680 °C(Fig. 8l).

The low-temperature demagnetization curves of the samples showtwo distinct types of behavior (Fig. 9). Low-temperature magnetictransitions are not clearly evident in the ZFC, FC or RT-SIRM curves forsample 421 (unit 4). The RT-SIRM cooling curve shows a continuousgradual increase between 300 and 5 K, and the warming curve re-sembles the cooling curve, implying either that magnetite particles arein the SP state or that the concentration of coarse-grained magnetite isundetectable (Dunlop and Ŏzdemir, 1997). A transition around 30–40 Kis clearly evident in the ZFC and FC curves for sample 443 (unit 7); inaddition, a Verwey transition is evident in the ZFC and FC curves

(Fig. 9). The maximum gradient in the RT-SIRM curves is reached at100 to 110 K, corresponding to the temperature interval that is in-dicative of the Verwey structural phase transition of magnetite (Dunlopand Ŏzdemir, 1997), confirming its presence (Verwey et al., 1947).

5. Discussion

5.1. Higher magnetic susceptibility values of the gouge in the Anxian-Guanxian Fault

Fig. 3 shows the results of surface scanning and low- and high-fre-quency magnetic susceptibility measurements for the north section ofJiulong trench. The results of surface magnetic susceptibility mea-surements in the field are consistent with the mass magnetic suscept-ibilities measured in the laboratory. In addition, comparison betweenthe surface scanning and laboratory measurements reveals similar

Fig. 6. FORC diagrams of representative samples. Samples 429, 432 and 435 are from units 5, 5 and 6, respectively; sample QQTG8 is from unit 6; and sample 444 isfrom unit 7.

Fig. 7. Day-plot (Day et al., 1977) of representative samples from the studied section. SD+MD and SP+SD mixing lines are from Dunlop (2002). The hysteresisparameters Mr/Ms and Hcr/H are plotted on a Day-plot (Day et al., 1977) with the theoretical mixing curves shown (Dunlop, 2002). Mr/Ms versus Hc is plotted on aSquareness-coercivity (SC) plot (Tauxe et al., 2002). A plot of low-field magnetic susceptibility (χlf) versus high-field magnetic susceptibility (χhf) is also shown.Squares in black, blue, red and orange represent black gouge (unit 4 and unit 5), gray-blue sandstone (unit 6), red gouge (unit 7) and gray-red sandstone (unit 8). (Forinterpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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trends, indicating that the field results are reliable (Fig. 3). As shown inFig. 3 and described in Section 4 (Results), the prominent magneticsusceptibility peak in unit 7, as well as the relatively higher values ofmagnetic susceptibility in unit 4, may depend on not only magneticmineralogy, but also the concentration of magnetic minerals and theirgrain size (Dunlop and Ŏzdemir, 1997; Nagata, 1961).

In general, fault gouge is generated from ambient protolith. In theJiulong trench, black gouge and red gouge should have originated from

the surrounding rock units, such as units 2, 6 or 7; furthermore, theconcentration of magnetic minerals should be relatively constant be-tween these rock units. However, there are substantial differences intheir magnetic susceptibility, with unit 4 and especially unit 7 havinghigh values. With regard to the grain size of the magnetic minerals, theDay plot and SC plot indicate PSD behavior; in addition, there areminimal differences between the various rock units, although the grainsize of the magnetic minerals in unit 7 is finer than that in unit 4. In

Fig. 8. Results of three-axis thermal demagnetization of IRM for selected samples. Samples 386, 401, 420, 427, 435 and 443 are from units 2, 3, 4, 5, 6 and 7,respectively;

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another words, the variations in magnetic grain size are unrelated tothe variations in the magnetic susceptibility of the rock units. In sum-mary, the significant peaks in magnetic susceptibility of units 7 and 4are related to magnetic mineralogy which is discussed below.

Previous studies of the same section reported slightly lower surfacemagnetic susceptibility values for the gouge than for the protolith, faultbreccia and hanging wall sediments, indicating that the magnetic sus-ceptibility of fault-related rocks can decrease during or after repeatedlarge earthquakes (Liu et al., 2014). However, our results do not sup-port this conclusion (Fig. 3 and Table 1). As shown in Fig.3, the mag-netic susceptibility of gouge (including unit 4 and unit 5) is higher thanthat of the fault breccia or protolith (unit 2) and cataclasite (unit 3).Therefore, the previous conclusion (Liu et al., 2014) that the averagemagnetic susceptibility value of the gouge is slightly lower than that ofthe potential protolith may be incorrect and may have resulted fromdifferences in sample weight, instrumental error, or the smoothing ef-fect of continuous U-channel measurements (Brachfeld et al., 2004;Roberts, 2006). Previous work on 16-cm of the fault gouge of theChelungpu Fault in Taiwan, also revealed anomalous rock magneticresults (Chou et al., 2012). In addition, previous studies of the LSTB alsoobserved anomalous rock magnetic properties but overlooked the

significance that the highest magnetic susceptibility occurs in unit 7;this is because the presence of high magnetic susceptibility values offault gouges in the same country rock could be considered as an in-dicator of earthquakes or seismic signatures (Pei et al., 2014a, 2014b,2010). Overall, the rock magnetic properties of fault gouge exhibitunique characteristics that distinguish it from the protolith.

5.2. Identification of magnetic minerals in lithological units

To understand the reasons for the observed variations in magneticsusceptibility, we need first to determine the magnetic mineral assem-blages in the various rock units. For units 2 and 3, the results suggestthat the fault breccia contains magnetite, with lesser amounts of goe-thite and Fe-sulfides (Liu et al., 2014). However, the IRM reaches sa-turation below 300mT, Hcr is< 50mT (Fig. 8a and b), the hard IRMcomponents are reduced to zero at ~300 °C, and the intermediate andsoft IRM components decrease gradually to zero by 580 °C (Fig. 8g andh). These characteristics indicate that no high coercivity magnetic mi-nerals are present, in contrast to the work of Liu et al. (2014). Thecomplex hysteresis behavior and noisy FORC diagrams result from lowconcentrations of ferrimagnetic grains relative to the paramagnetic

Fig. 9. Low-temperature demagnetization curves for selected samples. The left column shows the thermal demagnetization of a field-cooled (FC) and zero-field-cooled (ZFC) SIRM given in 2.0 T at 20 K, and the right column the thermal demagnetization of a low-temperature IRM imparted at 20 K in a 2.0 T field. Moments arenormalized by the value at 20 K.

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contribution (Fig. 7a). Overall, the main magnetic carriers identifiedare ferromagnetic minerals such as magnetite and pyrrhotite.

In the samples from unit 4, the κ-T curve of sample QQTG22 isasymptotic and follows the Curie Law up to 350 °C (Fig. 4a), indicatingthat the susceptibility is dominated by paramagnetic phases, which isalso indicated by the room temperature hysteresis loops; however, thehump in the heating curves, as well as the much higher cooling curvesabove 400 °C, probably reflects the neoformation of magnetite which isconfirmed by a Curie temperature at ~585 °C in the κ-T curves (Fig. 4jand l) (Dunlop and Ŏzdemir, 1997). This representative κ-T curve re-flects the occurrence of siderite and/or pyrrhotite, which will betransformed to magnetite during heating (Hunt et al., 1995; Deng et al.,2001; Chou et al., 2012). The magnetite in unit 4 could be a residualproduct of heating. Nevertheless, we can conclude that unit 4 pre-dominantly contains siderite and/or pyrrhotite rather than magnetite.The results of IRM demagnetization and three-axis thermal demagne-tization of IRM for sample 420 (Fig. 8c and i) and 427 (Fig. 8d and j)reveal a rapid decrease in back-fields below −100mT and a drop at~100 °C, which is regarded as reflecting the presence of goethite. Inaddition, a slight decrease at ~10 K is also indicative of goethite(Fig. 9c). In summary, the rock magnetic results indicate that the mainmagnetic minerals present in unit 4 are siderite, pyrrhotite, goethiteand possibly magnetite.

These findings differ from those of previous research which sug-gested that the main magnetic minerals in the gouge were Fe-sulfides.In contrast, we suggest that unit 4 is rich in paramagnetic phases, suchas siderite or goethite, while the main ferrimagnetic minerals includepyrrhotite or magnetite. The distribution of coercivities around 90mT(Fig. 6) suggests the occurrence of strongly interacting pyrrhotite inunits 4 and 5 (Wehland et al., 2005; Roberts, 2006). The magneticminerals of unit 5 are similar to those of unit 4, but the presence of ahigh-coercivity phase and a Curie point of ~680 °C indicate the pre-sence of a small quantity of hematite (Fig. 8d and j).

The significant difference between the magnetic susceptibility ofunit 6 and the gouges (Fig. 3) indicates the dominance of a ferromag-netic phase in the latter. The low coercivity (Figs. 6 and 8e) and~580 °C Curie point (Figs. 4p and 8k) confirm that magnetite is themain magnetic mineral. Nevertheless, a slight decrease in magneticsusceptibility below ~400 °C, as well as the much higher values of thecooling curves compared to the heating curves of sample QQTG18(Fig. 4p), which are evidences of siderite.

The magnetic susceptibility of unit 7 is the highest in the section.The κ-T results indicate a Hopkinson peak at ~580 °C (Fig. 4q), and thehysteresis experiments and intensity decrease at ∼120 K, confirmingthe presence of magnetite (Fig. 9); however, the high-coercivity phaseand ~680 °C Curie temperature reveal that hematite also occurs in unit7 (Figs. 5, 8f and l). The decrease in intensity at ~35 K (Fig. 9a) mayindicate pyrrhotite or siderite (Dunlop and Ŏzdemir, 1997; Frederichset al., 2003), whereas the increase in intensity below ~150 K (Fig. 9b)may result from the occurrence of siderite (Frederichs et al., 2003).

Unit 8 is the Jurassic dark-purple sandstone of the footwall. All theexperiments indicate the presence of high coercivity magnetic minerals,with a high Curie temperature, confirming that hematite is the mainmagnetic mineral.

In summary, except for unit 1 which was not sampled, the mainmagnetic minerals of the units have been identified. This enables theuse of the magnetic mineral transformations to be interpreted in termsof geological processes. The main magnetic minerals identified in units2–8 are listed in in Table 2.

5.3. Origin of magnetic minerals of the gouges

As mentioned above, the prominent magnetic susceptibility peak inunit 7 and the ‘hump’ in unit 4 and 5 displayed in Fig. 3 indicate theoccurrence of different magnetic minerals in the various lithologicalunits. The magnetic minerals in unit 4 consist of siderite, goethite,

magnetite and possibly pyrrhotite; in addition, it is reasonable to con-clude that magnetite is present in unit 4 given that siderite or pyrrhotitecan be transformed to ferrimagnetic minerals, such as magnetite,during heating. In addition, siderite is transformed to magnetite attemperatures of ~540–590 °C (Pan et al., 2000); and pyrrhotite istransformed to magnetite at a temperature of ~500 °C (Dunlop andŎzdemir, 1997). Notably, the temperature interval of the transforma-tion of magnetic minerals revealed by the κ-T curves is from 400 °C to530 °C, which is consist with the transformation of siderite or pyr-rhotite. On the other hand, the magnetic minerals in unit 4 may be amixture of magnetic minerals from the surrounding rock units, such asunit 6, due to faulting. The magnetite and siderite in unit 4 may bederived from unit 6 which also contained magnetite and siderite. If thisinference is correct, then higher magnetic susceptibility values wouldnot be observed in unit 4. Thus, the higher values of magnetic sus-ceptibility in unit 4 may not be result of the presence of magnetite orsiderite.

The goethite identified in units 4 and 5 is likely to have been pro-duce under the influence of fluids. Goethite is generally considered tobe produced by exogenous processes where fluids occur (Chou et al.,2012). The narrow zone of goethite in units 4 and 5 (black fault gouge),and its absence in units 3 and 6, implies that fluids may have flowedalong the boundary between units 5 and 6. Chou et al. (2012) pointedout that post-seismic weathering provided the conditions for pre-cipitation migration along the fault plane, leading to the formation ofgoethite in fault gouge (Kuo et al., 2012). He et al. (2018) also de-monstrated that inter-seismic fluids played important role on thetransformation of magnetic minerals. In terms of the origin of thegoethite in units 4 and 5, it may be the product of oxidation and thedecomposition products of Fe-containing minerals, such as siderite,magnetite or pyrrhotite. In addition, the formation of goethite, with lowmagnetic susceptibility, tends to decrease the magnetic susceptibility ofthe fault gouge in units 4 and 5, while the impact of weathering on theother magnetic minerals would have the same effect of reducing themagnetic susceptibility. Thus, we suggest that the formation of goethiteand the impact of weathering are responsible for the lower magneticsusceptibility values of units 4 and 5 compared to unit 7, which alsocomprises fault gouge but has the highest magnetic susceptibility.

The main magnetic minerals in unit 7 are magnetite, hematite,siderite and pyrrhotite. Like units 4 and 5, the magnetic minerals in unit7 may be derived from units 6 or 8, which also have the same magneticmineralogy (i.e. magnetite, hematite, siderite and pyrrhotite).However, as discussed above, the derived magnetic minerals don'tcause an increase in the magnetic susceptibility of the fault gouge.Thus, the substantially higher magnetic susceptibility of unit 7 was notcaused by the derived minerals. However, this is contrary to the con-clusion that higher values of magnetic susceptibility reflect the mag-netic mineralogy. To address this contradiction, we propose the hy-pothesis of the neoformation of magnetic minerals with a high magneticsusceptibility, such as magnetite.

Siderite (Pan et al., 2000) and pyrrhotite (Dunlop and Ŏzdemir,1997) are transformed to magnetite upon heating to temperatures of~540–590 °C and ~500 °C, respectively. In unit 7, siderite or pyrrhotitearguably were heated and transformed to magnetite during the latestrupturing process after the Wenchuan earthquake in 2008, marking thelatest rupture zone. The neoformation of magnetite, with a high mag-netic susceptibility, has contributed substantially to the prominentmagnetic susceptibility peak in unit 7. Moreover, there was little in-fluence of weathering on the neoformation of magnetite given the shortlength of time since the latest rupture event. The work of Chou et al.(2012) proposed this hypothesis to explain the occurrence of magnetitein the PSZ of the Chi-Chi fault.

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5.4. Implications of the magnetic properties of the fault gouge of the Anxian-Guanxian Fault

Our analyses reveal the existence of gouge zones with a high mag-netic susceptibility. Unit 7 (red gouge) is characterized by the highestmagnetic susceptibility and the mechanism responsible is most likelythe neoformation of magnetite. Moreover, the relatively high magneticsusceptibility of unit 4 could result from the formation of goethite thatwas produced during post-seismic weathering.

As suggested in Section 5.3, the neoformation of magnetite markingthe latest rupture zone resulted from the transformation of siderite orpyrrhotite at temperatures of more than ~400 °C. This implies that therupture process, i.e. faulting, may generate temperatures of at least400 °C which enable the transformation of siderite or pyrrhotite. Thisestimate is slightly higher than that of Liu et al. (2014). In addition, asshown by the Day plot and SC plot (Fig. 9), the grain size of the mag-netic minerals in unit 7 is finer than that of unit 4, indicating the role ofphysical grinding during the latest faulting that affected unit 7. Finer-grained magnetite was produced, whereas post-seismic weatheringprovided the necessary space and time for the growth of goethite in unit4. Overall, with regard to the generation of the finer-grained magnetitein unit 7 compared to unit 4, thermal pressurization is the most likelymechanism responsible, whereas the goethite in unit 4 was generatedby post-seismic processes.

Thus, if the most recent rupture process resulted in the neoforma-tion of magnetite, co-seismic faulting may have had the same ability toproduce magnetite in unit 4; however, this magnetite may have beenchemically altered to goethite during post-seismic weathering. Thus,the occurrence of goethite was not generated during faulting but wasthe product of post-seismic processes. Essentially, the occurrence ofgoethite in units 4 and unit 5 (black fault gouge) represents previousrupture events, such as paleo-earthquakes. It is also significant thatthere are several peak sin magnetic susceptibility in the Jiulong trench(Fig. 3), which may indicate that earthquakes have occurred repeatedlyalong the Wenchuan earthquake fault zone.

6. Conclusions

We have conducted a detailed investigation of the magnetic prop-erties of a trench excavated in the Anxian-Guanxian fault. The magneticsusceptibility of the black gouge (including units 4 and 5) is higher thanthat of the fault breccia or protolith (unit 2) and cataclasite (unit 3).The major magnetic minerals in the black gouge are siderite or pyr-rhotite, and magnetite and goethite. These findings differ from those ofprevious research which suggested that the main magnetic carriers ofthe gouge were Fe-sulfides (Liu et al., 2014). Goethite, which has a lowmagnetic susceptibility, was produced after paleo-earthquake and wasproduced by alteration of preexisting minerals by weathering; however,the detailed factors that influence the formation of the goethite, in-cluding temperature, source, dissolution, oxidation and reduction, arestill unknown and require further study. In contrast, minerals with ahigh magnetic susceptibility, especially neoformed magnetite, are re-sponsible for the maximum magnetic susceptibility values observed inunit 7. This magnetite is the product of the alteration of more weakly-magnetic minerals in the host rocks during the latest rupture event. In

addition, we suggest that thermal pressurization of faulting shouldgenerated temperatures of at least ~400 °C which ensured the neo-formation of magnetite.

If our conclusions regarding the formation of the magnetic mineralsof the black gouge and red gouge are correct, then the existence of theobserved thick black gouge (units 4 and 5) as well as the magneticsusceptibility peaks in unit 2 and unit 3 (Fig. 3) indicates the location ofprevious rupture zones that triggered by paleo-earthquakes which couldhave occurred repeatedly in the area. In fact, Chou et al. (2012) con-ducted rock magnetic experiments on samples from the Chelungpu faultin Taiwan, which also revealed the presence of goethite in fault gougeand magnetite similar to the PSZ. Both the Chelungpu fault and Anxian-Guanxian fault are thrust faults that have a similar magnetic miner-alogy to fault gouge. If the occurrence of goethite in fault gouge and theoccurrence of magnetite in the PSZ are representative of other types offault, then rock magnetic measurements are a potentially simple butpowerful tool for pinpointing the PSZ of a fault.

Acknowledgments

We thank the Wenchuan Earthquake Faults Scientific DrillingCentre for their help. This research was funded by the National ScienceFoundation of China (grants 41172177, 41572192, 41602226,41830217) and the Ministry of Science and Technology of the People'sRepublic of China through the Wenchuan Earthquake Fault ScientificDrilling Program (WFSD).

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Table 2Main magnetic mineral assemblages of different rock unis in Jiulong trench.

Lithoogical units Unit 2 (faultbreccia)

Unit 3 (cataclasite orultracataclasite)

Unit 4 (black faultgouge)

Unit 5 (faultgouge)

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SideriteGoethiteMagnetiteHematite

(pyrrhotite)

MagnetiteSiderite

MagnetiteHematiteSiderite

(pyrrhotite)

Hematite

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