39Ar dating of cataclastic K-feldspar: A new approach for establishing the chronology of brittle deformation

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39 Ar/ Ar dating of cataclastic K-feldspar: A new approach for establishing the chronology of brittle deformation Yu Wang, Liyun Zhou, Horst Zwingmann, Ching-hua Lo, Guowu Li, Jinhua Hao PII:

S0191-8141(19)30081-1

DOI:

https://doi.org/10.1016/j.jsg.2019.103948

Reference:

SG 103948

To appear in:

Journal of Structural Geology

Received Date: 21 February 2019 Revised Date:

13 November 2019

Accepted Date: 27 November 2019

40 39 Please cite this article as: Wang, Y., Zhou, L., Zwingmann, H., Lo, C.-h., Li, G., Hao, J., Ar/ Ar dating of cataclastic K-feldspar: A new approach for establishing the chronology of brittle deformation, Journal of Structural Geology (2019), doi: https://doi.org/10.1016/j.jsg.2019.103948. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

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Ar/39Ar dating of cataclastic K-feldspar: a new approach for

establishing the chronology of brittle deformation

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Yu Wang1, Liyun Zhou1, Horst Zwingmann2, Ching-hua Lo3, Guowu Li1, Jinhua Hao1

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1. Institute of Earth Sciences, China University of Geosciences, Beijing 100083, China

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2. Department of Geology and Mineralogy, Graduate School of Science, Kyoto University, Kyoto,

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606-8502, Japan

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3. Department of Geosciences, National Taiwan University, Taipei 106, Taiwan

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Corresponding author: Yu Wang

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Email: [email protected], Phone number: 8610-82323577

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Fax number: 8610-82321983

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Abstract

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Constraining the timing of brittle deformation, such as in high-angle normal or strike-slip faulting, is

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technically challenging. In this study we present a new approach of age determinations from cataclastic

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K-feldspars of brittle faults developed in rocks of the Tan–Lu fault zone and from Louzidian and Liaonan

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metamorphic core complexes in NE China. The attempt is based on field investigations, microstructural

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observations, back-scattered electron (BSE) imaging, electron-microprobe analyses, X-ray diffraction,

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scanning electron microscope (SEM) results, and

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brittle fault zones of cataclastic mylonite, granite and gneiss. SEM and BSE images of individual

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Ar/39Ar step-heating of K-feldspar separated from

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K-feldspar crystals reveal distinct grain deformation features, including grain-scale faults (fractures and

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microfractures) that occur parallel or perpendicular to each other. The

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well-defined plateau spectra for the cataclastic K-feldspar grains in the Tan–Lu fault zone (74.5 ± 1.3 Ma),

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and of brecciated gneisses and mylonites that formed at depth in the Louzidian and Liaonan

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metamorphic core complexes (~120–129 Ma and 116 ± 2 Ma). These data suggest complete resetting of

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the Ar isotope system of the cataclastic K-feldspars, allowing to constrain the timing of their deformation.

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Keywords: Cataclastic K-feldspar； ；40Ar/39Ar dating； ；fault rocks； ；brittle deformation; Microfractures

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Ar/39Ar dating results provide

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1. Introduction

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Isotopic dating of brittle deformation has been extensively reported in the literature (Dunlap et al.,

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1991; Kirschner et al., 1996; Dunlap and Fossen, 1998; Eide et al., 2002; Zwingmann and Mancktelow,

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2004; Rolland et al., 2009; Campani et al., 2010; Duvall et al., 2011). Previous studies have often used

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indirect dating methods to constrain timing of brittle deformation, such as dating dykes oriented parallel to

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fractures, normal faults, or strike-slip faults, and constraining cooling histories using biotite 40Ar/39Ar and

36

K-feldspar multi-diffusion domain modeling spanning the temperature interval appropriate for brittle

37

deformation (e.g., McDougall and Harrison, 1999; Reddy and Potts, 1999; Wells et al., 2005). Kralik et al.

38

(1987) and van der Pluijm et al. (2001) brought forth dating of shallow faults in the Earth’s crust, and laser

39

40

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Sherlock et al., 2003; O’Brien and van der Pluijm, 2012). Small micrometric sericite of cleavage planes

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and illite in fault gouges have also been dated, despite fine size (<2 µm or finer) and difficulty to separate

42

(Zwingmann and Mancktelow, 2004; Sasseville et al., 2008; Zwingmann et al., 2010, 2011; Duvall et al.,

Ar/39Ar dating of pseudotachylite has been reported in the literature (e.g., Sherlock and Hetzel, 2001;

2

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2011; Surace et al., 2011; Clauer et al., 2012, 2013; Fitz-Díaz and van der Pluijm, 2013; Fitz-Díaz et al.,

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2016; Viola et al., 2016). It is therefore timely to explore new potential approaches to directly date brittle

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deformation using minerals that occur in fault zones.

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The challenge addressed here is a consolidation of 40Ar/39Ar dating of original feldspars that were

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intimately subjected to cataclastic deformation, which potentially induced a complete reset of their Ar

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system. Theoretically, the closure temperature of K-feldspar ranges between 200 and 350 °C (Dodson,

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1973; McDougall and Harrison, 1999) for a common grain size. K-feldspar cooling histories and 40Ar/39Ar

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release spectra typically represent the diffusion features of Ar-release from K-feldspar (Tullis and Yund,

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1991), such as those predicted by multi-diffusion domain modeling (Lovera et al., 1989, 1991; Fitz Gerald

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and Harrison, 1993). However, these processes are still debated (e.g., Villa, 1998), and the purpose here

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is to provide data from natural samples, new arguments to the discussion, as well-defined

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spectra of detrital K-feldspar from brittle and recently reported fractured fault zones (Wang and Zhou,

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2009) for establishing a new approach of the brittle deformation dating. Newly formed authigenic mineral

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grains are produced during tectonic strain along fault zones, and in addition, sliding and cataclastic

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deformation over short periods of time can also affect the Ar system in minerals such as K-feldspar with

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the activation of diffusion domains that can potentially promote complete Ar resetting (Reddy et al., 1999;

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McLaren and Reddy, 2008). We propose a new direct method of dating brittle faulting using the 40Ar/39Ar

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dating system of shallow, fractured K-feldspar grains that assumes a full reset of the Ar system within a

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fault zone, during faulting.

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Ar/39Ar

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We collected cataclastic K-feldspar-rich samples from various structural locations within

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high-angle normal faults and in low-angle detachment faults associated with metamorphic core

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complexes, and separated for analysis. We utilize back-scattered electron (BSE) imaging,

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electron-microprobe analysis (EMPA), single crystal X-ray diffraction (XRD) analysis, and 40Ar/39Ar step

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heating to investigate the relationship between cataclasis and argon loss in K-feldspar. The 40Ar/39Ar data

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of the pristine and cataclastic K-feldspar samples were also compared with staircase Ar-diffusion

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experimental patterns of K-feldspar separates to analyze the Ar release of the K-feldspars as carefully as

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possible.

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2. Description of the deformed rocks and fractured K-feldspar

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The brittle faults investigated in this study affect granitic plutons, gneisses and mylonites at various

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structural positions. Optical microscopy, X-ray diffraction (XRD), scanning electron microscope (SEM)

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and field emission scanning electron microscope (FESEM) observations revealed that the investigated

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rocks contain cataclastic K-feldspar with microfractures that formed during brittle deformation.

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2.1. Geological framework and brittle deformation structure and fabric

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From the late Mesozoic through Cenozoic, deformation in eastern China was dominated by

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extensional tectonics leading to the formation of several rift-systems and metamorphic core complexes

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(e.g. Tian et al., 1992; Liu et al., 2005; Wang and Zhou, 2009) (Figure 1A). This deformation stage led to

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the formation of low- and high-angle normal faults along the northern and southeastern margins of the

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North China Craton (Figure 1B, C, D). Within the metamorphic core complexes along the northern margin

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of the NCC, the extension direction is indicated by the NW–SE trending stretching lineation in extensional

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shear zones (Liu et al., 2005). High-angle normal faults cut the magmatic intrusions and the metamorphic

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85

complex domes.

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Archaean gneisses affected by ~240–220 Ma old ultra-high pressure (UHP) metamorphism, as well

87

as granites, mylonites and Cretaceous andesites are exposed along the eastern margin of the Dabie

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orogenic belt in eastern China (Figures 1 and 2). A set of high-angle, east-dipping normal faults, including

89

the Tan–Lu Fault, cut these granites, gneisses, mylonites and Early-Cretaceous andesites (Figure 2A).

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An ~400 Ma old granite is deformed within a fault zone associated with the southern segment of the Tan–

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Lu fault zone (Wang and Zhou, 2009), where the DB-series samples of this study were collected,

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including breccia and cataclasite with fractured K-feldspar (Figure 1B). The fault zone comprises a 10–20

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cm thick zone of fault breccia and cataclastic material that contains fractured granitic material,

94

recrystallized chlorite, and pseudotachylites (Wang and Zhou, 2009). Fault striations steeply plunge on

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normal-fault surfaces. Down-dip fault striae, which plunge 55° to the ESE (~120°), are clearly visible on

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the fault surface, highlighted by chlorite (Figure 2A).

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A different locality than the previous one, along the Jinzhou and Louzidian detachment fault surfaces

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of varied metamorphic core complexes in NE China, down-dip fault striations indicate normal faulting.

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The Louzidian normal fault dips 64° SE (toward ~130°) and the Jinzhou detachment fault of the

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metamorphic core complex dips 40° WSW (toward ~265°) (Figures 1B–D, 2A–B, 3A–D, and 4A–B). The

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host rocks of the detachment faults are granites with ages ranging from ~400 to 160 Ma (Lin et al., 2011),

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together with 2500 to 1800 Ma old granitic gneisses (Wang and Zheng, 2005) in the Liaonan and

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Louzidian metamorphic core complexes. Samples of breccia and cataclastic rocks with fractured

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K-feldspar were collected from these Liaonan and Louzidian metamorphic core complexes (wys-series)

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(Figure 1C, D), where N-, NW-, and E-dipping normal faults are well developed. Cataclastic K-feldspar

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grains were collected from fault surfaces that developed in Paleozoic–Mesozoic granite, Proterozoic

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gneiss, and Paleozoic sedimentary rocks.

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Sliding on the fault surface and fracturing of the wall rocks produce gouges, cataclasites and

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breccias. Cataclasites in the study localities are commonly characterized by the occurrence of chlorite, no

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matter whether the protolith is granite, gneiss, or mylonite (Figures 1 and 3–4). The cataclastic material

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contains varied angular fragments up to 50 mm in size, and all fragments are embedded in a fine-grained

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matrix. The cataclastic rocks include non-foliated fault breccias, fault gouges, and pseudotachylites. No

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quartz veins are visible, but calcite veins occur locally along the detachment fault surfaces of the

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metamorphic core complexes.

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2.2. The microstructural features of the cataclastic rocks

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According to Passchier and Trouw (2005) and van der Pluijm and Marshak (2004), rocks change

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shape by brittle deformation at low temperatures or high strain rates; i.e., by fracturing and frictional

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sliding. Microcracks are planar discontinuities at the grain scale or smaller, commonly with some

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dislocation but with negligible displacement (Anderson et al., 1983; Passchier and Trouw, 2005). Brittle

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deformation shows a gradient evidenced by gradually increased microcrack density with fault

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displacement. Here, the cataclastic rocks and fault breccias show fractures or microfractures within

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grains along the brittle fault zones. Thin sections of the brittle fault rocks are cataclastic features such as

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fractured or fragmented feldspar grains with triangular outlines or fractured margins (Figures 2C–D, 3E–F,

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4C–D, and 5–6). Consisting of angular fragments of fractured protolith in a fine-grained matrix (Figures

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5–6), the cataclasites contain calcite veins and authigenic chlorite. The mineral grain fragments and

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crystals contain microfractures. These features confirm that the fault rock was subjected to brittle

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deformation along small intragrain fractures, and lack of recrystallized and/or authigenic minerals

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indicates little or no ductile deformation (Figures 2C–D, 3E–F, 4C–D, and 5–6).

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Cataclastic samples from Tan–Lu Fault do not exhibit evidence of quartz recrystallization, as no

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preferred orientation of quartz grains could be identified (Figure 5A). Plagioclase and K-feldspar appear

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to have been fragmented, and the grains outline microfractures (Figure 5B). Plagioclase was mainly

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subjected to brittle fracturing and cataclastic flow (Figure 5C, D), sometimes associated with formation of

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pseudotachylite. Characteristic structures in the cataclasites show angular grain fragments with a wide

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range of grain sizes (Figure 5). The larger fragments are dispersed in a matrix that is mainly fine-grained,

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containing in addition chlorite grains up to 0.5 cm across with no evidence of deformation (Figure 5E).

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About 5–30 cm below the fault surface, the host rock of granite shows no sign of deformation (Figure 5F).

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Cataclasites samples collected from a metamorphic core complex in NE China (Figures 3–4)

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typically contain angular grain fragments with a wide range of grain sizes dispersed in a fine-grained

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matrix (Figure 6A, B, D, F). Undulose extinction or recrystallization of quartz grains was not observed, but

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calcite veins are visible along fractures (Figure 6C). Besides, plagioclase and K-feldspar grains (Figure

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6E, F) have been deformed mainly by brittle fracturing and cataclastic flow, with fractures and

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microfractures within the grains (Figures 7–9), as well as bent cleavage planes.

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Chlorite that formed on the fault surface occurs adjacent to cataclastic rocks and fault breccia. Less

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than 1–3 cm beneath the fault surface, no chlorite is observed. Hence, this chlorite is inferred to have

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formed during brittle faulting and not after the deformation or during regional metamorphism. Within the

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footwall granite of the south Tan–Lu Fault and metamorphic gneiss in NE China, biotite and hornblende

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are unaltered, and the chlorite is therefore not a product of retrograde metamorphism or the alteration of

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these minerals.

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2.3. Common and Cataclastic K-feldspar composition: analytical methods and

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results

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Cataclastic K-feldspar represents the deformation of K-feldspar either on its own or together with

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other minerals. Electron microprobe (EMP) analysis can yield mineral compositions of deformed grains.

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In addition, cross-sections of single grains of cataclastic K-feldspar using EMP analysis can reveal the

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characteristics of microfractures as well as the composition of the material adjacent to or within healed

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fractures, allowing both of these compositional domains to be related to the results of Ar–Ar step-heating

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dating. Quantitative analysis of rock-forming minerals was conducted (Table 1, data repository 1; Fig. 6)

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using an electron microprobe (EMP) with a 15 kV accelerating voltage, a 10 nA beam current and a 1 µm

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beam size at the China University of Geosciences (Beijing), China.

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Data about cataclastic K-feldspar (Table 1 and data repository 1), indicate that there are mainly the

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K-feldspar compositions, besides a few quartz and calcite within healed microfractures. And on the fault

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surface, within some fractured K-feldspar, small albite grain is present. Compositional cross-sections by

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electron-microprobe show that in the center or at the edge of the cataclastic K-feldspar grains, the

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composition is similar for K2O and SiO2 etc. Compositions of healed fractures analyzed by EMP show

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that they consist of fine-grained K-feldspar fragments and subordinate calcite and quartz. However, these

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domains do not affect the data obtained from Ar–Ar dating. In addition, the EMP analyses indicate

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un-deformed K-feldspar grain has no any quartz and albite grains (data repository 1).

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2.4. SEM and BSE observation of cataclastic K-feldspar grains: analytical methods

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and results

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A microscope can be used to readily distinguish between fault breccia, cataclasite, or fault

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gouge within fault-zone materials. Microtectonic characteristics of cataclastic K-feldspar can be observed

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under a microscope, but intragrain microfractures are difficult to be detected optically. SEM and BSE

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image analyses can show both surface features and intragrain microfractures, and can be used to

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quantify the spatial distribution, length, and width of these microfractures.

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The clear microfractures were analyzed using a Zeiss SUPPA55 field-emission scanning

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electron microscope (FESEM) at the China University of Geosciences (Beijing) (Tang et al., 2014). SEM

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images observations were analyzed at the National Taiwan University, Taipei using field-emission

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scanning electron microscope (FESEM) and BSE images were collected at the China University of

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Geosciences (Beijing) during the EMP analysis.

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BSE and SEM images were obtained for K-feldspar separates (Figures 7–9). Most of the mineral

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grains are dissected by microfractures, which are locally parallel to one another and have an irregular

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arrangement with similarity in morphology, indicating a single stage of brittle deformation. Concentrated at

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grain margins and less often in the crystal interiors, these fractures contain small authigenic quartz grains,

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and minor chlorite. Minor authigenic albite crystals were also observed along or within the microfracture

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zones. BSE images of fragmented individual K-feldspar crystals reveal distinct intragrain deformation,

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including grain-scale fractures that occur in parallel sets or at right angles to each other. No twins were

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observed in the K-feldspar grains. Fractures and small angular grains occur along K-feldspar boundaries,

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and also penetrate grain boundaries. Very small-scale (<1 µm) fractures were also observed in SEM and

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BSE images (Figures 8–9). Authigenic albite and fine quartz grains occur at the summits of the

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microfractures, representing the formation of new minerals during the brittle deformation. The K-feldspar

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grains are surrounded by original fractured K-feldspar.

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2.5. XRD analysis of single crystals of cataclastic K-feldspar

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2.5.1. Analytical method

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Currently, crystal deformation or defects are analyzed using transmission electron microscope (TEM)

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and X-ray diffraction (XRD). XRD can reveal the occurrence of within-crystal deformation as well as

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microfractures. To investigate the deformation of the K-feldspars, single crystal XRD analyses were

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carried out in the X-Ray Laboratory of China University of Geosciences (Beijing), China. Single crystals,

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each about 0.3 × 0.2 × 0.05 mm in size, were carefully selected and mounted on a thin glass fiber with

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cyanoacrylate (superglue) adhesive. X-ray diffraction photos were obtained on a Bruker APEX

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SMART-CCD diffractometer equipped with a normal focus 2.4 kW sealed tube X-ray source (MoKα

204

radiation, λ = 0.71073 Å) operating at 45 kV and 35 mA. A matrix method was applied with ω scans (at a

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frame width of 0.30° and exposure time of 10 s per frame), and three sets of settings: (a) 2θ = –25°, Φ =

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0°, and ω = –25°; (b) 2θ = –25°, Φ = 90°, and ω = –25°; and (c) 2θ = 25°, Φ = 0°, and ω = 25°; a

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combination of 20 frames forming a diagram of 6° range in reciprocal space for each crystal. The

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laboratory procedure of single crystal analysis is similar to that described by Afanas’ev and Kohn (1971)

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and Dinnebier and Friese (2007).

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2.5.2. XRD results

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Determination of a crystal preferred orientation in polycrystalline aggregates is referred to as texture

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analysis, as micro-diffraction can differentiate crystals with different deformation histories and dislocation

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defects. The results presented here are based on single crystals rather than on powder diffraction Debye

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cones (Bish and Post, 1989; David, 2002). The images A, C, and E (from undeformed granitic plutons) of

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figure 10 summarize the undeformed K-feldspar crystal domains with no dislocations or defects, and the

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images B, D, and F the deformed K-feldspars (B-wys-253, D-DB54, and F-DB51-2) that were dated by

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the K-feldspar minerals. As the XRD method analyzes fine-grained particles (Bish and Post, 1989; David,

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2002), the deformed or fractured K-feldspars outline similar diffraction Debye cones, suggesting that

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these grains are deformed polycrystalline aggregates. The deformation influenced by fully or partly

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resetting the Ar isotope system, as recorded in preliminary gouge frictional shear-heating experiments

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(Zwingmann et al., 2013).

Ar/39Ar. Polycrystalline diffraction in images B, D, and F (Figure 10) show dislocation and defect cells of

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3. 40Ar/39Ar dating

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3.1. Mineral separation

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Samples of chloritized breccias and cataclasites were collected near (3.0–0.2 cm) to the fault

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surfaces (Figures 2–4). A total of 16 samples were analyzed including 14 of fault rocks from three

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metamorphic core complexes and a high-angle normal fault (Figure 1) and two of host rocks (Table 2).

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About 0.1-1 kg of each sample was collected. Individual K-feldspar grains of 0.3–0.45 mm in

231

diameter were separated using a Frantz magnetic separator, conventional heavy organic liquid

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separation techniques, and hand picking using a binocular microscope. SEM observations were also

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used to separate cataclastic K-feldspar with micrograins and intragrain microfractures (such as Figure 7A,

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B).

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3.2. Analytical methods

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K-feldspars were dated following step-heating procedures using a VG 1200S mass spectrometer

238

equipped with a double vacuum Mo furnace at the National Taiwan University, Taipei, and an MM-5400

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mass spectrometer at China University of Geosciences, Beijing, China.

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At the National Taiwan University, the samples were irradiated for 20 h, together with the biotite

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standard LP-6 (Odin et al., 1982) in the 5-C position at the Open-Pool Reactor in Hamilton (Canada). To

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monitor the neutron flux in the reactor, three aliquots of the LP-6 standard, weighed in the range of 6–10

243

mg, were stacked with the samples in each irradiation package of 9 cm length. After irradiation, the

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standards and samples were either incrementally heated or totally fused using a double-vacuum

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resistance furnace operated in continuous mode, and the gas was measured by noble-gas mass

246

spectrometry. The J values were calculated using argon compositions of the LP-6 standard, with a

247

40

248

biotite by assuming that it has the same age as the Fish Canyon sanidine (28.02 ± 0.28 Ma; Baksi et al.,

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1996; Renne et al., 1998; Lo et al., 2002). Ages were calculated using Ar isotopic ratios measured after

250

corrections for mass discrimination, interfering nuclear reactions, procedural blanks and atmospheric Ar

251

contamination, and the data were plotted as age spectra and in isotope correlation diagrams (Fig. 11 and

252

in the data repositories 2 and 3). The age data were calculated using the ArArCALC program (Koppers,

Ar/39Ar age of 128.4 ± 0.2 Ma (Odin et al., 1982), calibrated according to the age of the Fish Canyon

12

253

2002). At the China University of Geosciences (Beijing), the K-feldspar separates were dated by the

254 255

40

Ar/39Ar method using the MM-5400 micromass-spectrometer (Wang et al., 2005). The duration of

256

irradiation and the neutron dose were 9.5 h and 2.08 × 1017 n/cm2, respectively. The J factor was

257

estimated by a replicate analysis of the Fish Canyon Tuff sanidine with a known age of 27.55 ± 0.08 Ma,

258

as reported by Lanphere and Baadsgaard (2001), and a 1% relative standard deviation (Wang and Zhou,

259

2009). The ages were calculated using ISOPLOT 2.31 (Ludwig, 2000).

260 261

3.3. Analytical Results

262

Fourteen cataclastic K-feldspar samples were collected from four different structural domains: a

263

high-angle normal fault surface (Tan-Lu fault zone), a cataclastic mylonite and gneiss containing chlorite

264

(Louzidian fault zone), and fractured mylonites from two metamorphic core complexes (Wafangdian and

265

Jinzhou detachments). For samples collected from the Tan-Lu Fault, well-defined plateaus were

266

observed for the spectra of all 6 samples (Table 2, and data repository 3). No obvious staircase spectra or

267

Ar diffusion gradients were observed. The average age of the six (DB-49, DB-50, DB-51-1, DB-51-2,

268

DB-54, and DB-56; Figure 11; and data repository 3) cataclastic K-feldspar samples collected from

269

different positions along a single high-angle normal fault surface is 74.5 ± 1.3 Ma. For samples collected

270

from the Louzidian fault zone, cataclastic K-feldspar wys-250, wys-251, wys-253, wys-254, and wys-255

271

from different positions along a brittle normal fault surface of the Louzidian metamorphic core complex

272

yield similar plateau ages (120–129 Ma) (Figure 11; and data repository 3). The other three wys-324,

273

wys-327 and wys-328 (Figure 11; and data repository 3) cataclastic K-feldspar samples collected from

13

274

two normal fault surfaces in the Liaonan metamorphic core complex yield identical plateau ages of 110±

275

2 Ma, 117 ± 2 Ma and 116 ± 2 Ma, respectively. Small MSWDs (mean square of weighted deviates) were

276

found in inverse and normal isochron plots (included in the data repository); they range from 0.1 to 1.98.

277

The

278

excess or loss. All of these 14 cataclastic K-feldspar samples yield well-defined spectra, and cumulative

279

39

280

plateau ages (Table 2). No old ages or retentive ages can be seen in the plateaus (Figure 11 and data

281

repository 3).

40

Ar/36Ar intercept of isochron ages is similar to atmospheric ratios (of 295.5), indicating no

40

Ar

Ar for composition of spectra is larger than 92%. Their isochron and total fusion ages are similar to their

282

For comparison, the K-feldspar sample DB-52Kf was collected from the host rock approximately

283

40 cm below the surface of the Tan–Lu Fault. The results show increasing step ages from ~90 to 130 Ma.

284

The K-feldspar sample wys-318-1Kf was collected from the mylonite approximately 100 m below the

285

surface of the Jinzhou detachment fault. The results show increasing step ages from ~100 to 190 Ma.

286 287

3.4. Evaluations of closure temperatures

288

The flat spectra in the cataclastic K-feldspar shows that there is not an argon diffusion gradient in

289

the mineral. The Ar release spectra of the cataclastic K-feldspar from faulted rocks are distinct from the

290

general staircase Ar-diffusion spectra obtained for K-feldspar from granite, and mylonite. Hence, we see

291

very different cooling histories between the cataclastic rocks and the undeformed protolith (such as

292

DB-52 Kf and wys-318-1Kf).

293

We used the Closure Temperature (Tc) (Dodson, 1973) equation to calculate the Temperature (To)

294

and activation energy, and subsequently using the different cooling rates to estimate closure

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295

temperatures. The used parameters are listed in Table 3. Some samples were evaluated for the

296

activation energy (AE) E0, frequency factor log (D/a2) and log(D0/a2) (data repositories 4 and 5),

297

according to Dodson (1973), and different cooling rates such as 10, 20, 30, 40, and 50 °C /m.y. were

298

used to compare the potential closure temperatures. The calculated activation energy (kcal/mol) and

299

evaluated closure temperatures are listed in Table 3. Previous 40Ar/39Ar step-heating dating experiments

300

indicate no evidence of stair-case spectra and therefore no detailed multi-diffusion domain (MDD)

301

analysis was carried out. In addition, the Arrhenius data modeling indicate no fit for a rapid cooling

302

process in the investigated samples.

303

In this study we investigated the closure temperature within a single Arrhenius plot and up to five

304

different estimations for the closure temperature for some samples. The parameters of activation energy

305

(AE) E0, frequency factor log(D0/a2) and cooling rate for the collected data in these K-feldspar samples

306

could be directly estimated (Table 3). Estimates of the diffusion domains and parameters such as the

307

activation energy and closure temperatures, the first several step(s) were selected in three isothermal

308

sequences, such as sample DB-49Kf. For the cataclastic K-feldspar, DB-49Kf, an activation energy of 53

309

kcal/mol, with closure temperature at 281, 289, 294, 297 and 299°C from 10 °C /m.y. to 50 °C /m.y.

310

cooling rates (Table 3) was selected. The highest closure temperature of these samples is set at 380 °C.

311 312

3.5. Interpretation of closure temperatures vs the deformation temperature

313

Forster and Lister (2010) and Forster et al. (2014) suggest that porphyroclastic K-feldspar is not

314

preserved as single grains in cataclastic rocks. It is present as cataclastic K-feldspar, with glass, fine-grain

315

quartz and plagioclase. In this study, K-feldspar from relict gneiss and mylonite has been separated.

15

316

Cataclasis has caused grain-size communication of K-feldspar into varying sized fragments which can

317

subsequently almagate and form larger crystals. The almagated K-feldspar crystals document the real

318

closure temperature and not the earlier brittle deformation temperature. Pseudotachylite formation, as

319

well the cataclastic flow formation, suggests that the temperature partly up to 600 °C (e.g. Tagami, 2012;

320

Devès et al., 2014).

321

Forster et al. (2014) describe mylonitization at relative low 440-480 °C which are sufficient to reset

322

of the argon systematics of K-feldspar. The formation of high temperature pseudotachylite, authigenic

323

growth of chlorite, and well-defined Ar-Ar plateau and total fusion, isochron and plateau ages are similar,

324

suggesting that the rapid diffusion of Ar in cataclastic K-feldspar is related to the fractures and

325

microfractures of the grains and intragrains. Thus, argon systematics have been reset during brittle

326

deformation.

327 328

4. Discussion

329

4.1. Formation of cataclastic K-feldspars

330

K-feldspar crystals appear to have experienced rapid brittle deformation that induced significant

331

fractures (e.g., Figures 7–9 and 12), while no authigenic K-feldspar seems to have crystallized and grown.

332

The fractures are found in the K-feldspar grains within the fault damage zone, but not in K-feldspars

333

grains of the host rock located 20–30 cm from the fault surface. This observation differentiates the

334

cataclastic K-feldspar within the fault zone from the K-feldspar grains of the footwall or the host granite.

335

Most minerals in cataclasites are mechanically anisotropic, and microfractures commonly occur

336

along particular crystallographic directions such as a cleavage direction, as is the case in micas,

16

337

feldspars and amphiboles (Williame et al., 1979; Brown and Macaudiére, 1984; Tullis and Yund, 1998).

338

Microfractures are considered intragranular if they affect just one single grain, whereas fractures that

339

transect several grains are known as intergranular or transgranular. In a single K-feldspar grain, brittle

340

fracturing results in lattice defects and intracrystalline deformation features (Figures 8–9) associated with

341

cataclastic failure at sites of dislocation tangles (Tullis and Yund, 1987). Intracrystalline deformation is

342

also characterized by low temperatures and deformation lamellae with a high optical relief. In fact,

343

intracrystalline deformation (Lloyd, 2000) that is common in brittle K-feldspar and other minerals involves

344

breakage of grains (Figures 8–9). Alternatively, intracrystalline deformation does not involve pressure

345

solution, chemical reactions, or mineral transformations (Atkinson, 1982; Blenkinsop and Sibson, 1991),

346

nor fluid flow.

347

Here, the BSE images show numerous microfractures filled by small grains of K-feldspar (Figures

348

8–9), but no evidence of feldspar authigenesis confirms the lack of high-temperature chemical

349

interactions in the microfractures. Such microfractures and other microstructures are generally healed

350

and filled with secondary grains, most often of the same mineral phase, and in optical continuity with the

351

host crystal (Figures 7–9 and 12). This makes it especially difficult to identify tensional microcracks (Stel,

352

1981), except by BSE imaging, yet many larger fragments being crossed by healed fractures (Figures 7–

353

9). It is important to reiterate that the fractures are not overprinted by any later deformation.

354 355

4.2. Estimation of temperature and fluid flow during K-feldspar cataclasis

356

K-feldspar deformation is generally dependent on the metamorphic conditions (Deer et al., 2001;

357

Passchier and Trouw, 2005). Previous studies of naturally deformed feldspar have shown that the strain

17

358

rate of cataclastic flow decreases and diffusion creep increases, with increasing temperature (Wibberley,

359

1999; Blenkinsop, 2000; Rybacki and Dresen, 2004; O’Hara, 2007). Variably sized angular grain

360

fragments were observed in the resulting cataclasites at temperatures <400 °C.

361

The angular grain fragments yield a wide range of sizes that testify strong to intracrystalline

362

deformation including grain-scale fractures and bent cleavage planes (Figures 8–9). Cataclastic

363

K-feldspar with clear boundaries, as well as crystal microfractures, are visible in thin sections and in SEM

364

and BSE images. A TEM study of similar structures has shown that they are not due to dislocation

365

tangles or networks, but to very small-scale brittle fractures (Tullis and Yund, 1987). At temperatures

366

<400 °C, feldspars deform mainly by internal microfracturing, however with minor dislocation glide (Ji,

367

1998). Visible augen and matrix structures, and the absence of core–mantle structures confirm that the

368

deformation temperatures were <400 °C. Alternatively, cohesive fault breccias in a brittle deformation

369

environment, such as cataclasites and fault breccias with angular fragments of variable size, may not

370

result from intracrystalline fracturing that involves dislocation or recrystallization of quartz and K-feldspar

371

materials. Instead, cataclastic metamorphism along fault zones can result only from mechanical crushing

372

and granulation of the rocks. Fabric experiments show that such processes are favored by high strain

373

rates under a high shear stress at relatively low temperatures (Passchier and Trouw, 2005).

374

Deformed K-feldspars are characterized by cataclastic features, which usually occur at very low to

375

low (<300 °C) metamorphic grades without quartz undulose grain extinction. At higher temperatures,

376

intracrystalline microstructures such as undulose extinction and deformation lamellae may be absent

377

because of recovery and dynamic recrystallization (Passchier and Trouw, 2005). In this study, SEM and

378

BSE investigations did not reveal either internal microfractures, dislocations, bent twins, or deformation

18

379

bands. No augen-shaped grains or core–mantle structures could be identified as well. Thus, the

380

temperatures during deformation had to be within the brittle realm (~ <400 °C; Passchier and Trouw,

381

2005), consistent with the growth of chlorite during deformation (Arkai et al., 2000), which also suggests

382

temperatures likely ~300 °C during fracturing and cataclasis of the K-feldspar.

383 384

4.3. Strain effect for the Ar-system during K-feldspar cataclasis

385

The effective strain in cataclastic rocks causing disintegration of detrital K-feldspar and

386

subsequent amalgamation might be sufficient to significantly influence and reset the Ar isotope system,

387

and it is not simply related to the temperature (Forster and Lister, 2010; Forster et al., 2014). Our data

388

suggest: (1) the cataclastic K-feldspar changed its internal crystalline structure and texture due to

389

deformation, (2) this process increased the rapid Ar-diffusion and reset of the Ar-Ar system, (3) the

390

process involved deformation of the whole internal K-feldspar crystal and not only the grain boundary

391

(Forster et al., 2014), and (4) step-heating spectra are flat or well-defined with only a <10 Ma difference

392

for the >90% cumulative 39Ar.

393

The hanging wall sliding down of a normal fault is a decreasing temperature process from a critical

394

temperature, thus the sudden or the rapid temperature decrease and increase (such as formation of the

395

pseudotachylite at 600-650 °C), the rapid cooling of 50-100 °C /m.y. is easily to be reached (Kuo et al.,

396

2011; Tagami, 2012; Devès et al., 2014; Platt, 2015; Felicetti et al., 2017). Thus, the normal faulting is a

397

process for the decreasing temperature. Overall, a rapid diffusion and reset is controlled by the crystal

398

deformation and intracrystalline dislocation, and not only temperature. The pattern of age variation

399

between the most retentive and least retentive diffusion domain would be consistent with each other.

19

400

Forster et al. (2014) suggested that cooling of the K-feldspar during mylonitisation is required to produce

401

staircase spectra, however, in this study only well-defined plateau were obtained even within rapid uplift

402

and exhumation for the ductile shear zones (Wang et al., 2005). Brittle deformation is normally a

403

retrograde process with decreasing temperatures and obtaining thermal history is unlikely considering

404

the closure temperature for cataclastic K-feldspar. If the mylonite contains porphyroclastic K-feldspar, but

405

not the matrix, a stair-case cooling pattern would be documented (such as sample wys-318-1Kf).

406

Therefore, if some porphyroclastic K-feldspar is present in the proto-mylonite, the mylonitisation required

407

temperature of 450-500 °C would reset the K-feldspar. Brittle deformation occurs under high strain and

408

potential localized high temperature by shear heating in a very short time interval (such as cataclastic

409

rocks with pseudotachylite formation). If the internal deformation of K-feldspars is pervasive and can be

410

documented in BSE images and in combination with the crystalline deformation as recorded by single

411

crystal XRD investigations it is possible to obtain well-defined 40Ar/39Ar age spectra (<10 Ma differences).

412

If these fundamental conditions are met it is possible to date the brittle deformation event and constrain

413

the age of a cataclastic K-feldspar.

414 415

4.4. Resetting of the Ar-system for the cataclastic K-feldspar

416

Generally, temperature and fluid activity alter the mineral Ar-system. In the case of normal faulting

417

and strike-slip motions, the process leading to a healed fracture is of short time interval (Passchier and

418

Trouw, 2005). Thus, healed fractures potentially record the temperature decrease following a drop in

419

strain rates and tectonic stress (Figure 12). As high strain rate is associated with fracture healing in

420

minerals, rapid formation and healing of fractures is not affected by chemical interactions during brittle

20

421

deformation. As only a few authigenic mineral growths were detected in the fractures or the

422

microfractures, fluid flow and high temperatures were not the driving factors, especially after the

423

Ar-system was reset. The deformation influenced by fully or partly resetting the Ar isotope system, as

424

recorded in preliminary gouge frictional shear-heating experiments (Zwingmann et al., 2013).

425

If stress is released, the original shape of the cataclastic mineral grains is recovered, explaining the

426

common observation of oriented mineral fragments and microfractures; the systematic change in shape

427

only resulting from a change in the relative atomic positions inducing movement of lattice defects through

428

a crystal, as in quartz (Poirier, 1985). X-ray images suggest that intracrystalline textures have changed

429

(Figure 10), probably involving very fine powdering, these effects not necessarily detectable in

430

well-shaped single crystals.

431

Rapid brittle deformation occurs during normal and strike-slip faulting at high strain rates,

432

producing microfractures, while temperature changes suddenly from high during deformation to low after

433

deformation stops. Thus, microfractures of the grains and healing of the fractures change in the strain

434

rate. When K-feldspar undergoes cataclasis during high strain rates, the result is the extensive formation

435

and rapid sealing of the fractures. During an increase in strain rate, the temperature during sliding along a

436

fault zone potentially changes, with high temperatures during a relatively short time (pseudotachylite) as

437

discussed in experiments by Sato et al. (2009). The high temperatures during brittle deformation impact

438

the Ar-system, probably leading to fast Ar diffusion (Figure 13). Here, the strain rate favored formation of

439

intragrain microfractures, fractures and intracrystalline dislocations, potentially inducing Ar diffusion, thus

440

expectedly inducing a complete reset of the Ar-system (Figure 13). A rapid decrease in temperature and

441

strain rate result in a rapid healing of the fractures and microfractures within the grains. Thus, when the

21

442

K-feldspar Ar-system is closed again, it records the timing of the brittle deformation (Figure 13), if no later

443

fluid flow interactions occur.

444 445

4.5. Interpreting the age of cataclastic K-feldspar as timing of brittle deformation

446

Dating a brittle fault requires an extensive knowledge of the geological context and the occurrence

447

of minerals that are suitable for dating. The brittle fault zones studied here are characterized by

448

cataclastic rocks and fault gouges, pseudotachylite, authigenesis of chlorite, chlorite-bearing brecciated

449

mylonite and granite with cataclastic K-feldspar. As K-feldspar cataclasis was accompanied by chlorite

450

formation, it suggests that the deformation occurred at a temperature range of ~ 180 and 330 °C, which is

451

close to or even higher than the closure temperature of K-feldspar (170–380 °C; this study).

452

Previous studies have indicated a relatively simple relationship between intracrystalline Ar diffusion

453

and age spectra (McDougall and Harrison, 1999). Alkali feldspar crystals containing discrete diffusion

454

domains produce stairstepped age spectra with ages increasing during step-heating (Harrison et al.,

455

1991). Here, the analyses produced well-defined flat spectra associated with the fractured or cataclastic

456

K-feldspar.

457

The 40Ar/39Ar spectra of cataclastic K-feldspar are flat with no evidence of Ar diffusion gradients,

458

which is in contrast to K-feldspar grains from undeformed host granite adjacent to or at some distance

459

from the fault surface (i.e., footwall samples; Figure 1B–D). These data confirm that sudden brittle

460

deformation strongly altered the Ar system of the K-feldspar, resulting to a complete reset of the Ar

461

system of the cataclastic K-feldspars. Therefore, brittle deformation that produced the breccias and

462

cataclastic rocks can be dated by using the 40Ar/39Ar step-heating method on the cataclastic K-feldspars.

22

463

In addition, comparison of the ages of the K-feldspars along the Tan-Lu fault zone with preliminary ages

464

of illite-chlorite mixed-layers from the same fault surface, cataclasite, or fault breccia provided significantly

465

younger ages at 74 ± 1 Ma (Wang and Zhou, 2009).

466

The ages of the cataclastic K-feldspar of the normal fault surface of the Liaonan metamorphic core

467

complex are 2–3 Ma younger than the muscovite and biotite cooling ages obtained from metamorphic

468

core complex of the same area (Figure 14; Wang and Zheng, 2005; Lin et al., 2011). The ages are also

469

similar to the rapid cooling ages of pristine K-feldspar from mylonites and ductile shear zones (~110–115

470

Ma; Lin et al., 2011), and similar to the age of Early-Cretaceous volcanics that filled the basin in the

471

hanging wall of a normal fault next to the metamorphic core complex. Relative to the protolith of the fault

472

gouge and the breccia on the fault surface, these ages resulted from brittle deformation event rather than

473

from cooling of the granite due to rapid uplift (Figure 14).

474 475

4.6. Criteria for using cataclastic K-feldspar Ar–Ar ages to infer brittle faulting

476

chronology

477

To use cataclastic K-feldspar Ar–Ar ages to infer the chronology of brittle faulting, the following

478

criteria must be met: (1) samples should be collected from the fault surface with cataclasite and/or

479

chloritized fault breccia, with fractured K-feldspar being selected by hand-picking; (2) SEM and BSE

480

images should show crystals with microfractures; (3) single-grain XRD images should show a clearly

481

deformed crystal and (4) well-defined Ar–Ar spectra should be apparent without a stair-case pattern. If

482

these criteria are met, then the age data are likely to represent the age of brittle deformation.

483

As shown in Figure 13, the resetting of Ar by deformation/strain or by the healing of fractures

23

484

allows the development of a new daughter–parent (D–P) equilibrium that represents the age of the

485

deformation. The timing of normal faulting, strike-slip faulting, and thrust faulting can be constrained by

486

dating cataclastic K-feldspar in host materials such as granite, metamorphic rock, mylonite, and even

487

sandstone. Such dating can be used to reconstruct the history of paleo-earthquakes.

488 489

5. Conclusions

490

As reported in the literature, dating of brittle fault zones is complex, because the deformation is a

491

dynamic process that may last for a long time with the potential for numerous periods of reactivation

492

and/or tectonic events. It is therefore difficult to determine precisely its timing. Isotopic dating of brittle

493

faults is therefore challenging, depending on the specific lithologies of the cataclastic zones and the

494

authigenic minerals that potentially grew during faulting. The data obtained here suggest that the timing of

495

brittle deformation can, alternatively, be constrained by examining step-heating 40Ar/39Ar systematics of

496

cataclastic and fractured K-feldspar grains, if the fractured K-feldspars can be separated from brittle

497

deformed rocks, cataclasites and breccias, carefully identified, and characterized by complementary

498

SEM, BSE and XRD results.

499

Acknowledgments: We gratefully acknowledge the constructive comments and suggestions of Editor of

500

Journal of Structural Geology, Prof. Joao Hippertt, reviewer Dr. Kyle Min and an anonymous reviewer.

501

Prof. Michael Wells helped to clarify the manuscript and also gave some constructive suggestions. This

502

study was financially supported by the NSF of China (41430316 and 90914004)，the State Key Research

503

Development Program of China (973, 2011CB808901) and PhD Programs of the Foundation of Ministry

504

of Education of China (20120022110003).

24

505 506

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Peninsula metamorphic core complex. Journal of Asian Earth Sciences 42, 1048-1065.

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605 606 607 608 609

Lloyd, G. E., 2000. Grain boundary contact effects during faulting of quartzite: an SEM/EBSD analysis. Journal of Structural Geology 22, 1675-1693. Lo, C.-H. Chung, S.-L., Lee, T.-Y., Wu, G.-Y., 2002. Age of the Emeishan flood magmatism and relations to Permian–Triassic boundary events. Earth and Planetary Science Letters 198, 449-458. Lovera, O. M., Richter, F. M., Harrison, T. M., 1989. The 40Ar/39Ar thermochronometry for slowly cooled

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612 613 614 615 616 617 618 619 620 621 622 623 624 625

Lovera, O. M., Richter, F. M., Harrison, T. M., 1991. Diffusion domains determined by

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Passchier, C. W., Trouw, R. A. J., 2005. Micro-tectonics: Springer, pp. 1-366.

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Platt, J. P., 2015. Influence of shear heating on microstructurally defined plate boundary shear zones.

628 629 630

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deformation mechanisms and isotope systematics. Journal of Structural Geology 21, 1255-1265.

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Reddy, S.M., Potts, G.J., Kelley, S.P., Arnaud, N. O., 1999. The effects of deformation-induced

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Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating. Chemical Geology

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Rolland, Y., Cox, S. F., Corsini, M., 2009. Constraining deformation stages in brittle–ductile shear zones

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641 642

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646 647 648 649

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652 653 654

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656

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660 661 662 663 664 665 666 667 668 669 670 671 672

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680

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681

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683

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694 695 696 697

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702

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40

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703

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704

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705 706 707 708

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709

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710

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711

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712

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713

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714

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715 34

716

Figure Captions

717

Figure 1. Structural sample locations and normal faults in the field, with the ages of co-existing granite

718

and protolith (indicated on the footwalls of the fault surfaces). (A) Simplified structural map of eastern

719

North China. (B) Southern segment of the Tan–Lu fault zone (simplified from Wang and Zhou, 2009). (C)

720

Liaonan metamorphic core complex. (D) Louzidian metamorphic core complex. (E) Four structural

721

cross-sections. Age data are from Wang and Zhou (2009). Section positions are shown in Figure 1B, C,

722

D.

723 724

Figure 2. (A) Simplified structural cross-section of the southern Tan–Lu fault zone (Wang and Zhou,

725

2009). (B) Field photograph of a high-angle normal fault. (C) Microstructures of cataclastic rocks and

726

K-feldspar within breccia (sample DB-49 Kf). (D) Microstructures of cataclastic rocks and

727

syn-deformational chlorite within breccia (sample DB-50 Kf). Chl = chlorite, Kf = K-feldspar. Samples a to

728

f are from different sites in the same level within the fault damage zone and ages a to f are DB-series

729

from this study.

730 731

Figure 3. (A) Simplified structural cross-section of the Louzidian detachment fault. (B) Field photograph

732

of a high-angle normal fault footwall that comprises mylonite. (C) Field photograph of a high-angle normal

733

fault; its footwall and hanging wall consist of granite. (D) Fault surface of a high-angle normal fault in

734

gneiss. (E) Microstructures of cataclastic rocks with large cataclastic K-feldspar grains within breccia

735

(sample wys-250 Kf). (F) Microstructures of cataclastic rocks with fine cataclastic K-feldspar grains within

736

breccia (sample wys-254 Kf). Kf=K-feldspar. Samples a through e are from different sites in the same

35

737

level within the fault damage zone.

738 739

Figure 4. (A) Simplified structural cross-section of the Liaonan metamorphic core complex. (B) Field

740

photograph of a normal fault in a metamorphic core complex. (C) Microstructures of cataclastic rocks with

741

large cataclastic K-feldspar grains within breccia (sample wys-327 Kf, with relict mylonite). (D)

742

Microstructures of cataclastic rocks with fine cataclastic K-feldspar grains within breccia (sample wys-328

743

Kf, with relict granite).

744 745

Figure 5. Microstructural features of cataclastic rocks and undeformed granite from the Tan–Lu Fault

746

(DB-series of samples). (A) Fault breccia and cataclasite with fractured K-feldspar. The dark-colored

747

mass is cataclasite. (B) Microstructures of the cataclasites and syndeformational chlorite within the

748

breccia. Different types and sizes of angular feldspar, K-feldspar, and quartz are shown. Angular

749

fragments of variable size are set in a fine-grained matrix. (C) Fault breccia with fragments of granite and

750

pseudotachylite. (D) Fault breccia with smaller fractured grains of quartz and K-feldspar. Calcite veins are

751

present. (E) Chlorite within the layer of fault breccia; the surrounding materials are quartz, plagioclase,

752

and granitic rock fragments. (F) Undeformed granite from below the fault surface. Chl=chlorite,

753

Gr=granite, Kf=K-feldspar, Pl=plagioclase, Qz=quartz.

754 755

Figure 6. Microstructural features of cataclastic rocks from normal faults (wys-series of samples) in NE

756

China. (A) Microstructures of cataclastic rock containing different sized fragments of gneiss and granite.

757

(B) Fractured mylonite. (C) Fractured granite showing grains of K-feldspar. The calcite veins follow

36

758

fractures in the cataclastic rock. (D) Small-sized fractured grains of quartz and K-feldspar. The

759

dark-colored part is a mass of cataclastic material. (E) Cataclastic rock and syn-deformational chlorite

760

within the breccia. The protolith was gneiss. (F) Incohesive fault breccia in granite, and syndeformational

761

chlorite within the breccia. Angular fragments of variable size are set in a fine-grained matrix. Cal=calcite,

762

Chl=chlorite, Kf=K-feldspar, Pl=plagioclase, Qz=quartz.

763 764

Figure 7. Selected SEM images and data, showing the components of cataclastic K-feldspar grains. (A)–

765

(B) SEM images of K-feldspar. （C）Fractured K-feldspar. (D) Microfractures and grain fragments along

766

the margins of fractured K-feldspar in (C). In the images, spectrum (SEM spot) 1 has K = 12.01 wt %,

767

spectrum (SEM spot) 2 has K = 0 wt %. Albite is an impurity.

768 769

Figure 8. BSE images of fractured K-feldspar in breccia or cataclasite. Samples were collected from the

770

Tan–Lu Fault plane (DB-series). (A) Microfractures and grain fragments along the margins of fractured

771

K-feldspar. The microfractures have been healed. (B) Microfractures on the grain margin; the

772

microfractures have been healed. (C) Fractured K-feldspar and healed microfractures. (D) Fractured

773

K-feldspar (part of image C). (E) Fractured grains along the margin of the K-feldspar; microfractures

774

within the grain have been healed. (F) Microfractures within a K-feldspar grain (part of image E). Some

775

microfractures have been healed by small fractured K-feldspars. Fragments and small grains are found

776

along the margin and within the fractured K-feldspar. (G) Oriented microfractures; grain fragments can be

777

seen along the margin of the fractured K-feldspar grain. (H) Healed microfractures and grain fragments

778

along the grain margin.

37

779 780

Figure 9. BSE images of fractured K-feldspar in breccia or cataclasite. Samples were collected from the

781

surfaces of the normal fault plane of a metamorphic core complex in NE China (wys-series samples). (A)

782

Microfractures and grain fragments along a grain margin. The sub-fractures and some microfractures are

783

healed within the grain. (B) Fractures and grain fragments along the margin and within the grain. (C)

784

Fractured K-feldspar grain, similar to those found in fault breccia. The image also shows a quartz grain

785

(white). Grain fragments are found along the grain margin. (D) Microfractures along the grain margin and

786

within the grain. Within the grain the microfractures have been healed. (E) Microfractures within a

787

K-feldspar grain (a quartz grain is white). Microfractures are healed. (F) Microfractures parallel to each

788

other along the margins and within the grain. (G) Microfractures that have been healed within the grain.

789

Grain fragments can be seen along the margin of the grain. (H) Microfractures along the grain margin.

790

Microfractures have been healed within the grain.

791 792

Figure 10. Selected XRD images, showing the features of deformed and undeformed K-feldspar. (A), (C),

793

and (E) XRD images of undeformed single crystals of K-feldspar; no dislocations or defect cells are

794

present. (B), (D), and (F) XRD images of deformed single crystals of K-feldspar; polycrystalline diffraction

795

shows features with dislocations and defect cells in the K-feldspars. Their mineral compositions are listed

796

in data repository 1.

797 40

Ar/39Ar spectra and isochron plots for cataclastic K-feldspar samples. The five

798

Figure 11. Selected

799

samples were collected from (A) the Tan–Lu Fault, (B) the Louzidian metamorphic core complex, (C) the

38

800

Liaonan metamorphic core complex, (D) mylonite in Liaonan metamorphic core complex, and (E) granite

801

at the Tan-Lu fault zone. Other spectra and isochron plots are provided in the data repository 3. Kf =

802

K-feldspar.

803 804

Figure 12. Formation of cataclastic features in the K-feldspar. The features start to form from the moment

805

the brittle fracturing of the K-feldspar begins, and they progressively become extensive fractures and

806

micrograins and grain fragmrents. Microfractures may then be healed and the cataclastic K-feldspar

807

dated.

808 809

Figure 13. Summary of processes involved in the complete resetting of the Ar-system: (A) diffusion of Ar

810

outside the grains of K-feldspar, and (B) strain rates that allow intragrain microfractures, fractures, and

811

grain fragments to form, which result in the crystal textures observed and diffusion of the Ar. From the

812

moment the brittle fractures of the K-feldspar start to record the time, the reset of the Ar-system is rapid

813

and complete.

814 815

Figure 14. Summary and comparison of all available age data for the host rocks and the fault rocks. The

816

host rock ages and cooling age data are from 1 and 2, Wang and Zheng (2005; Louzidian metamorphic

817

core complex); 3 and 4, Lin et al. (2011; Liaonan metamorphic complex); 5, Wang and Zhou (2009; Tan–

818

Lu Fault) and this study; and 6, Grimmer et al. (2002, Tan–Lu Fault). Kf = K-feldspar, AFTA = apatite

819

fission-track age.

820

39

Table 1 Representative electron microprobe analysis on cataclastic K-feldspar Sample

SiO2

TiO2

Al2O3

0.021

17.086

Cr2O3 0.198

FeO

MnO

MgO

0

0

0.018

NiO

CaO

Na2O

P2O5

K2O

0.043

0

0.199

0.008

16.811

CoO 0.07

F

BaO

Total

mineral

0

0.206

99.146

Kf

DB-49-01-01

64.486

DB-49-01-02

66.043

0

15.571

0.575

0

0.018

0

0

0

0.208

0

15.922

0

0

0.517

98.853

Kf

DB-49-01-03

68.936

0.09

13.946

0.104

0.039

0

0.021

0.023

0

0.27

0

15.288

0.037

0.076

0.619

99.452

Kf

DB-49-01-04

63.74

0.045

17.774

0.047

0

0.012

0.01

0

0

0.275

0

16.667

0

0.09

0.512

99.172

Kf

DB-50-01-01

64.693

0.043

17.571

0.348

0

0

0.013

0

0

0.216

0

16.8

0

0

0.123

99.805

Kf

DB-50-01-02

63.437

0

18.493

0.284

0.006

0.017

0.003

0

0

0.228

0.013

16.544

0.001

0.017

0.187

99.229

Kf

DB-50-01-03

64.588

0.106

18.074

0.236

0

0

0

0.095

0

0.219

0.035

16.702

0

0.105

0.186

100.344

Kf

DB-50-01-04

63.167

0

18.343

0.224

0.125

0.024

0.033

0.055

0

0.243

0.059

16.191

0.022

0.071

0.228

98.784

Kf

DB-50-01-05

63.733

0

17.736

0.034

0.045

0

0

0.017

0

0.466

0.042

16.538

0

0

0.236

98.848

Kf

DB-50-01-06

64.871

0

17.66

0.086

0.027

0

0

0.026

0

0.407

0.079

16.488

0

0

0.168

99.812

Kf

DB-50-01-07

63.904

0

17.61

0.052

0

0

0.013

0

0

0.4

0.008

16.414

0

0.068

0.304

98.77

Kf

DB-50-01-08

66.328

0.006

16.04

0.163

0

0.052

0

0

0

0.258

0.045

16.637

0.011

0.069

0.319

99.926

Kf

DB-50-01-09

66.202

0

15.185

1.429

0

0

0.021

0.107

0

0.277

0

16.414

0

0

0.185

99.821

Kf

DB-50-01-10

63.558

0.085

17.803

0.836

0.125

0

0.028

0.027

0

0.31

0.021

16.656

0

0

0.31

99.761

Kf

DB-50-01-11

64.117

0.014

15.981

1.598

0.099

0.033

0.058

0.027

0.351

0.252

0

16.049

0.073

0.036

0.282

98.971

Kf

DB-50-02-01

62.679

0.045

18.432

0.695

0.334

0

0.024

0.038

0

0.274

0.011

16.473

0

0.054

0.407

99.466

Kf

DB-50-02-02

64.877

0.01

17.569

0.436

0

0.047

0.021

0.014

0

0.262

0.081

16.46

0.042

0

0.217

100.036

Kf

DB-50-02-03

64.281

0

17.441

0.102

0.015

0.005

0

0.093

0

0.229

0

16.5

0.003

0.099

0.172

98.94

Kf

DB-50-02-04

63.686

0

17.411

1.017

0

0

0.032

0.105

0

0.217

0

16.369

0

0

0.199

99.037

Kf

DB-50-02-05

65.554

0.034

17.057

0.234

0

0

0.025

0

0

1.267

0.005

14.537

0

0.119

0.264

99.095

Kf

DB-50-02-06

62.166

0

18.709

0.598

0.016

0

0.026

0.093

0

0.251

0.003

16.923

0

0.186

0.228

99.197

Kf

DB-50-02-07

67.3

0

13.324

0.307

0.039

0

0.024

0

0

0.106

0.019

17.492

0

0

0.035

98.646

Kf

DB-50-02-08

62.764

0.091

18.259

0.711

0

0.017

0.028

0

0

0.147

0

16.598

0.008

0.124

0.207

98.954

Kf

Table 2 Samples structural position and description Sampled

Sampled Site

Structural positions

Litho-petrology

Determined Minerals

Number

Plateau age

Isochron age

Total fusion age

(Ma)

(Ma)

(Ma)

DB-49

N30° 49.869', E116° 39.489'

Normal fault surface

Cataclastic granite

Cataclastic K-feldspar

75.49±0.71

76.00±1.12

75.75±3.39

DB-50

N30° 49.869', E116° 39.489'

Normal fault surface

Cataclastic granite

Cataclastic K-feldspar

72.33±1.29

72.00±2.21

72.04±1.43

DB-51-1

N30° 49.869', E116° 39.489'

Normal fault surface

Cataclastic granite

Cataclastic K-feldspar

74.10±1.29

73.16±2.20

73.30±1.32

DB-51-2

N30° 49.869', E116° 39.489'

Normal fault surface

Cataclastic granite

Cataclastic K-feldspar

73.17±1.25

72.29±1.83

72.71±1.26

DB-52

N30° 49.869', E116° 39.480'

Footwall of normal fault

Granite

K-feldspar

No plateau age

85.60±5.80

93.14

DB-54

N30° 49.869', E116° 39.489'

Normal fault subsurface

Cataclastic granite

Cataclastic K-feldspar

76.90±2.25

77.80±4.02

76.17±1.48

DB-56

N30° 49.869', E116° 39.489'

Normal fault subsurface

Cataclastic granite

Cataclastic K-feldspar

74.90±1.29

75.50±1.85

74.09±1.30

wys-250

N41° 58.819', E119° 04.642'

Normal fault surface

Cataclastic mylonite

Cataclastic K-feldspar

135.43±2.26

134.93±2.82

139.32±2.43

wys-251

N41° 58.819', E119° 04.642'

Normal fault surface

Cataclastic gneiss

Cataclastic K-feldspar

129.06±2.21

128.33±3.16

130.13±2.55

wys-252

N41° 58.819', E119° 04.642'

Subsurface of the normal fault

Cataclastic gneiss

Cataclastic K-feldspar

124.85±2.19

124.73±2.60

124.46±2.52

wys-253

N41° 58.819', E119° 04.642'

Subsurface of the normal fault

Cataclastic gneiss

Cataclastic K-feldspar

121.64±1.04

121.18±1.08

119.97±1.01

wys-255

N41° 58.819', E119° 04.642'

Subsurface of the normal fault

Cataclastic gneiss

Cataclastic K-feldspar

119.85±2.05

122.90±3.97

119.74±1.97

wys-318-1

N39° 18.273', E121° 50.667'

Footwall of normal fault

Mylonite

K-feldspar

Not well spectra

175.95±4.66

156.56±1.56

wys-324

N39° 18.334', E121° 50.495'

Normal fault surface

Cataclastic mylonite

Cataclastic K-feldspar

110.74±1.88

114.12±3.86

111.05±1.83

wys-327

N39° 38.880', E122° 13.996'

Normal fault surface

Cataclastic mylonite

Cataclastic K-feldspar

116.58±2.00

118.82±4.49

116.50±1.92

wys-328

N39° 38.880', E122° 13.996'

Normal fault surface

Cataclastic mylonite

Cataclastic K-feldspar

116.37±2.12

118.36±11.60

116.05±2.17

1

Table 3 The values of activation energy and estimated closure temperatures obtained from Arrhenius parameters of analysis of K-feldspar

Sample

log(D0/a2)

Activation

Closure T

Closure T

Closure T

Closure T

Closure T

Mean

Energy

(10°C

(20°C

(30°C

(40°C /m.y.)

(50°C

closure T

(kcal/mol)

/m.y.) (°C)

/m.y.) (°C)

/m.y.) (°C)

(°C)

(°C)

(°C)

/m.y.)

DB-49

5.6794

53.1921

281.16

288.9

293.53

296.86

299.46

291.98

DB-50

4.8929

48.8906

256.37

264.05

268.64

271.95

274.54

267.11

DB-51-1

6.7002

56.7111

290.77

298.29

302.79

306.01

308.54

301.28

DB-51-2

8.2238

63.3601

316.39

323.76

328.16

331.31

333.79

326.68

DB-52

4.803

54.3636

316.83

325.4

330.53

334.22

337.12

328.82

DB-54

11.359

77.0517

359.8

366.8

370.97

373.96

376.3

369.57

DB-56

10.325

72.3567

345.16

352.15

356.39

359.43

361.8

354.99

wys-250

8.9603

67.6616

337.13

344.53

348.95

352.12

354.6

347.47

wys-251

3.7443

44.9781

243.08

251

255.74

259.15

261.83

254.16

wys-252

0.3897

29.3326

135.77

143.33

147.88

151.18

153.76

146.38

wys-253

2.2734

35.9816

175

182.44

186.9

190.12

198.65

186.62

wys-255

2.2956

35.5926

169.53

176.87

181.28

184.45

186.95

179.82

wys-318-1

5.9973

53.5811

277.08

284.66

289.18

292.44

294.99

287.67

wys-324

2.0677

35.1991

170.33

177.77

182.24

185.46

187.99

180.76

wys-327

7.5352

59.4476

296.55

303.88

314.97

311.04

313.86

308.06

wys-328

6.6823

56.3222

287.46

294.94

299.41

302.63

305.14

297.92

In the table, samples DB-52 and wys-318-1 are un-deformed K-feldspar. Others are cataclastic K-feldspar.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

170 1800

Tan-Lu high-angle normal fault

150

1600

130 1400 1200

40Ar / 36Ar

Age (Ma)

110 72.33 +/-1.29 Ma 90 70

Total fusion=72.04+/-1.43 Ma Isochron age=72.00+/-2.10 Ma 40Ar/36Ar intercept=299.5+/-23.0 MSWD=0.54

1000 800 600

50

400

30

DB-50Kf

DB-50Kf

200

10

0

0

10

20

30

40

50

60

70

80

90

100

0

20

40

60

80

100

Cumulative 39Ar Released (%)

A

120

140

160

180

200

220

240

260

280

39Ar / 36Ar

3000 Louzidian metamorphic core complex 260

2500 124.85+/-2.19 Ma

2000

40Ar / 36Ar

Age (Ma)

210

160

1500

110

Total fusion=124.46+/-2.52 Ma Isochron age=124.73+/-2.60 Ma 40Ar/36Ar Intercept=296.4+/-13.1 MSWD=0.33

1000

60

500

wys-252Kf

wys-252Kf

0

10 0

B

10

20

30

40

50

60

70

80

90

0

100

40

60

80

100

120

140

160

180

200

220

240

260

39Ar / 36Ar

Liaonan metamorphic core complex

5000

260

4000

210

40Ar / 36Ar

116.58+/-2.00 Ma

Age (Ma)

20

Cumulative 39Ar Released (%)

160

110

60

3000 Total fusion=116.50+/-1.92 Ma Isochron age=118.82+/-4.49 Ma 40Ar/36Ar Intercept=244.4+/-89.2 MSWD=1.85

2000

wys-327Kf

1000

10

wys-327Kf

0 0

10

20

30

40

50

60

70

80

90

100

0

50

100

150

200

Cumulative 39Ar Released (%)

250

300

350

400

450

500

550

39Ar / 36Ar

C 300

6500

270 240

5500 5000

210

4500

180

4000

40Ar / 36Ar

Age (Ma)

6000

Mylonite, Liaonan metamoephic core complex

150 182.43 ± 2.17 Ma 120

3500 Total fusion 156.56 ± 1.56 Ma Normal isochron 175.95 ± 4.66 Ma 40Ar/36Ar intercept 431.5 ± 90.9 MSWD=5.17

3000 2500 2000

90

1500

60

1000

wys-318-1Kf

30

wys-318-1Kf

500 0

0 0

10

20

30

D

40

50

60

70

80

90

0

100

10

20

30

40

50

60

70

80

90 100 110 120 130 140 150 160 170 180 190 39Ar / 36Ar

Cumulative 39Ar Released (% )

300

12000

Undefromed granite, Tan-Lu fault zone

Age = 85.6±5.8 Ma Initial 40Ar/36Ar =669±380 MSWD = 12

250

10000 8000

36

Ar/ Ar

150

127.0±1.7 Ma

40

Age (Ma)

200

6000

100

4000 50

2000

DB-52Kf

DB-52Kf

0 0

10

20

30

40

50

60

Cumulative 39Ar Percent

E

Figure 11

70

80

90

100

0 0

100

200 39

Ar/36Ar

300

400

Figure 12

Figure 13

Figure 14

Highlights Direct Dating of Brittle Faulting; Dating of the cataclastic and breccia K-feldspar by 40Ar/39Ar step-heating methods; Relationship between fractured K-feldspar and Ar-diffusion.

Author Contribution Statement Yu Wang First

and

corresponding

author,

original

writing,

and

organization and support the research. Li Yunzhou Help to write, and field work, Ar-Ar lab analysis Horst Zwingmann Data curation and manuscript editing Ching-hua Lo Lab arrangement and Ar-Ar analysis Guowu Li XRD analysis and mineral analysis Jinhua Hao BSE and EMP analysis

1. Institute of Earth Sciences, China University of Geosciences, Beijing 100083, China 2. Department of Geology and Mineralogy, Graduate School of Science, Kyoto University, Kyoto, 606-8502, Japan 3. Department of Geosciences, National Taiwan University, Taipei 106, Taiwan

Best regards, Yu Wang and all of co-authors

Conflict of interest We have no any financial and personal relationships with other people or organizations that could inappropriately influence (bias) their work or state if there are no interests to declare. Best regards, Yu Wang and all of co-authors