磁化率各向异性与剪切带

华 天, 李金玺, 李智武, 冉 波, 童 馗, 王自剑, 袁梦雨, 蔡鸿燕, 陈 涛, 李 轲, 刘升武

磁化率各向异性与剪切带

华 天1, 李金玺2*, 李智武1, 冉 波1, 童 馗1, 王自剑1, 袁梦雨1, 蔡鸿燕1, 陈 涛1, 李 轲3, 刘升武1

(1.成都理工大学 油气藏地质及开发工程国家重点实验室, 四川 成都 610059; 2.成都理工大学 地球物理学院, 地球勘探与信息技术教育部重点实验室, 四川 成都 610059; 3.四川省煤田地质工程勘察设计研究院, 四川 成都 610072)

岩石组构记录了地壳形成与演化的关键信息, 提取这些信息对分析和恢复地球动力学过程具有重要意义。磁化率各向异性(AMS)是一种重要的岩石组构方法, 可以有效地揭示岩石的应变特征, 分析其地球动力学过程, 是研究构造变形性质以及应力作用方式的有效手段。本文在梳理AMS的研究历史、主要成果和最新进展的基础上, 系统阐述了AMS的基本原理以及在剪切带的应用: ①岩石组构具有复杂性, AMS作为一种间接组构手段受控于矿物的物理特性、含量以及变形变质等多方面因素; ②AMS可以提供剪切带的运动学以及不同部位应变状态的信息; ③对于剪切带, AMS主要受控于磁性矿物(矿物成分和粒度的变化导致全岩磁化率各向异性的变化)、构造变形强度(决定磁线理发展的重要因素)以及流体的作用(流体导致磁性矿物的类型与定向性的变化)。

磁化率各向异性; AMS; 剪切带; 岩石组构

0 引 言

磁性矿物颗粒结晶方向、形态和分布的差异导致岩石不同方向上磁化率的差异称为磁化率各向异性(anisotropy of magnetic susceptibility, AMS), 主要表现为磁晶各向异性和颗粒形状各向异性两个方面(吴汉林, 1988; 许顺山和陈柏林, 1998)。快速、准确、适用性强以及无损性使AMS成为应用最广泛的岩石组构方法之一(Tarling and Hrouda, 1993; 潘永信和朱日祥, 1998)。

Voight and Kinoshita (1907)发现磁性与晶体的取向之间存在对应关系, 可以通过磁性的变化识别晶体的取向。Ising (1942)在研究纹泥磁性时发现, 平行层面和垂直层面的磁化率存在较大差异, 据此提出了磁化率各向异性。Graham (1954)指出几乎所有的岩石都可以观测到磁化率各向异性, 磁化率椭球体可以反映岩石内部铁磁性矿物颗粒长轴的定向分布, 并将其用于地质研究。Nye (1957)将磁化率各向异性定义为一个二阶张量。Nagata (1961)根据张量的不变量性质, 认为磁化率量值椭球体的体积保持不变。至此, 人们才充分认识到主要磁化率轴与非正交晶轴之间的精确关系。Fuller指出铁磁性矿物的空间分布对AMS具有重要意义, 并指出矿物颗粒的定向分布诱发了磁化率各向异性(Fuller, 1961, 1963)。Owens (1974)进一步提出, 相同应变状态的岩石由于变形过程的不同会导致其内部铁磁性矿物分布的差异, 进而产生不同的AMS特征。Hrouda and Janák (1976)指出变质岩的最小磁化率主轴垂直于变质片理, 与岩石的最缩短方向平行。次年, Kligfield et al. (1977)将磁化率各向异性作为应变标志应用于加拿大安大略盆地黏土岩的研究中, 证明了应变椭球体的形状与磁化率椭球体一致, 并且引入了构造分析中使用的Flinn图(Flinn, 1962, 1965)表示磁化率椭球体的形状。Jelínek (1978)首次将张量统计方法引入AMS统计中。20世纪80年代初, Rathore (1980, 1981)和Kligfield et al. (1982)分别对英格兰湖区板岩和阿尔卑斯地区灰岩进行磁化率各向异性研究, 进一步探讨了AMS与应变的关系。Hrouda (1982)首次系统归纳了前人的研究, 具有里程碑意义。此外, Borradaile等进一步系统的对磁化率各向异性进行综述(Borradaile and Henry, 1997; Borradaile and Jackson, 2004, 2010)。Parés (2015)综述了先前变形沉积岩AMS研究。Biederman (2020)对AMS研究现存挑战和未来发展趋势展开论述。

有限应变和AMS都可以设想成一个量值椭球体。但在构造地质学研究中, 应变椭球体与磁化率椭球体存在较大差别。应变椭球体是无量纲, 轴被归一化, 椭球体与初始球体具有相同的体积。而AMS椭球体由于样本的平均磁化率差异, 导致其在形状(各向异度)和大小上各不相同。因此, 不同样本的应变椭球体是直接可比的, 而AMS椭球体则不行(Borradaile and Jackson, 2010)。大多数情况下, 磁化率椭球体和应变椭球体之间存在较好的相关性, 二者主轴方向相互平行(Borradaile and Tarling, 1981; Borradaile, 1987, 1988, 1991; Tarling and Hrouda, 1993; Borradaile and Henry, 1997; Parés et al., 1999)。但主轴大小之间没有可靠的相关性(Parés and van der Pluijm, 2004; Borradaile and Jackson, 2010)。

AMS在地质学领域可以广泛运用到以下研究领域: ①研究古流向(Chou et al., 2013; Novak et al., 2014)和古风向(Liu and Sun, 2012; Ge et al., 2014); ②恢复熔岩流向和侵位方向(孙靖鹏等, 2016; Wiegand et al., 2017; Nagaraju and Parashuramulu, 2019); ③揭示不同构造背景下应变机制: 前陆等弱变形区(Soto et al., 2009; Anchuela et al., 2010; Raposo et al., 2014; Dudzisz et al., 2018)、断裂带(Abdeen et al., 2014; Dudzisz et al., 2018; Casas-Sainz et al., 2018)以及构造叠加变形区(Mamtani and Sengupta, 2010; Mondal and Mamtani, 2013; Mondal, 2018)。本文就磁化率各向异性的基本原理、其在剪切带的应用情况及最新进展进行相对系统的论述。

1 磁化率各向异性的形成机制

1.1 单矿物磁化率各向异性

磁性矿物是AMS的载体, 准确识别磁性矿物是精确解读AMS所蕴含地质信息的前提。基于磁学性质差异, 磁性矿物可分为抗磁性、顺磁性、反铁磁性、铁磁性以及亚铁磁性等几种类型(图1)。一般来说, 岩石的磁化率和各向异性源于所有造岩矿物的贡献, 但具有较高磁化率或各向异性的矿物往往直接控制着整个岩石的磁化率和各向异性。铁磁性和亚铁磁性的磁化率比顺磁性矿物的磁化率大三个数量级, 对整个岩石磁化率的贡献最大(Hunt et al., 1995)。在不同磁畴状态下, 磁性矿物的贡献也存在一定的差异性(图2)。对于多畴颗粒, 磁化率主轴与颗粒的形态组构一致(包括一些顺磁性的黑云母和部分角闪石反磁组构); 而对于单畴颗粒, 情况则相反, 出现反磁组构现象(Stephenson et al., 1986; Hrouda and Faryad, 2017)。对超顺磁颗粒的研究表明, 在等量条件下超顺磁颗粒的磁化率要比稳定的单畴和多畴颗粒大得多, 该磁畴状态下的磁性颗粒与温度具有很强的相关性, 常温下表现出稳定的单畴颗粒所具有的铁磁性或亚铁磁性(Henry, 1983; Thompson and Oldfield, 1986)。

单矿物研究表明, 形状各向异性和磁晶各向异性是主要的控制因素。形状各向异性受限于强磁性矿物, 主要为磁铁矿。当固有磁化率较低时, 由形状引起的磁各向异性则很小。理论计算表明, 当固有磁化率的值为4π×10–2SI时, 形状各向异性最大为1.06, 如磁铁矿(Stacey et al., 1960; Bhathal, 1971)。对于一些晶体而言, 磁化倾向于沿着某些晶轴, 这种效应称为磁晶各向异性。磁黄铁矿的磁晶各向异性很大, 掩盖了静磁各向异性, 此时形状各向异性的影响可以忽略。对于一些常见的顺磁性矿物(如普通辉石、角闪石), 虽然磁化率较低, 但磁化率各向异性却较高。普通辉石的各向异性值在1.2~1.4之间, 平均值为1.26; 普通角闪石在1.08~1.30之间, 平均值为1.25(Uyeda et al., 1963; Bhathal, 1971)。

1.2 全岩磁化率各向异性

由于岩石内部不同矿物颗粒的形状、排列、空间分布、以及磁晶各向异性等因素的影响导致整体磁化率表现为各向异性。当全岩的磁化率值较高时, 不同磁化率主轴值往往会出现较大的差异。全岩磁化率各向异性成因主要有以下六点: ①矿物颗粒的形状各向异性。一个非等轴磁性颗粒在不同方向上有不同的退磁系数, 此时磁化率各向异性与矿物颗粒的形状有关; ②矿物晶体的磁晶各向异性。对一些晶体而言, 磁化倾向于沿着某些晶体轴。当岩石中主要磁性矿物为铁磁性的磁黄铁矿以及含铁的硅酸盐矿物时, 磁晶各向异性占据优势; 当主要磁性矿物为磁铁矿或钛磁铁矿时, 形状各向异性占据优势(Jackson, 1991; Raposo et al., 2008); ③磁畴排列的各向异性。任何一种铁磁性或亚铁磁性材料的固有磁化率是对其施加的磁场方向与磁畴方向的函数; ④磁性颗粒排列引起的磁各向异性。当岩石中包含数量众多的磁铁矿时, 此现象尤为明显。Grabovsky and Bradskaya (1958)将其定义为: “结构”磁各向异性。即由相互作用的磁性颗粒的平面分布或线状分布造成。20世纪60年代, Stacey在巴黎通过实验证实结构磁各向异性是由前二者中的某一个或者同时作用所造成的; ⑤交换各向异性(最初用来定义亚铁磁性和反铁磁性物质二者之间相互作用的性质。之后的研究表明, 在铁磁性和亚铁磁性物质之间也存在这种性质。当温度或磁场方向发生变化, 相应的磁滞回线会发生位移)(Bhathal, 1971); ⑥应力诱发的各向异性。它与磁致伸缩有关, 为磁化强度与晶轴方向的函数(Bhathal, 1971; Jackson, 1991)。

M. 磁化强度; H. 磁场强度; T. 绝对温度; Tc. 居里温度; Tn. 尼尔温度; Ta. 渐进温度; Tp. 临界温度。

(a) 磁畴分区(据Thompson and Öldfield, 1986); (b) 磁晶各向异性(据袁学诚, 1991); (c) 多晶样品内磁畴的排列情况(据Thompson and Öldfield, 1986); (d) 几类单畴磁化曲线(据Thompson and Öldfield, 1986)。

2 磁化率各向异性与剪切带

剪切带是面状高剪切应变带, 具有不同类型和形成机制, 反映了大陆岩石圈变形行为。据变形机制、变形行为以及发育的物理环境可分为: 脆性剪切带(断层和断裂带)、韧‒脆性过渡型剪切带以及韧性剪切带(Rasmay, 1980)。开展剪切带磁化率各向异性的研究, 可以在显微组构分析等的基础上进一步约束其运动学特征和变形机制(Ziegler, 1989; Kley and Voigt, 2008; Solum and Pluijm, 2009; Levi and Weinberger, 2011; Abdeen et al., 2014; Casas-Sainz et al., 2017; 王开等, 2017; 陈应涛等, 2019)。

2.1 脆性断层

脆性断层是地壳浅层次脆性变形的产物, 具有一个或多个清楚的不连续界面(曾佐勋等, 2008)。Casas-Sainz et al. (2017)对Cameros-Demanda逆冲断层的研究表明, 磁线理可以揭示断层的传播方向, 但断层不同部位存在差异性(平行或垂直传播方向)。当所采集的数据不足以产生强烈的簇状分布时, 磁线理的可靠性则降低。当铁磁性矿物含量增加时, 磁线理的可靠性则提高。此外, 他们认为变形强烈地区的磁组构不受大规模流体或深部成岩作用的影响。Braun et al. (2015)对死海西边界正断层的研究显示, 断层面处磁化率各向异度值较高。断层面处磁组构的定向性与断层活动所产生的局部应变相关。断层内应力场的局部差异性会导致磁化率椭球体发生偏转。随着黏土矿物的产生, 局部应变可能被放大(Medina-Cascales et al., 2019)。剪切应变的强度直接影响磁线理的类型(Marcén et al., 2020)。Nakamura and Nagahama (2001)研究发现, 弱碎裂化花岗岩处磁线理与破裂面的定向性近一致, 而强碎裂化花岗岩内由于绿泥石化的黑云母发生强烈挤压破碎, 导致磁线理的定向性与破裂面存在较大差异, 并且出现磁化率各向异度较低甚至各向同性的情况。Román- Berdiel et al. (2019)在对Rastraculos断裂带断层岩的研究中指出, 影响断层岩磁组构类型的主要因素是磁性矿物, 而不是露头尺度的构造变形强度。

2.1.1 增生楔部位磁组构

增生楔又称增生柱或增生杂岩, 是大洋板块沿海沟俯冲时, 被刮削下来的沉积盖层和洋壳碎片与原地沉积物堆积到海沟的向陆方向形成的楔形地质体(Stern, 2005)。增生楔内部应力分布具有差异性, 而磁组构可以有效识别应力的变化(Lin et al., 2010; Kanamatsu et al., 2012)。滑脱断层分割了强变形部位和弱/未变形部位(Moore, 1989)。滑脱层上下区域磁组构类型存在差异。滑脱层下部磁组构呈典型的沉积组构(Owens, 1993), 且磁化率各向异度值较低, 可能是由于随着流体压力的增高, 沉积物压实作用减弱甚至停止所导致(Yang et al., 2013)。滑脱层上部磁化率椭球体朝扁长形转变, 各主轴簇状分布明显(Owens, 1993), 磁线理垂直于板块俯冲方向(Yang et al., 2013)。增生楔内各向异性参数的差异性, 可能暗示其局部变形强度的差异性(Owens, 1993), 也可能反应泥质杂基中砂级碎屑含量的差异性(Ujie et al., 2000)。Kanamatsu et al. (2012)利用磁组构对Nankai Trough处增生楔内应变随时间和空间的变化展开研究发现, 钻井 C0001处, 增生楔内(C1U2)磁化率椭球体以扁长形为主, 是增生过程中平行层缩短所导致(Hrouda et al., 2009)。钻井C0002处(Kumano弧前盆地处)增生楔内磁化率椭球体趋于扁长形, 是受后期板块俯冲产生的侧向挤压力叠加改造的结果, 但挤压强度相对较弱(Ujiie et al., 2003)。磁组构指示NW-SE向挤压变形, 与现今NE-SW向最大水平主应力正交。应力方向的差异性暗示现今构造应力场未对其造成影响(Kanamatsu et al., 2012)。

2.1.2 断层岩磁组构

断层岩是断层带局部应力变化的产物(Sibson, 1977; Wise et al., 1984)。其中浅层次断层岩主要包括: 断层泥、碎裂岩以及断层角砾岩。其中断层泥是断层剪切滑动、碎裂、碾磨和黏土矿化作用的产物, 记录了断层活动的信息(Sibson, 1977)。相较围岩, 断层泥含有更多的顺磁性矿物(Solum and van der Pluijm, 2009)。Carboneras断层带内断层泥磁组构最小磁化率主轴垂直于断层面, 磁线理与断层走向平行。磁化率各向异度在断层泥处最低, 表明断层泥处组构发育程度较低(Solum and van der Pluijm, 2009)。对台湾车笼埔断裂带的磁组构研究发现, 断层泥具有较高的磁化率各向异度, 磁化率椭球体主要为扁平状, 磁组构与区域应力状态相关(Yeh et al., 2007)。对英国安格尔西岛北部沿海Porth-pisty11断层泥磁组构研究表明, 磁组构的定向性与断层运动方向一致(Hailwood et al., 1992)。然而, 来自台湾车笼埔断层科钻(TCDP-Hole B)所取得断层泥的AMS显示: 主滑动带(principal slip zone, PSZ)处磁面理和磁化率各向异度值最低, 可能受控于断层活动中高温和流体的作用; 而断层泥处磁面理和磁化率各向异度值则最高, 与热流体作用新生的针铁矿的分布有关; 地表露头处, 从岩墙到断层泥磁线理呈下降趋势, 而磁面理和磁化率各向异度值先增加后减小(Chou et al., 2014)。Nakamura and Nagahama (2001)对断层泥的研究也显示断层泥处磁化率值较高而磁化率各向异度较低, 他们认为, 断层泥处含铁氧化物的生长使硅酸盐矿物沿面理面的定向分布推迟。综上, 脆性断层岩处AMS可能受控于应力变化也可能受后期热化学反应等的影响。在确定是否存在次生矿物的基础上对其进行解释, 可能更可靠(Román- Berdiel et al., 2019; Yang et al., 2020)。

近年来通过大量科学钻探, 对断层岩(泥)磁学性质的认识取得了较大进展。如对台湾车笼埔断层的科钻研究发现, 1194 m和1243 m处发育的假玄武玻璃具有较高的磁化率值, 可能与顺磁性矿物热解为铁磁性矿物(Mishima et al., 2006)或铁磁性矿物的碾磨细化有关。研究表明, 铁磁性矿物颗粒在剪切作用下细化为超顺磁状态可以引起磁化率的增加(张蕾等, 2018)。高分辨率磁化率测量揭示断层泥处磁化率值出现波动(Hirono et al., 2006)。由于摩擦加热(>400 ℃), 顺磁性含铁矿物热解为磁铁矿, 从而导致黑色断层泥处磁化率值升高(Mishima et al., 2009)。高速摩擦实验表明, 短时间内的摩擦加热也会导致磁化率的变化(Tanikawa et al., 2007)。汶川科钻同样发现断层泥和假玄武玻璃具有高磁化率的现象(Pei et al., 2010, 2014; 张蕾等, 2017, 2018)。实验揭示, 当摩擦加热温度达1300 ℃, 含铁矿物就会发生还原反应, 从而生成大量单质铁球粒, 引起假玄武玻璃的高磁异常现象(Zhang et al., 2018; 张蕾等, 2019)。与之相反, 在针对安县‒灌县断裂带的科钻(何祥丽等, 2018)以及九龙探槽(Liu et al., 2014)和北川‒映秀断裂带的大沟探槽(Yang et al., 2012a)等的研究中出现断层泥处磁化率低于围岩的现象, 原因可能是: ①断层泥所经历的温度未超过300~400 ℃(Gillett, 2003; Yang et al., 2012b; Liu et al., 2014); ②当断层处于蠕滑变形时, 在流体的作用下, 水岩反应导致铁磁性矿物转变为顺磁性矿物(Kuo et al., 2012; Liu et al., 2014; 何祥丽等, 2018)(图3)。

2.2 韧性剪切带

Rathore and Becke (1980)对阿尔卑斯造山带中Periadriatic Line (P.L.)剪切带的AMS研究发现, 磁化率椭球体以扁圆形(Oblate)为主。最小磁化率主轴方向与云母C轴具有很好的一致性。利用AMS可以对剪切带建立分段运动模型。Hrouda (1982)指出磁面理、磁线理的分布与构造片理和线理具有一致性。Goldstein and Brown (1988)对阿巴拉契亚山脉南部Brevard剪切带的AMS研究发现, 磁化率椭球体以压扁形为主。糜棱岩化伴随着磁化率的降低。磁化率各向异性可以定性的描述糜棱岩的应变历史。随后, Mims et al. (1990)对北卡罗莱纳州Nutbush Creek韧性剪切带研究后指出, AMS可以用来估算韧性剪切带应变场的大小, 但不能估算剪切带内的应变历史或应力大小。Kankeu et al. (2009)在喀麦隆东部Bétaré-Oya剪切带, 利用磁化率椭球体形态、磁线理和磁面理产状识确定了三期构造变形, 并绘制出了区域上的磁线理分布迹线。Ferré et al. (2014)对韧性剪切带内AMS特征进行了系统综述, 并指出剪切带内AMS与应变的关系不仅取决于磁性矿物的类型也取决于变形机制。Vikas et al. (2016)利用AMS和岩组学对印度南部Achankovil剪切带开展运动学和显微构造演化研究。研究表明, 矿物晶格优选方向(LPO)指示剪切带内塑性变形主要通过晶内滑移完成, 剪切带的再活化造成先存组构的改变。AMS与LPO具有一致性, 反映了区域上应力/运动学与岩石内组构演化的关系。Marcén et al. (2018)通过对比比利牛斯Gavarnie剪切带内显微构造与磁化率椭球体, 确定了剪切带内部应力分布和运移方向。

图3 图示岩石碎裂化过程中流体的作用(据Yang et al., 2016)

相较国外, 国内学者利用AMS对剪切带也开展了大量的研究工作。施建宁等(1990)对闽浙碰撞造山带内部韧性剪切带展开研究发现: 磁化率椭球体以压扁形为主; 磁面理较磁线理发育; 最小磁化率主轴与岩石片理法线的产状基本一致; 剪切带中随着深度的增加, 由简单剪切作用向纯剪作用转变, 面状构造的发育不断增强。Zhou et al. (2002)对哀牢山‒红河韧性剪切带展开AMS研究, 具体分析了剪切带内外的构造变形性质。杨朝斌等(2006)认为磁化率各向异度()直接反映韧性变形的强度。他们利用磁面理对剪切带进行运动学研究, 指出雅鲁藏布江缝合带内部的两条韧性剪切带以逆冲为主, 兼具右旋扭动的运动学特征。AMS主要反映较强的压缩作用, 而非简单剪切作用。陈柏林等(2007)指出后期热事件会导致剪切带内AMS的均一化, 出现值降低甚至消失的现象。此外, 岩性和应力的变化也可能引起AMS的变化。用值来判别构造变形强度可能更适用于相似岩性的样品(梁文天等, 2008)。李阳等(2017)利用AMS、显微组构和运动学涡度对秦岭沙沟街韧性剪切带进行研究, 发现剪切带内部整体各向异度值较大(均>1.19, 最高可达2.41), 表明构造变形较为强烈。磁化率椭球体以压扁形为主。磁线/面理与矿物线/面理产状较为一致。结合边界断层和C面理产状, 认为剪切带具有左行走滑挤压的运动学特征。同时, 运动学涡度研究表明, 剪切带中纯剪切作用所占的比重大于简单剪切作用。上述研究表明, ①磁化率椭球体的形状可以反映岩石变形的性质(拉伸、压缩或剪切); ②磁化率各向异度能够反映韧性变形的强度。但岩性的差异、后期热事件以及应力的变化会引起值的变化; ③磁面理、磁线理和最小磁化率主轴的产状可以用来分析剪切带的运动方向。

2.2.1 韧性剪切带内面理与线理的研究

Berthé et al. (1979)在研究法国South Armorican韧性剪切带内面理构造时提出S-C组构(S-C fabrics)。他把一组平行剪切带边界的面理称为C面理, 把另一组平行变形矿物优选方向的面理称为S面理, 二者组合称为S-C组构(图4)。显微构造上, C面理为间隔排列的含有细小重结晶颗粒强剪切应变带。其内石英颗粒动态重结晶, 形成大致平行于S面理的倾斜组构。云母发生强烈变形和细粒化。由于较强的剪切应变, C面理表面形成类似于擦痕镜面的形态且发育“槽中脊”型擦痕(Lc)(Lin and Williams, 1992)。S面理由C面理之间先存矿物的形状优选定向所决定(Simpson and Schmid, 1983)。一般为层状硅酸盐、石英或长石等细小颗粒(Dell’Angelo and Tullis, 1989)。C面理相较S面理更离散但也更连续(Lin et al., 2007)。Berthé et al. (1979)认为S面理与C面理同时形成。Lister and Snoke (1984)认为S面理与C面理分别形成于两期构造事件。Lin and Williams (1992)认为以下两点对解释S-C组构的形成机制极为重要: ①形成S-C组构的糜棱岩表现为韧性变形, 围压对变形机制起到制约作用; ②晶粒级别的变形是不均匀的。

韧性剪切带内还发育C′面理(伸展褶劈理), 是以塑性剪切(Platt and Vissers, 1980)或脆性不连续为特征的面状构造。该面理与剪切带边界斜交, 与C面理夹角一般小于30°。与S-C组构共同组成S-C-C′复合组构(Lister and Snoke, 1984)。C′面理常以剪切面或狭窄剪切带的方式切割S面理(刘江, 2019)。内部可见强烈重结晶的细小石英颗粒(Law et al., 1984)。C′面理在低剪切应变下发育良好。随着剪切应变的增加, C′面理逐渐旋转至剪切边界或因重结晶加剧导致C′面理逐渐消失(Dell’Angelo and Tullis, 1989)。剪切带有时发育共轭的伸展褶劈理, 二者发育情况存在差异(Law et al., 1984)。Biot (1965)认为各向异性材料在变形过程中存在扰动, 而变形诱发的显微构造变化可能是形成C′的关键(Platt and Vissers, 1980)。White et al. (1980)认为伸展褶劈理是先存较发育面理后期破裂的产物, 是韧性剪切带晚期次生劈理。C′可能与剪切带非均匀变形有关(Behrmann, 1987)。Behrmann (1987)指出对于强各向异性的岩石, 多组共轭伸展褶劈理的形成可能受控于机械作用而非运动学因素。

除面理外, 韧性剪切带内还主要发育两类线理: “槽中脊”型擦痕(Lc)(Means, 1987)和拉伸线理(Ls)(许志琴, 1989)(图4)。Lc常发育在C面理上, 平行于剪切运动方向。若未受到后期改造, Lc可给出可靠的运动学信息(Lin et al., 2007)。随着变形的增强, Lc的脊(ridge)和槽(groove)变长和变浅(Lin and Williams, 1992a)。Ls位于S面理上(除真正的L构造岩: 只发育拉伸线理, 面理发育较弱或不发育)(林寿发等, 2007)。研究发现剪切带内Ls方位会因以下原因发生变化: ①单剪局部化, 纯剪趋向于广泛分布(Gordon, 1995; Lin and Jiang, 1998); ②应变量的变化(Lin and Williams, 1992); ③角的变化(简单剪切方向与纯剪切主分量方向的夹角(Jiang and Williams, 1998)。对于高剪切应变带, 拉伸线理方向与剪切方向之间不存在简单的关系。因此, 很难用拉伸线理的方向来推断剪切方向, 除非剪切带的运动学格架为单斜对称, 即拉伸线理在剪切带边界上的正交投影平行于剪切方向(Lin and Williams, 1992b; Lin et al., 2007; 林寿发等, 2007)。

(a) 韧性剪切带(右行剪切下)C′与S面理与C面理几何关系示意图(据Blenkinsop and Treloar, 1995); (b) 典型S-C糜棱岩构造简图。C面理平行剪切带边界, 沿C面的剥露面擦痕面, C面上槽中脊型擦痕平行于剪切方向, 拉伸线理发育在S面理上(据李阳等, 1992a修改); (c) 磁面理分布迹线(据Lin et al., 2009修改); (d) 共轭伸展褶劈理与S面理和线理几何关系示意图(据刘江, 2019修改)。

2.2.2 S-C(C′)组构与AMS

Aranguren et al. (1996)对S-C糜棱岩中AMS的研究表明, AMS同时受控于S面理和C面理, 主要反映了两个形状各向异性的面状组构的叠加效应。磁面理处于S面理和C面理的中间位置。随着剪切应变量的增大, 磁面理逐渐平行于C面理。磁线理垂直于S面理和C面理的交线, 近平行于拉伸线理, 与剪切运动方向一致。Tomezzoli et al. (2003)将S-C组构与AMS对比后发现, 磁面理偏离S面理约20°, 可能是由于S-C组构产生的叠加组构所导致。Ono et al. (2010)利用背散射电子图像分析技术对磁组构和S-C-C′关系展开研究, 发现磁组构主要反映了顺磁性单斜矿物特别是黑云母颗粒的形状优先定向。磁面理处于S面和C面的中间位置。磁线理平行于S面理和C面理的交线。Casas-Sainz et al. (2018)将Daroca剪切带内S-C组构与AMS进行对比后发现, 磁面理存在平行S面理或C面理的现象, 表明剪切面内层状硅酸盐矿物的重新定向或铁磁性矿物的富集影响了磁面理的定向性。受变形强度和矿物学控制, 磁线理平行于S面理和C面理的交线。Marcén et al. (2018)对比利牛斯Gavarnie剪切带内S-C-C′组构与AMS的研究发现, 磁面理存在三种定向性: ①平行于S面理; ②平行于C面理; ③平行于C′面理。磁线理具有两种定向性: ①与剪切运动方向一致; ②与剪切运动方向垂直。与剪切运动方向一致的磁线理受控于阿尔卑斯韧性变形构造。与剪切运动方向垂直的磁线理形成于早期华力西运动, 且未被后期变形改造。对磁线理来说, 当主要受顺磁性矿物控制时, 在变形早期, 磁线理表现为平行于S面理和C面理的交线。在更高级的变形阶段, 则表现为垂直于S面理和C面理的交线, 平行于剪切运动方向。当磁线理主要受控于铁磁性矿物时, 磁线理普遍表现为平行于剪切运动方向(Casas-Sainz et al., 2018)。

3 变形机制与磁组构

前人对剪切带内变形机制与磁组构的关系开展了大量研究。Borradaile and Alford (1987)通过实验发现, 磁化率各向异性的变化与应变具有很强的相关性。构造变形强度是决定磁线理发展的重要因素(Parés and van der Pluijm, 2002)。但是, Bikramaditya et al. (2017)对三期叠加变形片麻岩的研究表明, 磁组构结果与区域上优势面理(第二期变形事件)不一致, 主要反应了最后一期弱变形事件。剪切带内位错滑移、位错蠕变和扩散蠕变等变形机制引起磁性矿物的重新定向和晶体内部的变形, 导致磁化率各向异性的改变(Jackson et al., 1993; Ferré and Améglio, 2000; One et al., 2010; Till and Moskowitz, 2014; Ferré et al., 2014)。相较纯剪, 简单剪切更能引起磁化率各向异性的变化(Borradaile and Alford, 1988)。剪切过程中, 铁磁性矿物粒度的减小可以诱发磁化率(Billi, 2005; Sammos and Ben-Zion, 2008)和各向异度的变化(Jackson et al., 1993)。Yang et al. (2020)认为磁性矿物粒度的减小可能是造成日本Nojima断层处强碎裂花岗岩出现弱磁化率各向异性, 甚至各向同性的原因。

Parés and van der Pluijm (2002)认为磁性矿物的变化是影响磁化率各向异性的另一因素。高磁化率矿物含量的变化会引起磁化率各向异性较大的变化(Borradaile, 1988)。研究发现, 剪切带内由摩擦导致的热化学变化可以诱发含铁矿物的分解和转化, 形成磁铁矿等铁磁性矿物(Yang et al., 2019)。菱铁矿在400~580 ℃时可分解为磁铁矿(Koziol, 2004; Han et al., 2007); 纤铁矿在200 ℃左右会转变为磁赤铁矿, 进一步在300~350 ℃转变为赤铁矿(Gehring and Hofmeister, 1994)。

剪切带内存在大量流体(刘贵, 2020)。一方面, 构造运动可以对流体的运移和循环造成影响(Sibson and Scott, 1998; Robl et al., 2004)。对哀牢山韧性剪切带型金矿的研究表明, 强烈的构造变动形成的混合流体与已糜棱岩化的围岩发生水‒岩反应, 导致成矿流体物理化学条件的改变和矿物的沉淀(孙晓明等, 2007)。对夹皮沟金矿的研究同样表明由构造诱发的高压流体导致大气降水、变质水和岩浆流体混合, 含矿流体的失衡导致矿石的沉淀(Deng et al., 2009)。流体使含铁矿物发生溶解, 从中析出Fe2+(Humbert et al., 2012; Yang et al., 2016)。富含Fe2+的流体一方面被认为是新生铁磁性矿物的物质来源(Pechersky and Genshaft, 2001)。另一方面被认为是形成含铁黏土矿物(如绿泥石)的来源。并且随着流体与断层活动的继续, 更多的顺磁性含铁蚀变矿物出现在新生的断层岩中(Yang et al., 2016), 从而导致剪切带内磁化率及各向异性的变化(Yang et al., 2020)。Yang et al. (2020)指出, 新生矿物的磁化率各向异性可能并非与应变相联系。Saint-Bezar et al. (2002)研究显示, 平行于构造缩短方向磁线理的出现, 可能和富铁矿脉与层面的相交有关。磁组构的几何形态与定向性也可能受控于热液蚀变作用(Just et al., 2004)。地震发生两年后, 断层的性质就会因流体的作用发生显著变化(Brodsky et al., 2009)。此外, 这些新生成的磁性矿物可能形成于不同的断层运动阶段(事件), 并且具有不同的形成方式, 所以磁组构所记录的信息也可能不同(Yang et al., 2020)。对死海断层白垩样品的分离研究表明, 其中顺磁性黏土矿物保留初始沉积组构, 而抗磁性方解石则体现为构造组构(Issachar et al., 2018)。另一方面, 流体反作用于构造活动, 影响其发生、发展。段宝庆(2015)对龙门山断裂花岗岩质破裂带的研究表明, 流体不仅会因封闭产生的高压使断层弱化, 也会与围岩反应生成摩擦系数低的黏土等层状硅酸盐矿物。矿物成分和粒度的变化导致全岩磁化率各向异性的变化, 并随着构造运动的持续, 磁化率各向异性可能会持续变化(Ferré et al., 2014)。

4 结 论

全岩磁化率各向异性是磁性矿物类型、含量、大小以及分布等的综合反映, 受到其形成时(后)各种地质因素控制。但利用AMS可以在剪切带获得常规地质方法难以获得的一些构造信息。由于剪切带内应变具有复杂性, 并且渗透大量流体, 导致AMS具有复杂性。因此对剪切带开展AMS研究工作, 在确定磁性矿物的基础上, 识别AMS的形成阶段以及多期次AMS的叠加就显得十分关键。

致谢:由衷地感谢两位匿名审稿人专业而又中肯的建议, 使作者受益良多！

陈柏林, 张招宠, 闫升好, 何立新, 周刚, 李丽, 蒋荣宝, 王祥, 张小林, 杨文平. 2007. 阿尔泰山南缘东段变形岩石磁组构分析. 地学前缘, 14(3): 138–148.

陈应涛, 张国伟, 鲁如魁, 郭安林, 谢晋强, 朱伟. 2019. 龙门山南段盐井‒五龙断裂磁组构特征及其对几何学、运动学的制约. 大地构造与成矿学, 42(2): 199– 212.

段宝庆. 2015. 地震断层带流体作用的岩石物理和地球化学响应——以龙门山断裂花岗质破裂带为例. 北京: 中国地震局地质研究所博士学位论文: 1–9.

何祥丽, 李海兵, 张蕾, 王焕, 葛成隆, 曹勇, 白明坤, 李成龙, 叶小舟, 韩帅. 2018. 龙门山灌县‒安县断裂带断层泥低磁化率的矿物、化学响应和蠕滑作用环境. 地球物理学报, 61(5): 1782–1796.

李阳, 梁文天, 靳春胜, 董云鹏, 袁洪林, 张国伟. 2017. 秦岭沙沟街韧性剪切带的岩石磁学、磁组构和运动学涡度分析. 岩石学报, 33(6): 1919–1933.

梁文天, 张国伟, 鲁如魁, 裴先治, 袁四化, 姚安平. 2008. 西秦岭北缘武山‒鸳鸯镇构造带磁组构特征. 地学前缘, 15(4): 298–306.

林寿发, 姜大志, Williams P F. 2007. 剪切带运动学: 重点评述三斜模式和区分韧性滑动擦痕与拉伸线理的重要性. 地质通报, 26(9): 1139–1153.

刘贵. 2020. 韧性剪切带内得流体与岩石相互作用研究进展. 地质力学学报, 26(2): 1–15.

刘江. 2019. 伸展褶劈理地质现象、成因机制及其地质意义. 地质科技情报, 38(2): 1–13.

潘永信, 朱日祥. 1998. 磁组构研究现状. 地球物理学进展, 13(1): 52–56.

施建宁, 许同春, 董火根, 卢华复. 1990. 浙闽碰撞造山带中剪切带的磁性组构与碰撞动力学的研究. 南京大学学报, 26(1): 108–123.

孙靖鹏, 韩非, 邓成龙, 潘永信. 2016. 山东胶莱盆地鲁科一井上白垩统火山岩的岩石磁学和磁组构研究. 地球物理学进展, 31(6): 53–63.

孙晓明, 熊德信, 石贵勇, 王生伟, 翟伟. 2007. 云南哀牢山金矿带大坪韧性剪切带型金矿40Ar-39Ar定年. 地质学报, 81(1): 88–92.

王开, 贾东, 罗良, 董树文. 2017. 磁组构与构造变形. 地球物理学报, 60(3): 1007–1026.

吴汉宁. 1988. 岩石的磁性组构及其在岩石变形分析中的应用. 岩石学报, 4(1): 96–100.

许顺山, 陈柏林. 1998. 应用岩石磁性组构研究动力变形作用. 地球学报, 19(1): 19–24.

许志琴. 1989. 拉伸线理及其构造意义. 四川地质学报, 9(2): 9–15.

杨朝斌, 朱大岗, 孟宪刚, 邵兆刚, 韩建恩, 余佳, 孟庆伟, 吕荣平. 2006. 西藏阿里雅鲁藏布江缝合带韧性剪切带的磁组构特征. 地学前缘, 13(4): 168–173.

袁学诚. 1991. 古地磁学原理及其应用. 北京: 地质出版社: 26–28.

确定指标体系的常用方法包括层次分析法、专家咨询法、主成分分析法、熵值法、非模糊数判定矩阵法、优序图法等[12]．选择熵值法作为本文指标体系权重的计算方法,其基本思路是通过计算指标的信息熵,根据指标的变化程度来决定指标权重．信息量越小,不确定性就越大,熵也就越大;信息量越大,不确定性就越小,熵也就越小[13]．

曾佐勋, 樊光明, 王国灿, 李德威, 秦松贤, 张旺生, 索书田. 2008. 构造地质学. 武汉: 中国地质大学出版社: 210–225.

张蕾, 李海兵, 孙知明, 曹勇. 2019. 断裂岩岩石磁学研究进展. 地球学报, 40(1): 157–172.

张蕾, 李海兵, 孙知明, 周佑民, 曹勇, 王焕, 叶小舟, 何祥丽. 2018. 龙门山断裂带大地震孕震环境的岩石磁学证据. 地球物理学报, 61(5): 1715–1727.

张蕾, 孙知明, 李海兵, 赵来时, 曹勇, 叶小舟, 王雷振, 王焕, 何祥丽, 韩帅, 白明坤, 葛成隆, 赵越. 2017. 龙门山构造带WFSD-2钻孔岩心磁化率特征及其对大地震活动的响应. 地球物理学报, 60(1): 225–239.

朱岗崑. 2005. 古地磁学——基础、原理、方法、成果与应用. 北京: 科学出版社: 1–9.

Abdeen M M, Greiling R O, Sadek M F and Hamad S S. 2014. Magnetic fabrics and Pan-African structural evolution in the Najd Fault corridor in the Eastern Desert of Egypt., 99(1): 93–108.

Anchuela Ó P, Juan A P and Imaz A G. 2010. Tectonic imprint in magnetic fabrics in foreland basins: A case study from the Ebro Basin, N Spain., 492(1–4): 150–163.

Aranguren A, Cuevas J and Tubía J M. 1996. Composite magnetic fabrics from S-C mylonites., 18(7): 863–869.

Berthé D, Choukroune P and Jegouzo P. 1979. Orthogneiss, mylonite and non-coaxial deformation in granites: The example from South Armorican Shear Zone., 1(1): 31–42.

Bhathal R S. 1971. Magnetic anisotropy in rocks., 7(4): 227–253.

Biedermann A R. 2020. Current challenges and future development in magnetic fabric research., 795: 1–13.

Bikramaditya S R, Krishnakanta Singh A, Sen K and Sangode S J. 2017. Detection of a weak late-stage deformation event in granitic gneiss through anisotropy of magnetic susceptibility: Implications for tectonic evolution of the Bomdila Gneiss in the Arunachal Lesser Himalaya, Northeast India., 154(3): 476–490.

Billi A. 2005. Grain size distribution and thickness of breccia and gouge zones from thin (<1 m) strike-slip fault cores in limestone., 27(10): 1823–1837.

Biot A. 1965. Mechanics of Incremental Deformation. New York: John Wiley and Sons.

Blenkinsop T G and Treloar P J. 1995. Geometry, classificationand kinematics of S-C and S-C′ fabrics in the Mushandike area, Zimbabwe., 17(3): 397–408.

Borradaile G J. 1987. Anisotropy of magnetic-susceptibility- rock composition versus strain., 138: 327–329.

Borradaile G J. 1988. Magnetic susceptibility, petrofabrics and strain., 156(1): 1–20.

Borradaile G J. 1991. Correlation of strain with anisotropy of magnetic susceptibility (AMS)., 135(1): 15–29.

Borradaile G J and Alford C. 1987. Relationship between magnetic susceptibility and strain in laboratory experiments., 133: 121–135.

Borradaile G J and Alford C. 1988. Experimental shear zones and magnetic fabrics., 10(8): 895–904.

Borradaile G J and Henry B. 1997. Tectonic applications of magnetic susceptibility and its anisotropy., 42(1): 49–93.

Borradaile G J and Jackson M. 2004. Anisotropy of magnetic susceptibility (AMS): Magnetic petrofabrics of deformed rocks.,,, 238(1): 299–360.

Borradaile G J and Jackson M. 2010. Structural geology, petrofabrics and magnetic fabrics (AMS, AARM, AIRM)., 32(10): 1519–1551.

Borradaile G J and Tarling D H. 1981. The influence of deformation mechanisms on magnetic fabrics in weakly deformed rocks., 77(1): 151–168.

Brodsky E M, Kuo M F, Mori J and Saffer D M. 2009. Rapid response fault drilling past, present, and future., 8(8): 66–74.

Casas-Sainz A M, Gil-Imaz A, Simón J L, Izquierdo-Llavall E, Aldega L, Román-Berdiel T, Osácar M C, Pueyo- Anchuela Ó, Ansón M, García-Lasanta C, Corrado S, Invernizzi C and Caricchi C. 2018. Strain indicators and magnetic fabric in intraplate fault zones: Case study of Daroca thrust, Iberian Chain, Spain., 730(4): 29–47.

Casas-Sainz A M, Román-Berdiel T, Oliva-Urcia B, García- Lasanta C, Villalaín J J, Aldega L, Corrado S, Caricchi C, Invernizzi C and Osácar M C. 2017. Multidisciplinary approach to constrain kinematics of fault zones at shallow depths: A case study from the Cameros-Demanda thrust (North Spain)., 106(3): 1023–1055.

Chen X Y, Liu J L, Weng S T, Kong Y L, Wu W B, Zhang L S and Li H Y. 2016. Structural geometry and kinematics of the Ailao Shan shear zone: Insights from integrated structural, microstructural, and fabric studies of the Yao Shan complex, Yunnan, Southwest China., 58(7): 849–873.

Chou Y M, Song S R, Aubourg C, Song Y F, Boullier A M, Lee T Q, Evans M, Yeh E C and Chen Y M. 2013. Pyrite alteration and neoformed magnetic minerals in the fault zone of the Chi-Chi earthquake (Mw7.6, 1999): Evidence for frictional heating and co-seismic fluids.,,, 13(8): 1–17.

Chou Y M, Song S R, Tsao T M, Lin C S, Wang M K, Lee J J and Chen F J. 2014. Identification and tectonic implications of nano-particle quartz (~50 nm) by synchrotron X-ray diffraction in the Chelungpu fault gouge Taiwan., 619–620: 36–43.

Cifelli F, Mattei M, Chadima M, Hirt A M and Hansen A. 2005. The origin of tectonic ineation in extensional basins: Combined neutron texture and magnetic analyses on “undeformed” clays., 235: 62–78.

Dell’Angelo L N and Olgaard D L. 1989. Fabric development in experimentally sheared quartzites., 169: 1–21.

Deng J, Yang L, Gao B F, Sun Z S, Guo C Y, Wang Q F and Wang J P. 2009. Fluid evolution and metallogenic dynamics during tectonic regime transition: Example from the Jiapigou gold belt in Northeast China., 59(2): 140–152.

Dimanov A, Dresen G and Wirth R. 1998. High-temperature creep of partially molten plagioclase aggregates.:, 103(B5): 9651– 9664.

Dudzisz K, Szaniawski R, Michalski K and Chadima M. 2018. Rock magnetism and magnetic fabric of the Triassic rocks from the West Spitsbergen Fold-and- Thrust Belt and its foreland., 728(20): 104–118.

Enomoto Y, Akai M, Hashimoto H, Mori S and Asabe Y. 1993. Exoelectron emission: Possible relation to seismic geo-electromagnetic activities as a microscopic aspect in geotribology., 168(1–2): 135–142.

Faulkner D R, Lewis A C and Rutter E. 2003. On the internal structure and mechanics of large strike-slip fault zones: Field observations of the Carboneras fault in southeastern Spain., 367(3): 235–251.

Ferré E C and Améglio L. 2000. Preserved magnetic fabrics vs. annealed microstructures in the syntectonic recrystallised George granite, South Africa., 22: 1199–1219.

Ferré E C, Gébelin A, Till J L, Sassier C and Burmeister K C. 2014. Deformation and magnetic fabrics in ductile shear zones: A review., 629: 179–188.

Flinn D. 1962. On folding during three-dimensional progressivedeformation., 118: 385–433.

Flinn D. 1965. On the symmetry principle and the deformation ellipsoid., 102(1): 36–45.

Fossen H, Carolona G and Cavalcante G. 2017. Shear zones —A review., 171: 434–455.

Fukuchi T, Mizoguchi K and Shimamoto T. 2005. Ferrimagnetic resonance signal produced by frictional heating: A new indicator of paleoseismicity., 110, B12404.

Fuller M. 1961. Magnetic Anisotropy of Rocks. UK: Cambridge University: 1–20.

Fuller M. 1963. Magnetic anisotropy and paleomagnetism., 68: 293–309.

Ge J Y, Guo Z T, Zhao D A, Zhang Y, Wang T, Yi L and Deng C L. 2014. Spatial variations in paleowind directionduring the last glacial period in north China reconstructed from variations in the anisotropy of magnetic suscepti­bility of loess deposits., 629(26): 353–361.

Gehring A U and Hofmeister A M. 1994. The transformation of lepidocrocite during heating: A magnetic and spec­troscopic study., 42: 409–415.

Goldstein A G and Brown L L. 1988. Magnetic susceptibility anisotropy of mylonites from the Brevard Zone, North Carolina, USA., 51: 290–300.

Gordon R G. 1995. Plate motion, crustal and lithospheric mobility, and paleomagnetism: Prospective viewpoint., 100: 24367–24392.

Graham W J. 1954. Magnetic susceptibility anisotropy, an unexploited petrofabric element., 65: 1257–1258.

Hailwood E A, Maddock R H, Fung T and Rutter E H. 1992. Paleomagnetic analysis of fault gouge and dating fault movement, Anglesey, North Wales., 149(2): 273–284.

Han R, Shimamoto T, Ando J I and Ree J H. 2007. Seismic slip record in carbonate-bearing fault zones: An insight from high-velocity friction experiments on siderite gouge., 35(12): 1131–1134.

Henry B. 1983. Interprétation quantitative de l’anisotropie de susceptibilité magnétique., 91(1): 165–177.

Hirono T, Lin W, Yeh E C, Soh W, Hashimoto Y, Sone H, Matsubayshi O, Aoike K, Ito H, Kinoshita M, Masafumi M, Song S R, Ma K F, Hung J H, Wang C Y and Tsai Y B. 2006. High magnetic susceptibility of fault gouge within Taiwan Chelungpu fault: Nondestructive continuous measurements of physical and chemical properties in fault rocks recovered from Hole B, TCDP., 33(15): L15303.

Hirt A M and Gehring A U. 1991. Thermal alteration of the magnetic minerality in ferruginous rocks., 96(B6): 9947–9953.

Hrouda F. 1982. Magnetic anisotropy of rocks and its application in geology and geophysics., 5(1): 37–82.

Hrouda F and Faryad S W. 2017. Magnetic fabric overprints in multi-deformed polymetamorphic rocks of the GemericUnit (Western Carpathians) and its tectonic implications., 717(16): 83–89.

Hrouda F and Janák F. 1976. The changes in shape of the magnetic susceptibility ellipsoid during progressive metorphism and deformation., 34(1–2): 135–148.

Hrouda F, Krejčíc O, Potfajd M and Stráník Z. 2009. Magnetic fabric and weak deformation in sandstones of accretionary prisms of the Flysch and Klippen Belts of the Western Carpathians: Mostly off scraping indicated., 479: 254–270.

Humbert F, Robion P, Louis L, Bartier D, Ledésert B and Song S R. 2012. Magnetic inference ofopen microcracks in sandstone samples from the Taiwan Chelungpu Fault Drilling Project (TCDP)., 45: 179–189.

Hunt C P, Moskowitz B M and Banerjee S. 1995. Magnetic Properties of Rocks and Minerals. Rock Physics and Phase Relations: A Handbook of Physical Constants, 3: 189–204.

Ising G. 1942. On the magnetic properties of varved clay.,, 29A: 1–37.

Jackson M. 1991. Anisotropy of magnetic remanence: A brief review of mineralogical sources, physical origins, and geological applications, and comparison with susceptibility anisotropy., 136(1): 1–28.

Jackson M, Borradaile G, Hudleston P and Banerjee S. 1993. Experimental deformation of synthetic magnetite- bearing calcite sandstones: Effects on remanence, bulk magnetic properties, and magnetic anisotropy., 98(B1): 383–401.

Jelínek V. 1978. Statistical processing of anisotropy of magnetic susceptibility measures on groups of specimens., 22: 50–62.

Jiang D Z and Williams P F. 1998. High-strain zones: A unified model., 20(8): 1105–1120.

Johns M and Jackson M. 1991. Compositional control of anisotropy of remanent and induced magnetization in synthetic samples., 18(7): 1293–1296.

Just J, Kontny A, Wall D H, Hirt A M and Martín F. 2004. Development of magnetic fabrics during hydrothermal alteration in the Soultz-sous-Forets granite from the EPS1 borehole, Upper Rhine Graben.,,, 238: 509–526.

Kanamatsu T, Parés J M and Kitamura Y. 2012. Pliocene shortening direction in Nankai Trough off Kumano, southwest Japan, Sites IODP C0001 and C0002, Expedition 315: Anisotropy of magnetic susceptibility analysis for paleostress.,,, 13(1): 1–13.

Kankeu B, Greiling R and Nzenti J P. 2009. Pan-African strike-slip tectonics in eastern Cameroon-Magnetic fabrics (AMS) and structure in the Lom basin and its gneissic basement., 174: 258–272.

Kley J and Voigt T. 2008. Late Cretaceous intraplate thrusting in central Europe: Effect of Africa-Iberia Europe convergence, Not Alpine collision., 36(11): 839–842.

Kligfield R, Lowrie W and Priffner O A. 1982. Magnetic properties of deformed oolitic limestones from the Swiss Alps: The correlation of magnetic anisotropy and strain., 75: 127–157.

Koziol A M. 2004. Experimental determination of siderite stability and application to Martian Meteorite ALH84001., 89(2–3): 294–300.

Kuo L W, Song S R, Yeh E C, Chen H F and Si J L. 2012. Clay mineralogy and geochemistry investigations in the host rocks of the Chelungpu fault, Taiwan: Implication for faulting mechanism., 59: 208–218.

Law R D, Knipe R J and Dayan H. 1984. Strain path partitioning within thrust sheets: Microstructural and petrofabric evidence from the Moine Thrust zone at Loch Eriboll, northwest Scotland., 6(5): 477–497.

Levi T and Weinberger R. 2011. Magnetic fabrics of diamagnetic rocks and the strain field associated with the Dead Sea Fault, northern Israel., 33(4): 566–578.

Lin S F, Jiang D Z and Williams P F. 1998. Transpression (or transtension) zones of triclinic symmetry: Natural example and theoretical modelling.,,, 135: 41–57.

Lin S F, Jiang D Z and Williams P F. 2007. Importance of differentiating ductile slickenside striations from stretching lineations and variation of shear direction across a high-strain zone., 29: 850–862.

Lin S F and Williams P F. 1992a. The origin of ridge-in- groove slickenside striae and associated steps in an S-C mylonite., 14(3): 315–321.

Lin S F and Williams P F. 1992b. The geometrical relationship between the stretching lineation and the movement direction of shear zones., 14: 491–497.

Lin W, Doan M L, Moore C, McNeill L, Byrne T B, Ito T, Saffer D, Conin M, Kinoshita M, Sanada Y, Moe K T, Araki E, Tobin H, Boutt D, Kano Y, Buret C, Schleicher A M, Efimenko N, Kawabata K, Buchs D M, Jiang S, Kaneo K, Horiguchi K, Wiersberg T, Kopf A, Kitada K, Eguchi N, Toczko S, Takahashi K and Kido Y. 2010. Present-day principal horizontal stress orientations in the Kumano forearc basin of the southwest Japan subduction zone determined from IODP NanTroSEIZE drilling Site C0009., 37(L13303): 1–6.

Lister G S and Snoke A W. 1984. S-C mylonites., 6: 617–638.

Liu D, Li H, Lee T Q, Chou Y M, Song S R, Sun Z M, Chevalier M L and Si J L. 2014. Primary rock magnetism for the Wenchuan earthquake fault zone at Jiulong outcrop, Sichuan Province, China., 619–620: 58–69.

Liu W M and Sun J M. 2012. High-resolution anisotropy of magnetic susceptibility record in the central Chinese Loess Plateau and its paleoenvironment implications.:, 55(3): 488–494.

Mamtani M A and Sengupta P. 2010. Significance of AMS analysis in evaluating superposed folds in quartzites., 147(6): 910–918.

Marcén M, Casas-Sainz A M, Román-Berdiel T, Oliva-Urcia, Soto R and Aldega. 2018. Kinematics and strain distribution in an orogeny-scale shear zone: Insights from structural analyses and magnetic fabrics in the Gavarnie thrust, Pyrenees., 117: 105–123.

Marcén M, Casas-Sainz A M, Román-Berdiel T, Oliva-Urcia B, Soto R, García-Lasanta C, Calvin P, Gil-Imaz A and Aldega L. 2020. Magnetic fabric in brittle faults and ductile shear-zones: Examples from cataclasites from the Iberian Peninsula. EGU General Assembly: 1–2.

Mean W D. 1987. A newly recognized type of slickenside striation., 9: 585–590.

Medina-Cascales I, Koch L, Cardozo N, Martin-Rojas I, Alfaro P and García-Tortosa F J. 2019. 3D geometry and architecture of a normal fault zone in poorly lithified sediments: A trench study on a strand of the Baza Fault, central Betic Cordillera, South Spain., 121: 25–45.

Mims C V H, Powell C A and Ellwood B. 1990. Magnetic susceptibility of rocks in the Nutbush Creek ductile shear zone, North Carolina., 178: 207–223.

Mishima T, Hirono T, Nakamura N, Tanikawa W, Soh W and Song S S. 2009. Changes to magnetic minerals caused by frictional heating during the 1999 Taiwan Chi-Chi earthquake., 61: 797–801.

Mishima T, Hirono T, Soh W and Song S R. 2006. Thermal history estimation of the Taiwan Chelungpu fault using rock-magnetic methods., 33, L23311.

Mondal T K. 2018. Evolution of fabric in Chitradurga granite (south India) — A study based on microstructure, anisotropy of magnetic susceptibility (AMS) and vorticity analysis., 723(16): 149–161.

Mondal T K and Mamtani M A. 2013. 3D Mohr circle construction using vein orientation data from Gadag (southern India)-Implications to recognize fluid pressure fluctuation., 56: 45–56.

Moore G F. 1989. Tectonics and hydrogeology of accretionary prisms: Role of the décollement zone., 11(1-2): 95–106.

Nagaraju E and Parashuramulu V. 2019. AMS studies on a 450 km long 2216 Ma dyke from Dharwar craton, India: Implications to magma flow., 10: 1931–1939.

Nagata T. 1961. Rock Magnetism. Tokyo: Maruzen: 1–350.

Nakamura N and Nagahama H. 2001. Changes in magnetic and fractal properties of fractured granites near the Nojima fault, Japan., 10(3-4): 486–494.

Novak B, Housen B, Kitamura Y, Kanamatsu T and Kawamura K. 2014. Magnetic fabric Analyses as a method for determining sediment transport and deposition in deep sea sediments., 356(6): 19–30.

Nye J F. 1957. Physical Properties of Crystals. London: Oxford University: 1–60.

Ono T, Housomi Y, Arai H and Takagi H. 2010. Comparison of petrofabrics with composite magnetic fabrics of S-C mylonite in paramagnetic granite., 32: 2–14.

Owens W H. 1974. Mathematical model studies on factors affecting the magnetic anisotropy of deformed rocks., 24: 115–131.

Owens W H. 1993. Magnetic fabric studies of samples from hole 808C, Nankai Trough.,, 131: 301–310.

Parés J M. 2015. Sixty years of anisotropy of magnetic susceptibility in deformed sedimentary rocks., 3(4): 1–13.

Parés J M and van der Pluijm B A. 2002. Evaluating magnetic lineations (AMS) in deformed rocks., 350: 283–298.

Parés J M and van der Pluijm B A. 2004. Correlating magnetic fabrics with finite train: Comparing results from mudrocks in the Variscan and Appalachian Orogens., 2: 213–220.

Parés J M, van der Pluijm B A and Turell J D. 1999. Evolution of magnetic fabrics during incipient deformationof mudrocks (Pyrenees, northern Spain)., 307: 1–14.

Pechersky D M and Genshaft Y S. 2001. Petromagnetism of the continental lithosphere and the origin of regional magnetic anomalies: A review., 3(2): 97–124.

Pei J, Li H B, Wang H, Si J L, Sun Z M and Zhou Z Z. 2014. Magnetic properties of the Wenchuan Earthquake Fault Scientific Drilling Project Hole-1(WFSD-1), Sichuan province, China., 66(1): 23.

Platt J P and Vissers R L M. 1980. Extensional structures in anisotropic rocks., 2(4): 397–410.

Raposo M I B and Berquo T S. 2008. Tectonic fabric revealed by AARM of the proterozoic mafic dike swarm in the Salvador city (Bahia State): São Francisco Craton, NE Brazil., 167(3–4): 179–194.

Raposo M I B, Drukas C O and Basei M A S. 2014. Deformation in rocks from Itajaí basin, Southern Brazil, revealed by magnetic fabrics., 629(26): 290–302.

Rasmay J G. 1980. Shear zone geometry: A review., 2: 83–99.

Rathore J S. 1980. The magnetic anisotropy of same slates from the Borrow dale volcanic group in the English Lake district and their correlation with strain., 67: 207–220.

Rathore J S. 1981. The magnetic fabric of some slates from the Borrow dale volcanic group in the English Lake rocks., 77: 151–168.

Rathore J S and Becke M. 1980. Magnetic fabric analyses in the Gail Galley (Carinthia, Austria) for the determination of the sense of movements along this region of the Periadriatic Line., 69: 349–368.

Robl J, Fritz H, Stüwe K and Bernhard F. 2004. Cyclic fluid infiltration in structurally controlled Ag-Pb-Cu occurrences (Schladming, Eastern Alps)., 205(1–2): 17–36.

Román-Berdiel T, Casas-Sainz A M, Oliva-Urcia B, Calvín P and Villalaín J J. 2019. On the influence of magnetic mineralogy in the tectonic interpretation of anisotropy of magnetic susceptibility in cataclastic fault zones., 216(2): 1043–1061.

Saint-Bezar B, Hebert R L, Aubourg C, Robion P, Swennen R and de Lamotte D F. 2002. Magnetic fabric and petrographic investigation of hematite-bearing sandstones within ramp-related folds: Examples from the South Atlas Front (Morocco)., 24: 1507–1520.

Sibson R H. 1977. Fault rocks and fault mechanisms., 133(3): 191–213.

Sibsona R H and Scott J. 1988. Stress/fault controls on the containment and release of overpressured fluids: Examplesfrom gold-quartz vein systems in Juneau, Alaska; Victoria, Australia and Otago, New Zealand., 13(1–5): 293–306.

Simpson C and Schmid S M. 1984. An evaluation of criteria to deduce the sense of movement in sheared rocks., 94: 1281–1288.

Solum J G and van der Pluijm B. 2009. Quantification of fabrics in clay gouge from the Carboneras fault, Spain and implications for fault behavior., 475(3): 554–562.

Soto R, Larrasoaña J C, Arlegui L E, Beamud E, Oliva-Urcia B and Simón J L. 2009. Reliability of magnetic fabric of weakly deformed mudrocks as a palaeostress indicator in compressive settings., 31(5): 512–522.

Stacey F D, Joplin G and Lindsay J. 1960. Magnetic anisotropy and fabric of some foliated rocks from S.E. Australia., 47(1): 30–40.

Stephenson A, Sadikun S and Potter D K. 1986. A theoretical and experimental comparison of the anisotropies of magnetic susceptibility and remanence in rocks and minerals., 84(1): 185–200.

Stern R J. 2005. TECTONICS丨Ocean Trenches. Encyclopedia of Geology: 428–437. doi.org/ 10.1016/B0-12-369396-9/00141-6

Tanikawa W, Mishima T, Hirono T, Lin W, Shimamoto T, Soh W and Song S R. 2007. High magnetic susceptibility produced in high-velocity frictional tests on core samples from the Chelungpu fault in Taiwan., 34: L15304.

Tarling D H and Hrouda F. 1993. The Magnetic Anisotropy of Rocks. London: Chapman and Hall: 1–217.

Thompson R and Oldfield F. 1986. Environmental magnetism. London: Allen and Unwin: 1956: 1–227.

Till J L and Moskowitz B M. 2014. Deformation microstructures and magnetite texture development in synthetic shear zones., 629: 211–223.

Tomezzoli R N, MacDonald W D and Tickyj H. 2003. Composite magnetic fabrics and S-C structure in granitic gneiss of Cerro de los Viejos, La Pampa province, Argentina., 25: 159–169.

Ujiie K, Hisamitsu T and Soh W. 2000. Magnetic and structural fabrics of the melange in the Shimanto accretionary complex, Okinawa Island: Implication for strain history during decollement-related deformation., 105(B11): 25729– 25741.

Ujiie K, Hisamitsu T and Taira A. 2003. Deformation and fluid pressure variation during initiation and evolution of the plate boundary decollement zone in the Nankai accretionary prism.:, 108: 1–8.

Uyeda S, Fuller M D, Belshé J C and Girdler R W. 1963. Anisotropy of magnetic susceptibility of rocks and minerals., 68(1): 279–291.

Vikas C, Prasannakumar V and Pratheesh P. 2016. Role of microfabrics and magnetic fabrics in the tectonic evolution of the Achankovil shear-zone, South India., 131: 95–108.

Voight W and Kinoshita S. 1907. Bestimmung absoluter Werte von Magnetiserungszahlen, inbesondere für Kristalle., 24: 492–514.

White S H, Burrows S E, Carreras J, Shaw N D and Humphreys F J. 1980. On mylonites in ductile shear zones., 2(1–2): 175–187.

Wiegand M, Trumbull R B, Kontny A and Greiling R O. 2017. An AMS study of magma transport and emplacementmechanisms in mafic dykes from the Etendeka Province, Namibia., 716: 149–167.

Wise D U, Dunn D E, Engelder J T, Geiser P A, Hatcher R D, Kish S A, Odom A L and Schamel S. 1984. Fault-related rocks: Suggestions for terminology., 12(7): 391–394.

Yang T, Chen J Y, Wang H Q and Jin H Q. 2012a. Rock magnetic properties of fault rocks from the rupture of the 2008 Wenchuan Earthquake, China and their implications: Preliminary results from the Zhaojiagou outcrop, Beichuan County (Sichuan)., 530–531: 331–341.

Yang T, Chen J Y, Wang H Q and Jin H Q. 2012b. Magnetic properties of fault rocks from the Yingxiu-Beichuan fault: Constraints on temperature rise within the shallow slip zone during the 2008 Wenchuan Earthquake and their implications., 50: 52–60.

Yang T, Chen J Y, Xu H R and Dekkers M J. 2019. High-velocity friction experiments indicate magnetic enhancement and softening of fault gouges during seismic slip.:, 124: 26–43.

Yang T, Chou Y M, Ferré E C, Dekkers M J, Chen J, Yeh E C and Tanikawa W. 2020. Faulting processes unveiled by magnetic properties of fault rocks., 58: 1–60.

Yang T, Dekkers M J and Zhang B. 2016. Seismic heating signatures in the Japan Trench subduction plate-boundary fault zone: Evidence from a preliminary rock magnetic ‘geothermometer’., 205(1): 319–331.

Yang T, Mishima T, Ujiie K, Chester F M, Mori J, Eguchi N, Toczko S and Expedition 343 Scientists. 2013. Strain decoupling across the décollement in the region of large slip during the 2011 Tohoku-Oki earthquake from anisotropy of magnetic susceptibility., 381: 31–38.

Yeh E, Lee T, Lin Y, Chou Y and Lu C. 2007. Analysis of magnetic and grain fabrics across fault gouge in the Chelungpu Fault of Taiwan (Abstract T43B-1331). Paper presented at American Geophysical Union Fall Meeting 2007, San Francisco, USA.

Zhang L, Li H B, Sun Z M, Chou Y M, Cao Y and Wang H. 2018. Metallic iron formed by melting: A new mechanism for magnetic highs in pseudotachylyte., 46(9): 779–782.

Zhou Y, Zhou P, Wu S M, Shi X B and Zhang J J. 2020. Magnetic fabric study across the Ailao Shan-Red River shear zone., 346: 137–150.

Ziegler P A. 1989. Geodynamic model for Alpine intraplate compressional deformation in Western and Central Europe.,,, 44(1): 63–85.

Anisotropy of Magnetic Susceptibility and Shear Zone

HUA Tian1, LI Jinxi2*, LI Zhiwu1, RAN Bo1, TONG Kui1, WANG Zijian1, YUAN Mengyu1, CAI Hongyan1, CHEN Tao1, LI Ke3and LIU Shengwu1

(1. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, Sichuan, China; 2. MOEKey Laboratory of Earth Exploration and Information Technology, College of Geophysics, Chengdu University of Technology, Chengdu 610059, Sichuan, China;3. Sichaun Institude of Coal Field Geological Engineering Explortation and Designing, Chengdu 610072, Sichuan, China)

Rock records and preserves critical information about the transformation and evolution of the crust. Extracting such information is of great significance for analyzing and restoring geodynamic processes. Anisotropy of magnetic susceptibility (AMS) is an important rock fabric, which effectively records the rock strain characteristics. It can be used to analyze the geodynamic process and is an effective method to study structural deformation and stress effects. In this paper, the basic principle of AMS and its application in shear zone are systematically described on the basis of the research history, main achievements and the latest progress of AMS. We can come to conclusions as below: (1) The rock fabric is complex, and the magnetic fabric, as an indirect means of fabric, is controlled by the physical properties of minerals, the types and contents of magnetic minerals, deformation and metamorphism. (2) The determination of magnetic minerals is the key to the study of magnetic fabric. (3) In most cases, there is a coaxial relationship between the principal axis of the magnetic ellipsoid and the strain ellipsoid, thus the magnetic ellipsoid can be used as “display” of rock deformation. (4)In the shear zone, AMS can obtain some structural information which is difficult to obtain by conventional geological methods. However, due to the complexity of shear zone and magnetic fabric, this method should be used with caution. It is necessary to make a reasonable explanation for its origin using geologic evidence from different aspects. (5) Strain across shear zone is typically heterogeneous, which leads to the different orientation of the magnetic fabric. Moreover, deformation is commonly accompanied by fluid-rock interaction or mineral segregation. The interaction between fluid and rock induces changes in magnetic mineralogy. (6) In shear zone, the relationship betweenjand strain depends strongly on the deformation mechanisms and the mineral carriers of AMS as well.

anisotropy of magnetic susceptibility; AMS; shear zone; rock fabric

2019-08-05;

2020-06-01

国家自然科学基金项目(41602153、41472107、41230313)资助。

华天(1995–), 男, 硕士研究生, 构造地质学专业。Email: tianhuamag@163.com

李金玺(1981–), 男, 副教授, 从事构造地质学及地球物理研究与教学工作。Email: lijinxi23@qq.com

P545

A

1001-1552(2021)02-0280-016

10.16539/j.ddgzyckx.2021.02.002