地球物理学报  2018, Vol. 61 Issue (5): 1782-1796   PDF    
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
何祥丽1,2, 李海兵2, 张蕾2, 王焕2, 葛成隆2, 曹勇3, 白明坤2, 李成龙2, 叶小舟3, 韩帅2     
1. 地球科学与资源学院, 中国地质大学(北京), 北京 100083;
2. 自然资源部深地动力学重点实验室, 中国地质科学院地质研究所, 北京 100037;
3. 自然资源部古地磁与古构造重建重点实验室, 中国地质科学院地质力学研究所, 北京 100081
摘要:断层带内的流体不仅可以通过水岩反应改变断裂岩的矿物组成和化学成分,从而导致化学性质和物理性质的变化,而且可以影响或控制断裂带的变形行为.断裂带中岩石磁学特征是由特定化学环境下磁性矿物的种类和含量所决定的,因此,从矿物学和地球化学角度探讨断裂岩的磁性变化,对揭示断层的变形行为和环境具有一定的指示作用.本文以汶川科钻WFSD-3P钻孔中龙门山灌县—安县断裂带断裂岩为研究对象,运用高分辨率磁化率测试、XRD矿物成分半定量分析、XRF元素扫描以及不同价态Fe元素含量分析等多种方法开展断层磁学变化和变形环境的研究.磁化率测试结果表明灌县—安县断裂带断层泥的磁化率值普遍低于对应的围岩磁化率平均值.结合前人研究成果表明造成该断层泥低磁化率异常的原因是在间震期的长期流体作用下,铁磁性矿物(例如磁铁矿)转变成顺磁性矿物(铁硫化物、菱铁矿或含铁的黏土矿物).新生铁硫化物和含铁黏土矿物是在间震期缓慢形成的,而黏土矿物含量的增加弱化了断层强度,促进断层蠕滑,这说明断层泥低磁化率异常可能指示了该断裂在间震期长期缓慢活动,即为蠕滑变形.断层泥中黄铁矿的发育和高Fe2+和S元素、低Fe3+的特征显示灌县—安县断裂作用环境通常是在低温、还原环境中进行的.这些结果与低磁化率值的相关性暗示断层泥低磁化率异常可能对活动断层的低温还原环境具有指示意义.
关键词: 灌县—安县断裂带      断层泥      磁化率      蠕滑      WFSD-3P     
Mineral and chemical response to low magnetic susceptibility of the fault gouge from the Guanxian-Anxian fault zone and fault creep setting in the Longmen Shan
HE XiangLi1,2, LI HaiBing2, ZHANG Lei2, WANG Huan2, GE ChengLong2, CAO Yong3, BAI MingKun2, LI ChengLong2, YE XiaoZhou3, HAN Shuai2     
1. School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China;
2. Key Laboratory of Deep-Earth Dynamics of Natural Resources Ministry, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
3. Key Laboratory of Paleomagnetism and Tectonic reconstruction of Natural Resources Ministry, Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China
Abstract: Fluid infiltration within fault zones can not only alter the mineral and chemical composition of fault rocks by fluid-rock interactions and thus causes variation in chemical and physical properties, but can also influence or control the deformational behavior of a fault zone. Rock magnetic property in the fault zone is controlled by the type and content of magnetic minerals in a particular chemical environment. Therefore, studying the magnetic variation of fault rocks by mineralogy and geochemistry can reveal the deformational behavior and environment of an active fault. In this study, high-resolution magnetic susceptibility measurements, X-Ray Diffraction (XRD) minerals analyses, X-Ray Fluorescence (XRF) element scanning, and Fe element geochemical analyses were implemented on some representative cores from the Wenchuan earthquake Fault Scientific Drilling Hole-3P (WFSD-3P) located on the Guanxian-Anxian Fault zone (GAF) to study the magnetic variation and deformational environment of the fault. The measurement results show that the magnetic susceptibility values of the fault gouge in the GAF are commonly less than the average of the corresponding host rocks. The low magnetic susceptibility of the fault gouge results from the transformation from ferromagnetic minerals (such as magnetite) to paramagnetic minerals (Fe-sulfides, siderite, or iron-bearing clay minerals) under long-term fluid infiltration during interseismic periods based on Scanning Electron Microscope (SEM) observation and previous results. New Fe-sulfides and iron-bearing clay minerals were formed slowly during interseismic periods. An increase in clay minerals content weakens the fault and facilitates the creeping of the GAF, which suggests that low magnetic susceptibility of the fault gouge may be an indicator of a slowly slipping active fault during interseismic periods (i.e. creep deformation). The development of pyrite, relatively high contents of Fe2+ and sulfur element, and the low content of Fe3+ in the fault gouge implies that the faulting of the GAF is commonly at a low temperature and reducing environment. The relationship between the above observations and the low magnetic susceptibility of the fault gouge shows that the latter may attest to the interseismic low-temperature and reducing environment along an active fault.
Key words: Guanxian-Anxian fault zone    Fault gouge    Magnetic susceptibility    Creeping fault    WFSD-3P    
0 引言

流体渗透在断层带的物理、化学演化中起着至关重要的作用(Ishikawa et al., 2008; Niwa et al., 2015).断层带通常发育密集破裂和次级断层,无论是在同震、震后,还是间震期,流体都可能分解或溶解断层岩矿物,转化其磁性矿物集合,从而影响其磁学性质(Chou et al., 2012aYang et al., 2013, 2016).然而,这些流体的迁移通道和成分随时间和空间的变化发生显著变化(Goddard and Evans, 1995),这导致断层的氧化还原环境也随之改变(Yamaguchi et al., 2011).流体的循环可能会导致细粒的铁氧化物、氢氧化物或硫化物(如黄铁矿),在不同的物理化学条件下(温度、氧化还原性、酸碱度、流体-岩石比率等)沉淀(Goldstein and Brown, 1988; Grosz et al., 2006; Guichet et al., 2006).因此,与含铁矿物种类、磁性和含量密切相关的岩石磁化率特征(张锡濂,1991刘天佑,2007郎元强等,2011张川等,2013)可以指示断裂作用过程中的物理化学变化及环境(Orlický,1990Kapička et al., 2003Hounslow and Morton, 2004刘青松等,2007刘青松和邓成龙,2009).

前人在对断层作用的研究中发现大地震过程中摩擦热作用下新生铁磁性矿物造成断层泥呈高磁化率异常(Hirono et al., 2006Mishima et al., 2006Tanikawa et al., 2007Pei et al., 2014a, b).而Liu等(2014)发现在2008年汶川地震中破裂的灌县—安县断裂带地表九龙剖面的断层泥的磁化率值较围岩磁化率值低.岩石磁化率值的高低与矿物、化学成分有关(Xu et al., 2009a张川等,2013).例如,Chou等(2012a)阐明集集地震断裂带断层泥高磁化率特征与黄铁矿的缺失和磁铁矿的新生有密切关系.那么低磁化率的断层泥有着怎样的矿物、化学特征呢?暗示着怎样的断裂作用环境呢?并且有证据表明灌县—安县断裂具有非地震蠕滑性质(He et al., 2018李海兵等,2018),其断层泥低磁化率特征是否与其蠕滑性质有关?前人认为非地震蠕滑能够释放构造应力,不易造成灾害的断裂活动(Gratier et al., 2013).然而,具有蠕滑性质的灌县—安县断裂却在汶川特大地震(MS8.0)中发生破裂,这冲击了人们之前对蠕滑断裂的认识.虽然已有地质学家提出蠕滑断裂也可能发生大地震(Chang et al., 2009Noda and Lapusta, 2013Maurer and Johnson, 2014Jolivet et al., 2015Chen and Bürgmann,2017Harris,2017),但对蠕滑断裂的形成和作用过程尚未清楚.解决上述问题有助于揭示蠕滑断裂的变形和破裂机制,从而推动地震断裂作用的研究,为预报地震和防御地震灾害提供科学依据.

本文以穿过灌县—安县断裂带的汶川地震断裂科学钻探工程3P钻孔(WFSD-3P)岩心为主要研究对象,通过磁化率测试、X射线衍射(XRD)矿物分析,X射线荧光光谱(XRF)元素扫描以及XRF粉末样品Fe元素分析,结合扫描电镜(SEM)观测来讨论灌县—安县断裂岩磁化率异常原因及其对蠕滑断裂作用的指示意义,并探讨其断裂作用的环境特征.

1 地质背景

龙门山逆冲带位于四川盆地和青藏高原过渡带(图 1),在中国南北地震带中段,以巨大的地形起伏为特征,海拔高差达3000~4500 m(Densmore et al., 2007),大约长500 km,宽30~60 km,由三条近平行的北东—南西走向的活动断裂组成(李勇等,2006Zhang et al., 2010).其中,前山断裂即灌县—安县断裂是龙门山与四川盆地(侏罗纪前陆盆地)的边界,是一条倾向北西的铲式断层,在约1250 m深处倾角约38°,在地表倾角约为60°(Li et al., 2016).映秀—北川断裂为龙门山中央断裂,倾角大于60°(Li et al., 2013),具黏滑特征,多次发生大地震(Wang et al., 2015Zhang et al., 2017).汶川地震分别沿着映秀—北川断裂和灌县—安县断裂产生地表破裂约270 km和约80 km,其走向方向为10°N—30°E(李海兵等,2008Liu-Zeng et al., 2009Xu et al., 2009bFu et al., 2011).沿映秀—北川断裂分布的地表破裂带的最大垂直位移约11 m,位于北川县沙巴村.沿灌县—安县断裂分布的地表破裂带最大垂直位移约4 m,出现在绵竹九龙镇清泉村(Li et al., 2016).

图 1 龙门山构造带的地质简图(据Li et al., 2016修改) 图中显示了汶川地震地表破裂带和WFSD钻孔位置;F1:汶川—茂县断裂;F2:映秀—北川断裂;F3:灌县—安县断裂. Fig. 1 Geological map of the Longmen Shan fault zone (modified from Li et al., 2016) The figure shows Wenchuan earthquake surface rupture zones and WFSD drilling sites; F1:Wenchuan-Maoxian fault; F2:Yingxiu-Beichuan fault; F3:Guanxian-Anxian fault.

灌县—安县断裂上盘地层主要由三叠纪沉积岩组成,包括灰岩、白云岩、生物碎屑、砂岩、粉砂岩、泥岩和含碳页岩(四川省地矿局,1996).断层下盘的地层主要为侏罗纪(棕红色和灰绿色沉积岩)和第四纪冲积物及坡积物组成的地层. WFSD-3P钻孔深551.51 m,其岩心主要由三叠纪须家河组浅灰色粉砂岩和砂岩、侏罗纪棕红色和灰绿色砂岩以及断层岩组成(图 2).在WFSD-3P钻孔的440~511 m深度处分布灌县—安县断裂岩带,包括4个次级断裂带,分别为FZ451、FZ464、FZ490和FZ507(He et al., 2018).上盘岩石主要是三叠纪须家河组浅灰色粉砂岩和砂岩.下盘岩石是由侏罗纪棕红色和灰绿色砂岩或粉砂岩组成,岩心深度约为511~551m(图 2). FZ451、FZ464和FZ490的断层岩主要呈黑灰色或灰色,源于三叠纪须家河组沉积岩;而FZ507的断层岩则有两种组成(图 2),一种黑灰色断层岩,原岩为须家河组沉积岩,另一种红色断层岩,源于侏罗纪棕红色砂岩、粉砂岩.

图 2 WFSD-3P钻孔岩性柱状图和采样位置 Q:第四纪;T:三叠纪;J:侏罗纪. Fig. 2 Lithology chart along WFSD-3 cores and sampling locations Q:Quaternary; T:Triassic; J:Jurassic.
2 样品采集和测试方法 2.1 样品采集

本文磁化率测试和XRF元素扫描的样品是采自WFSD-3P钻孔岩心中灌县—安县断裂带次级断裂带FZ464(463.11~469.04 m)和FZ507(501.79~510.02 m)的断层岩和部分围岩,包括三叠纪须家河组灰色粉砂岩(230.94~232.83 m)和侏罗纪棕红色(522.75~524.33 m)和灰绿色砂岩(516.77~517.28 m).

我们从WFSD-3P钻孔岩心的不同深度采集了170个XRF粉末地球化学分析的样品,样品位置如图 2所示.共采集31个围岩样品,样品采集间隔约为10 m,其中3P-1~3P-25为三叠纪须家河组沉积岩,3P-165~3P-170为侏罗纪砂岩.破裂带样品共18个,本文中未对其进行讨论.次级断裂带FZ451(3P-44和3P-45)、FZ464(3P-46~3P-96)和FZ507(3P-96~3P-164)中共采集样品121个,其中3P-45~46、49~66、68~75、77~82、86~87、90~91、102~105、110~122、126~130、139~143、148~152、164样品为断层泥,采集间隔平均约为10 cm.由于FZ451样品太少,未进行讨论.另外,我们从中选择8个代表性样品做矿物物相半定量分析(表 2),分别为三叠纪粉砂岩(190.2 m)、FZ464灰色断层角砾岩(464.33 m)、FZ464黑灰色断层泥(464.23 m)、FZ507黑灰色断层泥(502.725 m)、FZ507灰色断层角砾岩(503.41 m)、FZ507红色断层泥(507.595 m)、FZ507红色断层角砾岩(508.69 m)、和FZ507侏罗纪红色砂岩(550.65 m).

表 2 WFSD-3P岩心样品矿物相对含量 Table 2 Mineral relative content of samples from WFSD-3P cores
2.2 测试方法

为了了解灌县—安县断裂的物理、化学性质及其断裂作用环境,本文对WFSD-3P断层泥、断层角砾岩和围岩进行了磁化率测试、XRF元素扫描、XRF粉末地球化学分析、和XRD矿物分析,具体说明如下:

(1) 磁化率测试

垂直于面理方向将代表岩心切成两半,沿岩心平整面中线测试其磁化率.避免因钻探过程中混入的泥浆、铁屑等表面污染物的干扰.岩心的磁化率测试工作在汶川地震断裂科学钻探实验中心完成,所用仪器为Bartington MS2K磁化率仪,灵敏度为1×10-6 SI,测试间隔为0.5 cm,共获得约3200多个数据.

(2) XRF元素扫描

我们采用X射线荧光光谱法(XRF)的无损检测技术对代表岩心的新鲜切面进行元素扫描,扫描间隔为1 mm,每个点扫描时间为5 s.该测试在中国科学院青藏高原研究所完成。

(3) XRF粉末地球化学分析

该测试是在国家测试中心完成,采用的检测仪器是X射线荧光光谱仪(PW4400),Fe3+检测方法是GB/T 14506.28-2010,以Fe2O3形式呈现,Fe2+的检测方法是GB/T 14506.14-2010,以FeO形式呈现.本文主要呈现FZ464和FZ507的断裂岩主量元素Fe不同价位的含量变化及与对应围岩的比较.我们将断层岩样品的实验数据归一化,即将每个断层岩样品的Fe2O3和FeO百分含量除以对应围岩Fe2O3和FeO的平均值,数值小于1的代表其含量小于围岩的对应价位Fe离子的含量,大于1则代表大于围岩的Fe2+或Fe3+含量.

(4) XRD分析

8个代表性样品做矿物物相半定量分析,采用的方法是SY/T 5163-2010沉积岩中黏土矿物和常见非黏土矿物X射线衍射分析方法,测试仪器为X射线衍射仪(D/max-rA).该实验在北京北达燕园微构分析测试中心有限公司完成.

3 结果分析 3.1 断层岩磁化率测试结果

WFSD-3P钻孔岩心次级断裂带FZ464和FZ507的断层岩分布与磁化率值测试结果见图 3图 4.黑灰色断层泥的磁化率值低于灰色断层角砾岩,而红色断层岩则无此特征,甚至有的红色断层泥磁化值略高于红色断层角砾岩(图 3图 4).黑灰色断层泥中有两段更为深色的含碳断层泥的磁化率值最低,一般小于20×10-6 SI(图 3).图中可以看到黑灰色断层泥有些位置的磁化率值出现异常高现象,根据观察岩心对应部位,发现异常高值一般由于对应位置的断层泥含有块状碎屑物质.但是,在深度为504.9 m附近的异常似乎不是由碎块造成的,该处断层泥呈揉皱状,含有黄白色条带,具体成分有待进一步测试.

图 3 FZ464断层岩分布与磁化率值变化曲线 磁化率沿岩心中间白线测试;灰色条框指示断层泥位置;白色虚线是断层角砾岩和断层泥的分界线. Fig. 3 Fault rocks distribution and variation curves of magnetic susceptibility in FZ464 Cores were measured along thewhite lines to obtain magnetic susceptibility values. Gray bars indicate the positions of fault gouges. White dotted lines represent the boundaries between fault breccia and fault gouge.
图 4 FZ507断层岩分布与磁化率值变化曲线 磁化率沿岩心中间白线测试;灰色条框指示断层泥位置;白色虚线是断层角砾岩和断层泥的分界线. Fig. 4 Fault rocks distribution and variation curves of magnetic susceptibility in FZ507 Cores were measured along thewhite lines to obtain magnetic susceptibility values. Gray bars indicate the positions of fault gouges. White dotted lines represent the boundaries between fault breccia and fault gouge.

排除这些异常值,我们将所测的WFSD-3P钻孔岩心不同岩性约3200个磁化率值进行统计,对每种岩性样品的有效磁化率值进行平均且得到每组数值的标准偏差(表 1).三叠纪须家河组浅灰色粉砂岩的磁化率平均值约为28.8×10-6 SI. FZ464和FZ507灰色断层角砾岩的磁化率平均值分别约为28.4×10-6 SI和27.6×10-6 SI,近似于围岩的磁化率平均值. FZ464和FZ507黑灰色断层泥的磁化率平均值分别约为20.6×10-6 SI和23.1×10-6 SI,较其围岩和断层角砾岩的磁化率平均值偏低.含碳断层泥的磁化率平均值约为12.1×10-6 SI,明显低于其围岩和断层角砾岩. FZ507红色断层泥和断层角砾岩的磁化率平均值分别约为24.1×10-6 SI和22.2×10-6 SI,大约只有其围岩(侏罗纪棕红色粉砂岩)磁化率平均值(52.6×10-6 SI)的一半.

表 1 WFSD-3P岩心主要岩性磁化率值统计结果 Table 1 Magnetic susceptibility of main rocks in WFSD-3P cores

FZ464和FZ506断层岩样品的磁化率值与对应围岩磁化率平均值的关系呈现在归一化磁化率值(断层岩样品的磁化率值除以对应围岩磁化率平均值)随深度变化的曲线中(图 5b5g).从图中,我们可以看出断层岩归一化磁化率值大部分小于1,说明断层岩,尤其是断层泥磁化率普遍小于围岩,而归一化磁化率值大于1的一般为断层角砾岩以及断层泥的异常值.

图 5 FZ464和FZ507断层岩分布及其物理-化学性质 (a)FZ464岩性柱;(b) FZ464断层岩归一化磁化率值;(c) FZ464断层岩归一化Fe2O3含量;(d) FZ464断层岩FeO含量;(e) FZ464断层岩S元素富集强度;(f) FZ507岩性柱;(g) FZ507断层岩归一化磁化率值;(h) FZ507断层岩归一化Fe2O3含量;(i) FZ507断层岩FeO含量;(j) FZ507断层岩S元素富集度指标.粉色条框指示断层岩指标值小于对应围岩的平均值;黑色圆点或方点代表断层泥样品,白色圆点或方点代表断层角砾岩样品;曲线下方的数值为三叠纪灰色粉砂岩对应指标的平均值,括号里是侏罗纪红色粉砂岩对应指标的平均值;图(e)和(j)中的灰色条框指示断层泥对应位置;cps:每秒的计数. Fig. 5 Correlation between fault rocks distribution and the physical and chemical properties in FZ464 and FZ507 (a) Lithological chart of FZ464 fault rocks; (b) Host-normalized magnetic susceptibility of FZ464 fault rocks; (c) Host-normalized Fe2O3 contents of FZ464 fault rocks; (d) Host-normalized FeO contents of FZ464 fault rocks; (e) Sulfur element enrichment index of FZ464 fault rocks; (f) Lithological chart of FZ507 fault rocks; (g) Host-normalized magnetic susceptibility of FZ507 fault rocks; (h) Host-normalized Fe2O3 contents of FZ507 fault rocks; (i) Host-normalized FeO contents of FZ507 fault rocks; (j) Sulfur element enrichment index of FZ507 fault rocks. The pink bars indicate the index values of fault rocks are less than the averages of corresponding host rocks; black dots represent fault gouge samples and white dots represent fault breccia samples; the values at the bottom of curves are the corresponding index means of Triassic gray siltstone and Jurassic red siltstone (in parentheses); the grey bars in (e) and (j) indicate the locations of fault gouges; cps: counts per second.
3.2 XRD矿物成分分析

WFSD-3P钻孔岩心中8个样品XRD图谱剖面显示灌县—安县断裂带围岩和断裂岩的主要矿物成分均为石英、斜长石、微斜长石和黏土矿物(图 6).围岩和断层角砾岩中石英含量比断层泥中高,而断层泥的黏土矿物含量则较高. FZ464黑灰色断层泥、FZ507黑灰色断层泥和红色断层泥的黏土矿物含量分别为39%、49%和51%.不同的是,FZ464和FZ507黑灰色断层泥中不含方解石,含有菱铁矿(1%~8%),而FZ507红色断层岩和围岩(须家河组粉砂岩和侏罗纪红色砂岩)不含菱铁矿(表 2图 6).而且,只有红色断层岩和红色砂岩含有赤铁矿(2%~4%),灰色粉砂岩中含有铁白云石(4%;表 2图 6).

图 6 WFSD-3P岩心样品XRD图谱剖面 Qz:石英,Cal:方解石,Ab:斜长石,Mc:微斜长石,Sd:菱铁矿,Hem:赤铁矿,Ank:铁白云石,Dol:白云石,Il:伊利石,Kln:高岭土,Chl:绿泥石,Mnt:蒙脱石.图最右边为样品号. Fig. 6 XRD spectrum profiles of WFSD-3P cores samples Qz:quartz, Cal:calcite, Ab:albite, Mc:microcline, Sd:siderite, Hem:hematite, Ank:ankerite, Dol:dolomite, Il:illite, Kln:kaoline, Chl:chlorite, Mnt:montmorillonite. The sample number is in the right side of the picture.
3.3 Fe和S元素化学分析

FZ464和FZ507断层岩分布和归一化Fe2O3和FeO百分含量随深度变化的结果呈现在图 5. 图 5c显示FZ464断层岩的Fe2O3含量通常小于围岩Fe2O3含量平均值,即归一化后的Fe2O3百分含量值小于1;而FZ464断层岩的FeO的含量则一般大于围岩平均值,局部断层泥的FeO的含量表现为小于围岩平均值(图 5d).在次级断裂带FZ507中,501.79~505.2 m的断层岩呈现出Fe2O3百分含量小于围岩平均值,在505.2 ~510.02 m则相反(图 5h). FZ507断层岩FeO百分含量在红黑断层泥交界处发生显著变化,即黑灰色断层岩的归一化FeO百分含量大于1,红色断层岩的归一化FeO百分含量则小于1(图 5i).断层泥与断层角砾岩的Fe2O3和FeO百分含量变化无明显规律. FZ464和FZ507断层岩归一化Fe2O3百分含量与磁化率值随深度变化的大致趋势相似(图 5b5c5g5h).

FZ464和FZ507断层岩分布与S元素含量XRF扫描结果如图 5e5j所示.从图中可以看出,在两个断裂带中黑灰色断层泥的S元素含量要明显高于灰色断层角砾岩.对应红色断层泥和断层角砾岩的S元素含量则没有明显的区别.

4 讨论 4.1 断层泥低磁化率值的成因分析及其对蠕滑作用的响应

前人研究表明断裂作用可以在物理、化学上改变断层岩中的磁性矿物,比如地震活动通过碾磨和摩擦升温改变矿物颗粒大小和铁磁性矿物的种类及数量(Tanikawa et al., 2008Mishima et al., 2009Yang et al., 2016),从而导致断层岩磁化率值异常.目前普遍认为映秀—北川地震断裂带断层泥高磁异常是大地震过程中摩擦热作用下顺磁性矿物热解,形成铁磁性矿物造成的(裴军令等,2010Yang et al., 2012a, 2012b刘栋梁等,2015Liu et al., 2016).其顺磁性矿物可能来源于硅酸盐或黏土矿物,因此黏土矿物含量变化与大地震摩擦热作用下矿物热解有关(裴军令等,2016).由于大地震断裂带主滑动带才能产生高温摩擦热,所以断层岩高磁化率异常通常指示大地震活动(Tanikawa et al., 2008Yang et al., 2016张蕾等,2017).然而,根据上述WFSD-3P钻孔岩心磁化率测试结果显示灌县—安县断裂带断层泥普遍低于或略低于其围岩磁化率平均值,尤其含碳断层泥的磁化率平均值呈现低磁化率异常,这与地震产生的断层岩高磁化率异常明显不同.刘栋梁等(Liu et al., 2014)在灌县—安县断裂带地表探槽所测结果也显示断层泥的磁化率平均值小于原岩、断层角砾岩的磁化率平均值,与本文测试结果相似.那么造成灌县—安县断裂带断层泥磁化率值降低的原因是什么呢?

灌县—安县断裂地表九龙探槽的断层岩及围岩的等温剩磁、天然剩磁强度和K-T曲线等结果显示断层泥主要磁性矿物为铁硫化物(黄铁矿、磁黄铁矿和硫复铁矿)和一些N型铁磁性矿物(如钛赤铁矿),断层角砾岩所含主要磁性矿物为磁铁矿、针铁矿和铁硫化物,而侏罗纪灰绿色砂岩的主要磁性矿物为磁铁矿,有的含有赤铁矿和N型铁磁性矿物,棕红色砂岩的主要磁性矿物则为赤铁矿和磁铁矿(Liu et al., 2014).磁铁矿是一种磁化率值要高于铁硫化物的铁磁性矿物.因此,他们认为在九龙探槽中部分磁铁矿在同震中或大地震后转变成铁硫化物,这是造成断层泥的磁化率平均值略低于原岩的原因(Liu et al., 2014).

通过对灌县—安县断裂带断层泥扫描电镜下观察,发现地表和钻孔岩心的断层泥中存在铁硫化物(图 7).其中,九龙探槽的断层泥中的黄铁矿呈自形结晶颗粒(图 7b),多个颗粒集合成球体生长在黏土矿物中.相似的结晶程度极好的黄铁矿颗粒也存在于WFSD-3P钻孔岩心的两个次级断裂带FZ464和FZ507中(图 7c7d),扫描电镜能谱仪(SEM-EDX)测试也显示其为铁硫化物颗粒.从图 7c7d中还可以看出该黄铁矿颗粒与层状硅酸盐矿物共生(白色箭头指示).另外,我们还可以从图 7a中看到含铁黏土矿物从富铁的铝硅酸盐矿物边缘新生长出来,矿物所含元素可见图 7底部SEM-EDX能谱图. XRF元素扫描结果也显示断层泥S元素要明显高于断层角砾岩. XRD半定量分析结果还显示灰黑色断层岩中含有菱铁矿,而围岩中没有.根据以上测试结果,灌县—安县断裂带断层泥低磁化率异常很可能是由于某种条件下部分铁磁性矿物(例如磁铁矿)转变成顺磁性矿物(铁的硫化物、菱铁矿或含铁的黏土矿物).然而,灌县—安县断裂从地表到钻孔岩心的断层泥普遍显示低磁化率特征,而不是局部分布.并且黄铁矿结晶程度较好,在不同深度的断层泥中均有发育,说明黄铁矿不是在大地震发生时快速形成的,而是在间震期,大气流体渗入并渗透到断层中,导致含铁矿物的溶解、沉淀和重结晶时形成(Yang et al., 2013),即该黄铁矿是在间震期流体长期作用下缓慢结晶形成的.

图 7 灌县—安县断裂带断层泥扫描电镜照片及成分能谱图 Qz:石英;Py:黄铁矿. Fig. 7 SEM images and SEM-EDX results of fault gouge in the Guanxian-Anxian fault zone Qz:quartz; Py:pyrite.

另外,XRD半定量分析显示围岩石英含量要高于断层岩(尤其是断层泥)中的石英含量,而其黏土矿物的含量较断层泥低(表 2).这也表明了原岩在某种条件下,如压溶作用和流体作用下,石英、铝硅酸盐矿物溶解形成黏土矿物,同时铁磁性矿物转变成顺磁性矿物(Liu et al., 2014),包括含铁的粘土矿物.例如,有学者提出在震后流体、水和其他条件下,一些磁性矿物发生转变(Chou et al., 2012b),而且在存在水的条件下,矿物可能向更加稳定的黏土矿物转变(Kuo et al., 2012).黏土矿物的增多有助于减小岩石强度、降低渗透性、增大孔隙度,从而弱化断层强度(Fisher and Knipe et al., 2001Aringhieri,2004Tembe et al., 2010),且发生蠕滑变形.综上所述,灌县—安县断裂带断层泥低磁化率异常可能指示了该断裂在间震期长期缓慢活动,即蠕滑变形时流体渗透通过水岩反应导致铁磁性矿物转变成顺磁性矿物,引起断层泥低磁化率异常.

4.2 灌县—安县断裂作用环境分析

岩石磁性矿物在不同温度下发生不同的转化过程(李海燕和张世红,2005Moreau et al., 2005Aubourg and Pozzi, 2010),因而可记录断层作用下的温度信息.许多实验研究已经对磁性矿物之间的转化温度进行了限制(Lowrie and Heller, 1982Pan et al., 2000Cairanne et al., 2004),如热磁学实验(Cairanne et al., 2004Moreau et al., 2005Aubourg and Pozzi, 2010).上文讨论得出灌县—安县断裂岩中的磁性矿物转化很可能是铁磁性矿物转变成顺磁性矿物,包括铁硫化物.根据前人在自然条件下的研究,从磁铁矿转化新生成铁硫化物的温度范围是小于300 ℃(Schill et al., 2002Gillett,2003).如此低的温度可能是由于断层低速滑动造成的(Liu et al., 2014),暗示了灌县—安县断裂具有蠕滑性质. Liu等(2014)认为磁铁矿转化为铁硫化物不仅需要满足低温条件,而且需要还原环境,但这只是根据前人经验推断所得,却缺少直接来自断裂岩的证据.

WFSD-3P钻孔岩心中FZ464、FZ507断层岩的Fe2O3和FeO百分含量与对应的围岩的比较研究发现,断层岩Fe2O3含量通常小于围岩Fe2O3含量平均值,而断层岩的FeO的含量则一般大于围岩平均值. Fe2O3含量代表的是Fe3+离子的含量,而FeO代表的是Fe2+离子的含量.该结果说明围岩中的Fe3+在断裂作用过程中被还原成Fe2+,对应铁磁性矿物转化为铁硫化物,即断层泥磁化率降低特征.况且,断层泥中的黄铁矿在氧化带是不稳定存在的,通常是在还原环境中形成的.此外,XRF元素扫描也发现不同深度的黑灰色断层泥都表现出S元素明显高于断层角砾岩的特点,对应其磁化率值低于断层角砾岩和围岩,这些说明了灌县—安县断裂作用环境可能是在还原环境中进行的,并且断层泥低磁化率值可能指示断裂的还原环境.然而在507 m深度附近,即红黑断层泥边界处有异常存在,在505.2 ~510.02 m的断层岩的Fe2O3含量高于围岩,近是围岩的平均值的2倍,而红色断层岩的FeO含量也异常地少于围岩的平均值.红色断层泥与断层角砾岩的S元素含量也没有明显区别.由于灌县—安县断裂带地表破裂带显示汶川地震主滑动带在断裂带底部,即三叠纪与侏罗纪地层交界处(Li et al., 2016),推断WFSD-3P钻孔中汶川地震的主滑动带也在地层交界处.在地震中,可能是含Fe3+离子流体的加入导致了主滑动带附近Fe2O3和FeO百分含量的异常.但是,具体造成异常的原因和过程还需进一步研究.

5 结论

本文通过对穿过龙门山灌县—安县断裂带的WFSD-3P钻孔部分岩心进行高分辨率磁化率测试、XRD矿物成分半定量分析、XRF S元素扫描以及不同价态Fe元素含量分析,并结合扫描电镜的观察和前人的研究成果得出以下结论:

(1) WFSD-3P钻孔岩心中灌县—安县断裂带的断层泥的磁化率值普遍低于对应的围岩.造成灌县—安县断裂带断层泥低磁化率异常的原因是在间震期的流体长期作用下,铁磁性矿物(例如磁铁矿)转变成顺磁性矿物(铁硫化物、菱铁矿或含铁的黏土矿物).

(2) 灌县—安县断裂带断层泥新生铁硫化物和含铁黏土矿物是间震期缓慢形成的,而黏土矿物的富集弱化了断层强度,促进断层蠕滑.因此断层泥低磁化率异常可能指示了该断裂在间震期长期缓慢活动,即灌县—安县断裂以蠕滑变形为主.

(3) 黑灰色断层泥中发现的黄铁矿和高Fe2+和S元素,低Fe3+的特征,表明灌县—安县断裂作用环境可能是在低温、还原环境中进行的.这些特征与磁化率值的相关性暗示断层泥低磁化率值可能指示断裂的低温、还原环境.

致谢

汶川地震断裂科学钻研工程实验中心魏金川在薄片制备方面给予帮助,吴建国在采集样品时给予帮助,中国地质科学院地质研究所施斌博士在观察扫描电镜时给予帮助,在此一并表示衷心的感谢.同时特别感谢两位审稿专家对改善该文章提出重要建议!

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