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大洋板块俯冲带地震波各向异性及剪切波的成因机制

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卷(Volume)35,期(Number)4,总(SUM)131 页(Pages)628~647,2011,11(November,2011) 大地构造与成矿学 Geotectonica et Metallogenia 大洋板块俯冲带地震波各向异性 及剪切波的成因机制 孙圣思 ,嵇少丞 (1.加拿大蒙特利尔大学工学院民用、地质、采矿工程系,蒙特利尔H3C 3A7;2.中国地质科学院地质研究 所,国土资源部动力学重点实验室,北京100037) 摘要:大洋板块俯冲带是许多重要地质作用(例如脱水、部分熔融、岩浆和地震活动)发生的场所。对位于俯冲带 之上的地震台站所检测到的不同剪切波的数据解析,可以获得源于上覆板块、地幔楔、俯冲板块和板下地幔的地震 波各向异性的关键信息。本文系统总结了世界各地大洋俯冲带的剪切波样式,对目前国际上流行的大洋俯冲 带的地震波各向异性的主要成因模式(例如地幔楔拐角流、与海沟迁移有关的平行海沟的地幔流、橄榄石位错蠕变 形成各类组构以及蛇纹石化的影响等)进行了较为详尽地评述。由橄榄石(010)[100]、(010)[001]、(100) [001]、{0kl}[100]、(001)[100]和{110f[001]位错蠕变形成的晶格优选定向(LPO)分别称之为A型、B型、c型、 D型、E型和F型组构,其中A型、D型和E型组构总是导致剪切快波的偏振方向( )平行于地幔流的方向,而B 型组构则导致(b垂直于地幔流的方向。c型组构虽然也能使 平行于地幔流方向,但快慢波之间的延迟时间 (8£)则不如同等条件下A型组构形成的那么大。F型组构导致剪切波在垂直于地幔流动面的方向上传播时几乎 不发生。叶蛇纹石是俯冲板块地幔和地幔楔中最主要的含水矿物,具极低的流变强度、很低的地震波速和很 大的弹性各向异性。蛇纹石化程度越高,变形地幔岩的各向异性就越大,则弧前地幔楔的剪切波愈强。只要 蛇纹石的含量超过10%~20%,则变形地幔岩的地震波各向异性特征将由蛇纹石的LPO主导。地幔楔的剪切波 特征主要取决于其蛇纹石化程度与俯冲角度,陡倾的俯冲和高程度的蛇纹石化有利于形成平行于海沟的 。 关键词:大洋俯冲带;地震波各向异性;剪切波;橄榄石组构;海沟迁移;蛇纹岩化;地幔楔 中图分类号:P541;P315.2 文献标志码:A 文章编号:1001—1552(2011)04—0628—020 0 引 言 在各向异性介质中传播的剪切波(S波)会 成两个偏振方向相互垂直、速度不同的子波,速度 快的是快波( ),速度慢的是慢波(Vs ),两者之 间的走时差或称延迟时间(8t)是剪切波穿越途径上 介质的弹性各向异性的度量。快波的偏振方向( ) 定的几何关系。 和8t是目前量化地壳和上地幔 剪切波各向异性的两个重要参数。 板块俯冲带不仅是地球上构造活动极为活跃的 地区,而且是相变、脱水、部分熔融、岩浆作用和地 震活动的重要场所,因而理所当然地成为剪切波分 裂研究的重要对象。通过分析俯冲带之上地震台站 所检测到的不同剪切波数据可以区分各向异性的来 平行于该介质中与剪切波传播方向垂直的面上的速 度最快的方向,与有限应变椭球主轴( ,y,Z)或 主切面(例如构造线理和面理或剪切面)方位具一 收稿日期:2011—07—08 源:上覆板块、地幔楔、俯冲板块或俯冲板块之下的 地幔(图1)。 项目资助:加拿大自然科学与工程委员会(NSERC)和中国地质调查局地质调查项目基金资助。 第一作者简介:孙圣思(1985一),女,博士研究生,主要从事岩石物理、构造地质学研究。 通讯作者:嵇少丞,Email:sji@polymt1.ca 632 犬’斟挝二j宙i矿il} 第35卷 2000),这与一些弧后地区的剪切波的资料吻 合,如汤加一克马德克俯冲带的西部(Fischer and Wiens,1996)和伊豆一小笠原的弧后地区(Fouch and Fischer,1996)。从理论上说,靠近海沟即接近 地幔楔的楔角顶部(图4),地幔可能会出现滞流, 形成的LPO强度也就弱,因为那里的应变量并不 大,甚至会出现平行海沟的侧向流动,使得 平 行于海沟,形成较为复杂的LPO样式及其随空间位 置的变化,导致4) 和8t 数据的离散。 Buttles and Olson(1998)对拐角流进行了简易 的物理模拟实验,说明板块俯冲角度对地幔楔内矿 物的LPO强度有着较为重要的影响。Fischer et a1. (2000)做过一些数值模拟实验,其结果表明由拐角 流所形成的橄榄石和斜方辉石的LPO样式可以解 释汤加俯冲带之上地幔楔的剪切波的特征。然 而,二维拐角流模型目前尚无法解释某些地区地幔 楔 平行于海沟的特征,甚至对4) 垂直于海沟 的地幔楔的解释也仍需进一步完善。此外,二维拐 角流模式预言的8£ 与板块运动速度呈正相关,这 与实际观测数据的统计结果尚不吻合(Long and Sil— ver,2008)。 3 海沟迁移 Russo and Silver(1994)首次在剪切波的解 释中注意到板块后退(Slab rollback)和海沟迁移 (Trench migration)的影响(图5),认为它们是引发 平行海沟的地幔流(图4)、形成南美洲平行于海沟 走向的 的主要因素。对此,Buttles and Olson (1998)还进行了物理模拟实验,实验结果证明俯冲 板块后退导致的平行海沟的地幔流足以形成所观察 到的平行海沟的 ,他们预言地震波速各向异性 的强度与海沟的总迁移量有关。其他学者也对平行 海沟的三维地幔流开展了一系列物理模拟(Kincaid and Grifiths,2003;Funiciello et a1.,2006)和数值 模拟(Piromallo et a1.,2006;Stegman et a1.,2006; Schella ̄et a1.,2007;Becker and Faccenna,2009)。 最近,Long and Silver(2008,2009)又对海沟或俯 冲板块迁移引发的平行于海沟的地幔流的概念做了 进一步的发展和完善。当大洋板块的俯冲角度由缓 变陡(图5a),位于大洋一侧的板下地幔必然受到 俯冲板块的挤压,导致平行海沟的塑性流动,形成 橄榄石[100]方向平行于海沟的强烈的LPO,其结 果导致 平行于海沟走向。大洋板块俯冲角度的 由缓变陡可能是一种较为普遍的地质现象(Hsui et a1.,1990;Houseman and Gubbins,1997),俯冲洋 壳(玄武岩和辉长岩)先转变成密度大的榴辉岩然 后再转变成密度更大的石榴子石岩(Ringwood, 1991;ji and Zhao,1994);在地幔转换带(Transition zoDe)内,橄榄石也会转变成密度更大的尖晶石( 相),俯冲的大洋板块在重力作用下必然要发生由 缓变陡的旋转;另一种地质作用也可以导致平行海 沟的地幔流,即海沟迁移。若俯冲板块后退,则迁 移的板块挤压着板下地幔(图5b),导致 平行于 海沟走向(图4)。若俯冲板块前进,则迁移的俯冲 ————— 图5 俯冲板块旋转(a)、后退(b)和前进(c)的模式示 意图。虚线和实线分别表示变化前后俯冲板块的 位置,箭头表示板块的运动方向 Fig.5 Schematic diagrams of rotating(a),retreating (b)and advancing(C)slabs.Dashed and solid lines indicate slab locations before and after the change,respectively.Arrows indicate the mo- tions of the trench and the subducting slab 634 据。例如,在伊豆一小笠原俯冲带,海沟迁移速率 约为5 cm/a,v,/Vo比值约为1,Long and Silver (2008,2009)把该俯冲带归人海沟迁移主导的类 型,即俯冲板块之上的地幔楔的 平行于海沟, 然而实际观察到的却是垂直海沟。阿留申俯冲带的 海沟几乎静止不动,既不前进也不后退,Long and Silver(2008,2009)将之划归二维拐角流主导的类 型,然而,实际观察的4) 却是平行海沟的(图2)。 此外,世界上大多数俯冲带的海沟迁移速度 总 比板块俯冲速度 要小得多,即V /V <<1,理应 在大多数情况下二维拐角流强于三维地幔流,即垂 直于海沟的 要比平行于海沟的 更为常见, 这个问题尚有待进一步探讨。 另外,Long and Silver(2008,2009)提出的三维 地幔流的可行性的三个必要条件皆有疑问。第一个 条件是,俯冲板块与其板下地幔之间不发生流变学 耦合,这样大洋板块在俯冲过程中才不会拖曳着板 下地幔向着俯冲方向一起流动,板下地幔才能作平 行于海沟的侧向流动。为了满足上述条件,Morgan et a1.(2007)假设在俯冲板块与板下地幔之间存在 着一个流变强度极低的薄层(10~30 km),它可能 是高温的、含水量高的软流圈的物质,被强拖到俯 冲板块与板下地幔之间,该假说还有待证实。三维 地幔流可行性的第二个条件要求在地幔转变带深度 (410 km)或上下地幔边界(660 km)位置存在高强 度的力学阻隔层(例如石榴子石岩层,Ringwood, 1991),低温高强度的俯冲板块能够通过而高温低 强度的板下地幔流却不能通过,这样,地幔流就可 以在俯冲板块迁移的驱赶下作平行于海沟的侧向水 平流动。目前,震源和地震波层析数据已证实俯冲 板块可以穿越转变带和660 km不连续面并插入下 地幔(van de Hilst et a1.,1997;Li et a1.,2008)。 但是,上地幔的板下地幔流能否进入下地幔以及转 变带内地幔流的强度与规模,目前尚无定论,争议 依然很大(Tackley,2008)。三维地幔流可行性的第 三个条件要求,在垂直俯冲带方向的远处存在某种 强大的力量,驱赶着热的、流变强度低的板下地幔 物质向着俯冲带方向运动,靠近俯冲板块时,由于 冷的高强度的俯冲板块的阻挡,地幔流被迫作平行 于海沟的侧向水平流动(Buttles and Olsen,1998; Kincaid and Grififths,2003)。上述神秘的力量或许 就是热一浮力驱动的上升地幔流或地幔柱。据此推 理,靠近洋中脊的南美洲西海岸的板下地幔的8£ 应该大于远离洋中脊的汤加一克马德克一新西兰俯 第35卷 冲带的板下地幔的8£ ,然而,事实并非如此,可 见上述的第三个条件并非必要条件。 4橄榄石B型组构的特殊性 传统的实验资料(Carter and Ave Lallemant, 1970)表明,在实验室应变速率条件下,橄榄石的 位错滑移系随温度升高逐渐由(100)[001]过渡到 {110}[001],然后再变为{0kl}[100],最后到 (010)[100](图7a),这4个滑移系之间相互转变 的临界温度随围压增加而逐渐减小。在正常的上地 幔温压条件下,最流行的橄榄石滑移系应该是 (010)[100],这与世界上许多地方玄武岩或金伯 利岩中地幔包体的橄榄石组构是一致的(Nicolas and Christensen,1987;Mainprice and Silver,1993; Ji et a1.,1994,1996;Saruwatari et a1.,2001)。但 是,Caner and Ave Lallemant(1970)的实验是在固 体围压介质的Griggs装置上完成的,由于当时技术 条件的局限性,差应力的测量精度不够,岩石试样 中甚至还可能存在较大的温度不均匀性,同时水的 影响亦没有得到有效地控制。 近年来,美国耶鲁大学唐户俊一郎(Shun-ichiro Karato)教授领导的研究组(Jung and Karato,2001; Karato,2002;Katayama et a1.,2004;Skemer et a1., 2006;Jung et a1.,2006)针对水对橄榄石LPO类型 的影响做了一系列的实验探讨(图7b),他们认为, 在正常差应力(<350~400 MPa)作用下,随着水含 量的增加,橄榄石的位错滑移系从(010)[I100]先 转变成(001)[100],然后再转变到(100)[001]。 但在高差应力(>350~400 MPa)作用下,橄榄石在 低水含量和中一高水含量的情况下分别出现{0kl} [100]和(010)[001]滑移系。由滑移系(010) [100]、(010)[001]、(100)[001]、{0kl I[100]、 (001)[100]和{110}[001]位错蠕变形成的LPO分 别称之为A型、B型、C型、D型、E型和F型组构 (图7c)。 然而,学界对唐户俊一郎等的结论目前尚存很 大的争议,主要因为他们的实验结果尚未被其他实 验室重复验证(Li et a1.,2003a,b;Couvy et a1., 2004;Li et a1.,2004;Raterron et a1.,2004,2007; Ji et a1.,2007)。例如,美国纽约大学石溪分校的 矿物物理研究所和法国里尔大学固体结构与性质 实验室的研究人员更强调围压对橄榄石位错滑移 系转变的重要性。Couvy et a1.(2004)报道,在围压 636 知斟键二 第35卷 石晶体中波速最大的方向(图3)。所以,这三种类 型中任一种LPO总会使得剪切快波的偏振方向平 行于地幔的流动方向。但是,若在地幔流动过程 中,橄榄石作{110}[001]滑移,[001](中间波速) 的最大集密平行于拉张线理( ),而[100](最大 波速)和[010](最小波速)方向皆形成垂直于拉张 线理的环带,构成F型组构,当剪切波在垂直于地 幔流动面(XY面)方向上传播时几乎不发生。 若在地幔的塑性流动过程中,橄榄石作(100) [001]滑移,[001](中间波速)的最大集密平行于 拉张线理,[100](最大波速)方向垂直于面理, [010](最小波速)方向平行于面理且垂直于拉张 线理,构成C型组构,当剪切波在地幔流动面的垂 直方向上传播时,快波的偏振方向依然平行于地幔 流方向,但是此时快慢波之间的延迟时间就不如同 等条件下A型组构形成的那么大。 橄榄石的B型组构(图7)造成的地震波各向异 性的样式和其他类型组构的明显不同,有必要在此 作重点讨论。B型组构是由(010)[001]位错滑移 造成的,[001](中间波速)方向的最大密集平行于 拉张线理,[010](最小波速)方向垂直于面理, [100](最大波速)方向平行于面理且垂直于拉张 线理。如果地幔作近乎水平的流动,则最小波速近 乎垂直,而在水平面上最大波速则垂直于地幔流动 方向。当剪切波在地幔流动面的垂直方向上传播 时,快波的偏振方向就会垂直于而不是平行于地幔 流的方向,这和所有其他类型橄榄石组构造成的地 震波各向异性是截然不同的。 目前最关键的问题是A型与B型组构之间准 确的转变条件,迄今尚不清楚。如果B型组构形成 的必要条件是高水含量(>200×10 H/Si)和高差 应力(>320 MPa,Jung and Karato,2001;Jung et a1.,2006;Karato et a1.,2008),那么我们可进行如 下探讨。在地幔岩的部分熔融过程中,水会优先进 入熔体(Karato,1986;Hirth and Kohlstdt,1996;嵇 少丞等,2008),水在熔体和橄榄石之间的分配系 数是10 :1(Grant et a1.,2007)。所以,部分熔融 程度越高,其难熔的残余组分如橄榄石就愈“干 燥”,也就愈不容易形成B型组构。大洋岩石圈地 幔相对于其下的软流圈地幔经过了更高程度的部分 熔融,洋中脊就是地幔发生部分熔融的场所,大洋 岩石圈地幔可以看成是由经过较高程度部分熔融之 后的残余组分(方辉橄榄岩和纯橄岩)构成的,其中 橄榄石的含水量甚少,所以,B型组构在大洋岩石 圈地幔中是不可能大规模存在的。在岛弧之下,地 幔楔也发生了较为强烈的部分熔融,水优先进入玄 武岩熔体并侵入地壳或喷发到地表,岩浆源区的方 辉橄榄岩中的橄榄石就相对变干,所以也不会形成 B型组构。有的学者认为最有可能形成B型组构的 地区是弧前地幔楔(Katayama and Karato,2006; Kneller et a1.,2005,2007,2008),因为由俯冲板 块脱水作用释放出来的水扩散进入地幔楔内的橄榄 石晶体。然而,这些水也能与弧前地幔楔中的橄榄 岩发生水化反应生成蛇纹石,蛇纹石的流变强度比 橄榄石的低得多(图8,Brodie and Rutter,1987; Escartin et a1.,2001;Hilairet et a1.,2007;Chenak and Hirth,2010),应变将发生局部化并优先集中到 蛇纹石之中。所以,在蛇纹石化橄榄岩中差应力是 不可能很高的,那么需要在很高差应力条件下才能 形成的橄榄石的B型组构也不可能出现在蛇纹石 化的弧前地幔楔中。 羔 一 图8 橄榄石和蛇纹石流变强度的比较及其蛇纹石化的 作用。曲线边上的数字表示应变速率。建图数据 取自Brodie and Rutter(1987) Fig.8 Comparison of flow strengths between olivine and serpentine.Strain rate(in S )is indica- ted for each curve.Dashed curve shows the effect of serpentinization. (Data from Brodie and Rutter。1987) 638 (1)镁橄榄石+顽辉石+水一叶蛇纹石 14Mg2SiO4+20MgSiO3+31 H2O Mg48Si34085(OH)62 (2)镁橄榄石+滑石+水一蛇纹石 6Mg2SiO4+Mg3Si4010(OH)2+9H20—} 5Mg,Si20 (OH) 或18Mg2SiO4+4Mg3Si40l0(OH)2+27H20 Mg48Si34085(OH)62 (3)镁橄榄石+二氧化硅+水一低温蛇纹石 3Mg2SiO4+SiO2+4H2O一2Mg3Si205(OH)4 (4)镁橄榄石+水一蛇纹石+水镁石 2Mg2SiO4+3H2O—Mg3Si205(OH)4+ Mg(OH) 或34Mg2SiO4+5IH20_÷Mg48Si34085(OH)62+ 20Mg(OH) (5)顽辉石+水一叶蛇纹石+滑石 90MgSiO3+45H20 Mg48Si34085(OH)62+ 14Mg3Si40l。(OH)2 (6)铁镁橄榄石+水+二氧化碳一低温蛇纹石 +磁铁矿+甲烷 (Fe,Mg)2SiO +H20+CO2一Mg3Si205(OH)4 +Fe 04+CH4 (7)铁镁橄榄石+水十二氧化碳一低温蛇纹石 +磁铁矿+菱镁矿+二氧化硅 (Fe,Mg)2SiO4+H20+CO2_Mg3Si205(Oil)4 +Fe 304+MgCO3+SiO2 (8)镁橄榄石+水+二氧化碳一滑石+菱镁矿 4Mg2SiO4+H20+5CO2—}Mg3Si40lo(OH)2+ 5MgCO3 (9)滑石+水一低温蛇纹石+二氧化硅 Mg3Si4010(OH)2+H20一Mg3Si205(OH)4+ 2SiO2 (10)滑石+菱镁矿+水一低温蛇纹石+二氧 化碳 Mg3Si4010(OH)2+3MgCO3+3H2O一2Mg3Si205 (OH)4+3CO2 低温的利蛇纹石和纤蛇纹石向高温叶蛇纹石转 变的反应式如下: (1 1)低温蛇纹石一叶蛇纹石+水镁石 17Mg3Si205(OH)4一Mg48Si34085(OH)62+ 3Mg(OH) (12)低温蛇纹石+二氧化硅一叶蛇纹石+水 16Mg3Si205(OH)4+2SiO2 Mg48Si34085(OH)62 +H 0 第35卷 (13)低温蛇纹石一叶蛇纹石+镁橄榄石+水 20Mg3Si205(OH)4一Mg 8Si34085(OH)62+ 6Mg2SiO4+9H20 反应式(1~2)和(1 1)的温压条件见图10,其 他反应式的温压条件尚有待进一步的实验确定。蛇 纹石的脱水反应是式(1~10)水化反应的逆反应。 近年来蛇纹岩对于俯冲带动力学的意义受到了 越来越多的关注,主要原因如下:(1)蛇纹岩具有 很特别的物理性质,例如低的P和s波速(图1 1), 高的地震波各向异性和剪切波(图l2,表1), 以及高的 / 比值或泊松比(Ji et a1.,2002;De・ wandel et a1.,21J03;Watanabe et a1.,2007;Ji et a1.,2009),利用这些特性就可研发出研究俯冲带 特性的地球物理方法(Bostock et a1.,2002;Faceen— da et a1.,2008;Boudier et a1.,2009;Katayama et a1.,2009);(2)蛇纹石作为俯冲带中最重要的水 的载体(~13%,Schmidt and Poll,1998;Ulmer and Trommsdorff,1995),在深部高温条件下通过脱水作 用释放出水,造成地幔楔的部分熔融,形成岛弧的 岩浆作用(Hyndman and Peacock,2003;Wada et a1.,2008);(3)蛇纹岩具有极低的流变强度(图8, Brodie and Rutter,1987;Eseartin et a1.,2001;Hi— lairet et a1.,2007;Chenak and Hirth,2010)和较小 的摩擦系数(Moore et a1.,1996;Moore and Lock— tier,2007),所以俯冲板块蛇纹石化的程度直接影 响俯冲带的热产出、地震活动性以及俯冲板块与地 幔楔之间的力学耦合,蛇纹石化程度高的区段以蠕 滑为主,难以产生强震,热产出也少,甚至影响俯 冲板块内部的热结(Hirauchi and Yamaguchi, 2007)。 大洋板块俯冲到一定的深度,含水矿物(如蛇 纹石、滑石、水镁石)开始脱水,释放出大量的含水 流体,进入地幔楔,一方面会造成地幔楔的部分熔 融,形成岛弧的岩浆作用,另一方面在地幔楔内形 成蛇纹石矿物,在深部主要是叶蛇纹石,但在弧前 地幔楔的浅部,由于那里的温度相对较低,利蛇纹 石和纤蛇纹石也可能稳定存在(Evans,1977, 2004)。已有的地质(如在伊豆一小笠原一马里亚 纳群岛出现大量的蛇纹石泥火山)和地球物理(如 低的地震波速和高的 / 比值)数据表明,弧前 地幔楔的蛇纹石化是一个全球性的普遍现象。例 如,北美洲西海岸的卡斯卡迪亚(Bostoek et a1., 2002;Brother et a1.,2003)、日本西南部(Kamiya and Kobayashi,2000;Seno et a1.,2001)、南美洲的 第4期 孙圣思等:大洋板块俯冲带地震波各向异性及剪切波的成因机制 639 O P=600MPa 0 O 168 (m. { 一● 雠(  . s\舀 m 5.0  ,十蛇纹石 n二 。 -水镁石 纤蛇纹石 绿泥石 滑石 0 2 837 +8.062 -利蛇纹石 R 0 892 4 0【-——----・-----J---—---—-----JL---・----—----l-------・---—-t--------—----J—-------・・---JL---------・-_L_ O.0 0 l 0.2 0.3 0.4 0.5 0.6 0.7 蛇纹石体含量 (b) 5.O 檄榄石 古铜辉石— 4.5 i o cP P=6N00 MPa 1 39 透辉石 4.o 三3.5 3.0 羝n 0 0 0\ 0 。 =一滑石 叶蛇纹石 D 0 水镁石 纤蛇纹石 绿泥石 2.5 2 118 +4.504 o o o l 1.O 利蛇纹石 R一=0.898 2.O ————j—————i—————L—————————— ————— —————L————J————— ———一 0.0 0.1 0.2 0.3 0,4 0.5 O.6 O.7 0.8 0.9 蛇纹石体含量 图l1 地幔岩的P波(a)和s波(b)速度随蛇纹石化程度的增加而呈近线性地减小。波速测量的围压为600 MPa,N为样 品总数。地幔岩的主要矿物橄榄石、古铜辉石和透辉石及其主要水化矿物叶蛇纹石、纤蛇纹石、利蛇纹石、滑石、水 镁石和绿泥石的多晶集合体的波速也标注到图上,以便比较 Fig.1 1 (a)and V (b)as a function of serpentine content for mantle rocks.Seismic velocities were measured at 600 MPa,N indicates sample numbers.Velocities of main rock・forming minerals(olivine,bronzite,and dioposide) and hydrous minerals(antigorite,lizardite,chrysotile,talc,brucite,and chlorite)are also indicated for eom- parison 安第斯山脉中部(Graeber and Asch,1999)和中美 洲的哥斯达黎加(DeShon and Schwartz,2004)的弧 前地幔楔均已发生了蛇纹岩化。 1997),在经受强烈大变形的剪切带(如俯冲板块边 界层)内,上述的剪切面(即俯冲板块边界)和剪切 方向(即板块俯冲方向)分别近乎平行于变形岩石 的面理和拉张线理。一般来说,由橄榄石的A型 LPO形成的地震波各向异性仅为3%~5%,而且具 有正交对称性:最大、中间与最小P波速度分别平 行于有限应变椭球的三个主轴方向,即X(矿物拉 张线理)、Y(平行于面理且垂直于线理)和z方向 (垂直面理)。而由蛇纹石LPO形成的各向异性总 是很强(>10%),且具有轴对称性:最小波速垂直 于面理,但在面理面上波速近乎各向同性。所以, 橄榄石单晶体的P和s波的各向异性分别为 ~23%和一20%(图3,Kumazawa and Anderson, 1969;Ji et a1.,2002),而叶蛇纹石单晶体的各向 异性比这大得多(图l2),其P和s波速的各向异 性分别高达46%和66%(Bezacier et a1.,2010)。 加之,叶蛇纹石流变强度ql ̄d,(图8),极易发生塑 性变形,形成强烈的LPO:(001)面和[100]方向分 别平行于蛇纹岩的剪切面和剪切方向(Katayama et a1.,2009;Hirauchi et a1.,2010;Kern et a1., 蛇纹石含量的增加总是增强变形蛇纹石化橄榄岩在 640 第35卷 5.00\\{ y/ 图12 叶蛇纹石单晶体中 。、 ,、 。和av,(: 一 )的等值线分布图(单位为km/s),下半球赤平投影;图中。和b分 别表示0和b晶轴,C 表示(001)的法线方向。叶蛇纹石单晶体的弹性系数由Bezacier et a1.(2010)测定 Fig.1 2 Seismic velocities of antigorite single crysta1.V口,V V 2 and 6y (=V。l—V 2)are shown in equal area stereo- graphic projection with respect to the crystallographic orientations of a,b,and C ,where C is perpendicular to (001)plane.Elasticity data of antigorite are from Bezacier et a1.(2010) 表1 常温常压下叶蛇纹石单晶体的弹性模量(实验数据来 自Bezaeier et a1.,2010) Table 1 Elastic constants(GPa)and properties of an- 平行和垂直于面理方向上地震波速的差别,即各向 异性。只要蛇纹石的含量超过~20%,则变形蛇纹 石化橄榄岩的地震波各向异性特征将由蛇纹石的 GPa tigorite single・crystal at ambient conditions(Data from Bezacier et a1..2010 c C1 1 LPO主导,在面理上利用地震波速将无法区别 和 l,方向。 C22 C33 C44 C55 Katayama et a1.(2009)在围压1.0 GPa和温度 300—400℃的实验条件下简单剪切了叶蛇纹石多 C66 Cl2 C13 C23 C15 C25 晶集合体,在剪切应变 一2.0时该蛇纹岩的剪切 波各向异性(A )高达32%,这与自然变形的蛇纹 岩的各向异性相当(Kern et a1.,1997;Ji et a1., 2002;Christensen,2004;Wang et a1.,2005;Watan— 2.5 C35 C46 abe et a1.,2007)。由此可见,蛇纹石化必然会对俯 13 Ks(GPa) G(GPa) 68.5 38.5 2.6 6.76 3.83 冲带及其地幔楔的地震波速各向异性大小与样式影 响很大,可惜以前的理论模式往往忽视了蛇纹石化 的贡献。 Density(g/cm ) V. (km/s) (km/s) 例如,在西太平洋的琉球岛弧,剪切快慢波之 642 斟挝启 第35卷 换。物理模拟和实验观察证实大洋俯冲板块上层发 育的张断裂近乎垂直或陡倾(Feccenda et a1., 2008;Jiao et a1.,2000;Ranero et a1.,2005)。每条 断裂的两壁上超基性岩的主要造岩矿物橄榄石和辉 石经水化形成蛇纹石、滑石和水镁石等层状矿物。 Boudier et a1.(2009)利用透射电镜(TEM)和电子背 散射衍射(EBSD)技术研究了Utah-Farallon俯冲带 内超基性岩中叶蛇纹石和橄榄石组构之间的定向关 系,发现了两种拓扑关系:(1)橄榄石的(100)面平 行于叶蛇纹石的(001)面,橄榄石的[001]方向平 行于叶蛇纹石的[010]方向,橄榄石的[010]方向 平行于叶蛇纹石的[100]方向,这样拓扑关系记着: (100)0l∥(001)atg,[001]ol//[010]atg,[010]ol// [100]atg;(2)橄榄石的(010)面平行于叶蛇纹石的 (001)面,橄榄石的[001]方向平行于叶蛇纹石的 嵇 A , t [010]方向,橄榄石的[100]方向平行于叶蛇纹石的 [100]方向,这样的拓扑关系记着(010)ol//(001) atg,[001]0l∥[010]atg,[001]0l∥[100]atg。 其中第一种拓扑关系在俯冲板块中更为常见, 橄榄岩发育A型组构(图7),橄榄石[100]的优选 方向平行于橄榄岩的拉张线理,而张裂隙则垂直于 该线理,裂隙内叶蛇纹石的(001)面平行于裂隙面 生长,故叶蛇纹石的(001)面垂直橄榄石[100]的 优选方向。因为橄榄石的快波极化方向平行其 [100]方向,而蛇纹石的快波极化方向平行于 (001)面方向,所以具第一种拓扑关系的蛇纹石化 橄榄岩的剪切波特征取决于岩石中橄榄石的 LPO强度、蛇纹石的含量及其LPO强度。一般来 说,由橄榄石LPO形成的地震波各向异性仅为3% ~5%,而由蛇纹石LPO形成的各向异性总是很强 (甚至可以高达15%~25%)。所以,只要蛇纹石 的含量超过20%,则蛇纹石化橄榄岩的剪切波 就由蛇纹石的LPO主导。如图13b所示,由蛇纹石 LPO主导的剪切快波偏振方向平行于张裂隙的走 向,俯冲板块上层中这些陡倾的张裂隙又垂直于板 块的总体运动方向或平行于海沟走向,这就很容易 解释为什么剪切快波偏振方向平行于海沟的走向。 地幔楔的地震波各向异性特征应是叶蛇纹石 LPO和A型橄榄石LPO相互竞争的结果。在地幔 楔内的拐角流作用下,橄榄石形成A型组构,理应 使得 垂直于海沟走向。但是,蛇纹石化作用将 改变地幔楔各向异性的样式。例如,占全岩体积约 10%~20%的蛇纹石化作用几乎能全部抵消由橄榄 石LPO形成的各向异性,使整体岩石近乎各向同 性。如果地幔楔橄榄岩中蛇纹石的体积分数大于 20%,且具有前述的第一种拓扑关系(Boudier et a1.,2009),那么地幔楔整体的 则受蛇纹石 LPO的控制。由于蛇纹石的(001)面垂直于海沟走 向,故测量到的 是平行于海沟的。如果上述解 释是正确的,那么 与海沟走向的几何定向关系 就是地幔楔中蛇纹石化程度与分布的度量。 参考文献(References): A A A B B t ( ( C C B Z 第4期 孙圣思等:大洋板块俯冲带地震波各向异性及剪切波的成因机制 643 M A.2003.Seismic evidence for widespread serpentinized forearc upper mantle along the Caseadia margin.Geology, 31:267—270. 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On the Formation of Seismic Anisotropy and Shear Wave Splitting in Oceanic Subduction Zones SUN Shengsi and JI Shaocheng ' (1.D@artement des Ggnies Civil,Gdologique et des Mines,dcole Polytechnique de Montrdal,Montrdal H3 C 3A7, Canada;2.Institute Geology,Chinese Academy Geological Sciences;Key Laboratory 0厂Continental Dynamics, Ministry o厂Land and Resources,Beijing 1 00037,China) Abstract:Subduction zones are critically important regi ons where significant geological processes(e.g.,phase transition,dehydration,partial mehing,volcanism,and seismic activity)take place.Seismic anisotropy formed by different parts of subduction system(i.e.,the overriding plate,the mantle wedge,the subducting slab,and the subslab mantle)can be distinguished by analyzing seismic wave raypaths.Here we provide a state—of—art overview on shear wave splitting patterns measured from global oceanic subduction zones,and on mechanism models[e.g., 2 D corner flow,3 D trench—parallel flow induced by trench migration,olivine lattice preferred orientations(LPO) and serpentinization].OliVine LPOs formed by(010)[100],(010)[O01],(1O0)[001],{0kl 100],(001) [100]and{110}[001]slip systems are identified as A,B,C,D,E and F-type fabrics,respectively.The A, D,and E-type fabrics cause fast polarization directions( )parallel to the mantle flow while formed by B—type fabric is perpendicular to the mantle flow.Olivine C—type LPO also results in a parallel to the mantle flow,but the resultant delay time(8t)is much smaller than that of A—type.F-type fabric results in almost no splitting in the direction normal to the mantle flow plane.In mantle wedge and subducting lithosphere mantle,the most important hydrous mineral is antigorite,which is characterized by extremely low flow strength,low seismic velocities,and high elastic anisotropy.Accordingly,the extensively serpentinized mantle wedge rocks usually have relative high seismic anisotropy and shear wave splitting.If more than 1 0%~20%serpentinization occurs,serpentine LPO would control the seismic anisotropy of the deformed mantle rocks.As the shear wave splitting in mantle wedge de— pends on both the degree of serpentinization and the slab dip,those highly serpentinized and steeply dipped sub— duction systems are more likely to produce a trench—parallel小. Keywords:oceanic subduction zones;seismic anisotropy;shear wave splitting;olivine fabric;trench migration; serpentinizati0n; mantle wedge 

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