大陆岩石圈流变与地震波速各向异性

岩石的流变决定了地球各时空尺度的变形,是理解大陆岩石圈构造演化的关键.岩石流变学的研究主要通过高温高压流变学实验和天然变形岩石的多尺度观测来实现,目前已经积累了大量的数据.本文总结了岩石圈不同深度主要造岩矿物,包括橄榄石、斜方辉石、单斜辉石、角闪石、斜长石、石英和云母的流变机制、组构类型以及地震学性质;介绍了岩石圈地幔橄榄岩、榴辉岩、基性麻粒岩、斜长角闪岩和长英质岩石的高温高压实验和天然变形观测进展,包括流变学强度和行为、地震波速和各向异性等;以青藏高原为例,讨论了岩石流变学研究在解译地震波速各向异性的定量化约束作用.将矿物变形组构与地震波各向异性相结合,有望在岩石圈流变学机制和结构的研究中取得重要突破.     

先存组构对各向异性岩石流变强度的影响

地壳深部岩石普遍存在变形组构,花岗质岩石中的变形组构不仅影响岩石强度,而且对后期变形具有显著控制作用。近年来,先存组构对各向异性岩石的流变强度影响成为高温高压实验研究的热点之一。文中对前人给出的各向异性岩石(包括云母片岩-片麻岩、石英-钙长石均匀混合体与层状组构样品)半脆性-塑性流变实验数据进行了重新整理与分析,结合作者开展的不同组构条件下花岗片麻岩与糜棱岩流变实验结果,讨论了先存组构对各向异性岩石流变强度的影响。实验数据表明:1)各向异性岩石的面理与最大主应力方向的角度是影响强度的主要因素。在半脆性破裂域,样品压缩方向垂直于面理(PER)和平行于面理(PAR)的强度基本相同,在压缩方向与面理呈30°夹角时,岩石破裂强度最小;在塑性流变域,垂直于面理方向的强度显著高于平行于面理方向的强度,当面理与最大主应力方向的角度为45°时,岩石强度最小。2)后期变形对原有组构的继承与改造程度,决定了各向异性岩石强度高低。3)样品中矿物的含量、分布与粒度对各向异性岩石强度有显著影响。理论模型预测结果与云母片岩实验结果比较吻合,但其他类型各向异性岩石的流变比理论模型结果要复杂得多。因此,进一步开展具有先存组构的各向异性岩石的流变实验,并将实验变形与实际地质条件下更为复杂的岩石变形进行对比分析,是认识各向异性岩石流变和变形机制最有效的方法。     

基性岩流变实验揭示出大陆下地壳流变的复杂性

基性麻粒岩流变实验研究对认识大陆下地壳流变、板内构造变形和强震孕育等具有重要意义.大量基性岩流变实验数据表明,基性麻粒岩流变实验研究集中在单相矿物和两相矿物集合体,对天然基性麻粒岩流变研究很有限.基性麻粒岩流变实验结果的影响因素众多,除了实验条件(如温度、压力、应变速率)外,实验样品的矿物组分、样品颗粒粒度、微量水、熔体、矿物反应等都会影响其流变特性,这导致实验结果的复杂性.因此,根据单相或两相矿物集合体流变讨论大陆下地壳流变结构时,存在很多不确定性,不能满足下地壳流变研究的实际需求.而根据端元组分流变参数和经验方程估计由多相矿物组成的麻粒岩的流变强度,只是一种简单近似,无法完全取代麻粒岩流变实验研究.因此,开展多相矿物基性麻粒岩的流变实验,以获得更接近真实大陆下地壳流变的实验数据,并且用于定量研究下地壳流变,是大陆下地壳流变实验未来发展的方向.在基性麻粒岩流变实验研究中存在的科学问题与技术难题是多相矿物流变、高温部分熔融的影响、矿物反应-流变相互作用,这些问题都有待通过高温高压流变实验进行深入研究.     

组构对花岗片麻岩高温流变影响的实验研究

深部岩石先存的变形组构对流变特性影响的实验研究是新的研究热点之一,然而目前相关的实验研究非常有限.本文利用3 GPa固体介质熔融盐三轴高温高压容器,选择华北克拉通北部辽东拆离断层带中具有变形组构的花岗片麻岩样品,在温度600~840℃、围压800~1200 MPa、应变速率1×10^-4~2.5×10^-6/S条件下,对不同组构方向的样品(实验压缩方向分别垂直和平行花岗片麻岩的面理)开展高温高压流变实验.实验结果表明,在相同的应变速率和温度条件下,垂直面理的岩石强度比平行面理的岩石强度要高.两组实验样品在600~700℃时,应力指数平均值为6.5,为半脆性流变;在800~840℃时,应力指数平均值为2,垂直面理样品的激活能为Q=380 kJ/mol,平行面理样品的激活能为Q=246.4 kJ/mol,以塑性变形为主,局部存在黑云母和角闪石的脱水熔融.微观结构研究表明,垂直面理的样品,在变形过程中形成了新的变形条带,把原有的面理破坏改造;而平行面理的样品,在实验变形过程中新的变形带主体继承了原有组构.EBSD分析显示花岗片麻岩原岩中石英轴极密区位于Z轴附近,为底面滑移;压缩方向垂直面理的样品,石英组构轴极密区位于X轴附近,为柱面滑移;压缩方向平行面理的样品,石英组构轴极密区位于Z轴附近,伴有少量的Y轴极密,底面滑移和柱面滑移.这表明垂直面理的样品中石英变形改造比平行面理的样品更彻底,这与微观结构分析结果一致.显然实验样品的非均匀组构对样品强度和石英轴定向等具有显著影响,但对样品的脆塑性转化和塑性变形机制没有实质影响,这对理解地壳深部普遍存在的形态各向异性岩石流变具有重要参考价值.     

天然麻粒岩高温流变实验研究

本实验在气体介质三轴高温流变仪上,采用怀安瓦窑口麻粒岩,在温度900～1200℃、围压300 MPa、应变速率10~(-4)～10~(-6)/s条件下,开展高温流变实验.实验样品麻粒岩由斜长石(52%)、单斜辉石和斜方辉石(40%)、石英(3%)、磁铁矿和钛铁矿(5%)组成,矿物平均粒度为:斜长石294μm、单斜辉石和斜方辉石282μm、石英97μm、磁铁矿和钛铁矿109μm.利用傅里叶变换红外光谱仪分析获得变形后样品的水含量约为0.17±0.05wt%.实验样品的强度随温度升高而降低,随应变速率降低而降低.基于力学数据,采用稳态流变方程,获得实验样品在900～1000℃时的应力指数为8.1～12.9,在1050～1150℃时的应力指数为4.8～5.8,平均值5.2.应力指数随着温度升高而降低.显微结构和成分分析表明,在900℃时麻粒岩出现矿物压扁与定向拉长特征,样品以位错滑移和微破裂变形为主;在950～1000℃时,麻粒岩样品中颗粒边界变得圆滑,表现出位错攀移特征,辉石和磁铁矿边缘出现微量熔体;在1050～1200℃时麻粒岩出现部分熔融,而且随着温度和实验时间(应变)增加,熔体含量增加,熔体结晶出微粒斜长石、辉石和橄榄石,部分辉石通过固体反应生成橄榄石.颗粒边界熔体和矿物反应促进了扩散作用,导致位错攀移和熔体引起的扩散蠕变共同控制了麻粒岩的高温流变.     

大青山高级变质岩中复晶石英条带成因的显微构造和流体包裹体证据

大青山高级变质岩中复晶石英条带是地壳深部变形的特征性显微组构, 记录了地壳深部变质变形作用信息. 边界平直的复晶石英条带平行片麻理分布、延伸稳定, 穿切周围其他矿物颗粒,内部包有透镜状、定向排列的细粒围岩矿物颗粒, 条带末端经常出现分叉现象. 流体包裹体分析结果表明, 复晶石英条带中流体包裹体类型复杂, 分布不均, 它们无论在成分和物理条件方面与麻粒岩相条件下捕获的包裹体明显不同. 依据显微组构和流体包裹体分析, 复晶石英条带是富SiO₂流体沿着平行片麻理的显微裂隙充填形成的, SiO₂来源于变形岩石内部. 流体存在对岩石流变特征、破裂作用和石英条带形成起着重要的作用.     

地壳主要岩石流变参数及华北地壳流变性质研究

岩石流变参数和变形机制是根据断层摩擦和岩石幂次流动本构关系建立岩石圈强度剖面的基础。近 30年来 ,高温高压实验取得了很大进展 ,获得了大量地壳矿物和岩石流变资料。本文系统总结了这些流变实验资料 ,并应用流变数据结合地震震源深度分布 ,对华北地壳流变性质进行了研究。结果表明 ,以花岗岩和低级变质岩为代表的上地壳为脆性破裂 ,其强度受断层摩擦约束 ,以长英质片麻岩为主的中地壳和以中性麻粒岩为主的下地壳上层处于塑性流变状态 ,由干的基性麻粒岩组成的下地壳下层处于脆性向塑性流变的过渡状态。华北地壳的这种物质组成和流变为地壳不同层次的解耦和强震孕育提供了力学条件 ,也构成了不同尺度块体的底边界     

华北太古宙长英质岩石的地震波速度及其在地壳中的位置.

研究了两项内容:① 在室温高压和高温高压条件下,华北地区长英质结晶岩的压缩波速度;② 在常压和石英-相变温度下,长英质岩石的加热声发射．结果表明:在相同的温压条件下,长英质结晶岩的波速明显低于基性岩,而与变质相的关系不大;含石英岩石的波速曲线具有与其它任何岩石不同的曲线形态,显示了-相反应峰;在常压下加热岩石至石英相变点时,长英质深变质岩和岩浆花岗岩产生声发射,另一种交代成因的花岗岩不产生声发射．其地质意义表明:结合其它地学资料解释了华北克拉通区的地壳组成,阐明了石英和石英的存在空间;说明了新生代构造沉降区下地壳岩石中石英的-相变以及角闪石和黑云母的脱水熔融是沉降区和隆起区间断裂活动（张性断裂）和地震活动的动力．      

组构对花岗质岩石流变影响的实验研究

正华北克拉通减薄是近年来地学界的热点问题,拆离断层形成与地壳伸展减薄是华北克拉通岩石圈减薄的浅部响应,而拆离断层内岩石的流动及变形机制直接受控于岩石流变学状态。对于华北克拉通地壳流变研究,流变实验和天然样品流变分析多数集中在下地壳,对于与拆离断层对应的中上地壳流变实验研究比较少。目前为止,有关于地壳伸展和拆离断层形     

岩石磁组构对剩磁稳定性的制约探讨:以印支地块中生代碎屑岩和拉萨林周盆地设兴组红层为例

岩石磁组构因能提供磁性矿物晶体形状、排列方式等赋存信息而被广泛应用于判别岩石剩磁是否受到了后期构造应力的显著影响;但常规岩石磁化率各向异性(AMS)是否能够准确限定岩石剩磁的稳定性,目前尚无深入探讨.本文以印支地块Nakhon Thai盆地中生代Nam Phong、Phu Kradung和Phra Wihan组三套碎屑岩样品及拉萨地块林周盆地设兴组红层样品为例,通过岩石磁化率组构和剩磁组构的对比分析发现,尽管Nam Phong组绝大多数样品和Phu Kradung组全部样品的AMS组构显示其具有铅笔状至强劈理过渡型构造变形组构特征,但高场等温剩磁各向异性(hf-AIR)显示其高矫顽力赤铁矿所携带的特征剩磁组构仍具有典型沉积组构特征,表明其以赤铁矿为主的载磁矿物未遭受后期构造应力的显著影响,仍然能够准确记录岩石形成时期的古地球磁场方向.另一方面发现有且仅有剩磁组构才是判别碎屑沉积岩特征剩磁是否遭受了后期构造应力影响的充分必要条件.也就是说,如果岩石剩磁组构(如以赤铁矿为主要载磁矿物的岩石hf-AIR组构)指示其原始沉积组构已被构造组构显著叠加或取代,则必然说明该岩石剩磁方向已受到构造应力...     

中下地壳温度压力条件下岩石半脆性蠕变的实验研究

本文着重介绍了在中下地壳温度、压力条件下进行的几种岩石半脆性蠕变的实验研究结果。通过对这些岩石的蠕变曲线、蠕变类型、蠕变速度和时间、蠕变应变的关系、稳态蠕变速度-流动应力和蠕变破坏特征的观测、分析,阐述了地壳岩石半脆性蠕变的基本规律,并在温度、围压-流动应力座标系中给出了岩石半脆性蠕变的分区     

δ objects as a gauge for stress sensitivity of strain rate in mylonites

Our understanding of the flow properties of deforming rocks in the Earth's lithosphere is mainly based on theoretical work and on the extrapolation of high-strain-rate experimental data to the low strain rates of rock deformation in nature. The geometry of structures in naturally deformed rocks can be an additional source of information on the rheology of the lithosphere. Flow experiments show that the geometry of a mantle of recrystallised material around a rigid object can be used to distinguish between a linear or power-law relation of differential stress and strain rate in strongly deformed rocks such as mylonites. This means that it is possible to use geometrical patterns in deformed rocks to constrain aspects of the rheology and to check the applicability of theoretical models and the extrapolation of laboratory data on creep in rocks to nature.     

花岗岩非稳态流变实验

脆塑性转化带对于研究岩石圈变形、断层强度和变形机制以及强震的孕育和发生具有重要意义。文中采用汶川地震震源区彭灌杂岩中具有代表性的细粒花岗岩样品,在固体压力介质三轴实验系统上开展了高温高压非稳态流变实验研究。实验设计模拟了汶川地震区地壳10~30km深度的实际温度和压力,温度为190~490℃,压力为250~750MPa,应变速率为5×10^-4s^-1,利用扫描电镜对实验样品进行微观结构观察。实验力学数据、微观结构及变形机制分析表明,在相当于地壳浅部10~15km深处的低温低压条件下,表现为应变强化,样品具有脆性破裂-半脆性流动的变形特征;在相当于地壳15~20km的深度条件下,随着应变量增加,应力趋于稳态,样品具有脆塑性转化特征;在相当于地壳20~30km的深度条件下,样品具有塑性流动特征。当样品处于半脆性域时发生非稳态流变,主要变形机制为碎裂作用,同时激活了动态重结晶作用、位错蠕变等塑性变形机制。样品强度随着深度不断增大,在深度为15~20km时达到极大值,深度为20~30km时强度逐渐减小。因此,花岗岩的强度随深度的变化规律与微观结构及变形机制均表明,在实验温度和压力条件下,花岗岩具有非稳态流变特征,在15~20km深处,龙门山断裂带处于脆塑性转化带,花岗岩强度达到最大值,该深度与汶川地震的成核深度一致,显示出彭灌杂岩的强度和变形对汶川地震的孕育和发生具有控制作用。     

Structural superplastic creep and linear viscosity in the earth's mantle

The rheology of dry polycrystalline olivine is examined by adopting a hyperbolic sine flow law (which reduces to a power law below 3 kbars) for high stress behavior, and a model for diffusion accommodated, coherent, grain boundary sliding (structural superplastic creep) for low stress behavior. The model for superplastic creep gives a linear relation between stress and strain rate and is consistent with the behavior of polycrystalline olivine during ductile faulting experiments (Post, 1973). For any given stable grain size, linear superplastic creep is promoted by relatively low stress and temperature. For a 1 -cm grain size and a homologous temperature between 0.6 and 0.8, superplastic creep dominates below transition stresses between 402 and 25 bars, respectively. Transition stresses are higher for smaller grain size and lower temperature. If grain size is stress dependent, superplastic creep is non-linear and dominates above a stress of 300 bars. Below that stress, relatively lower temperatures promote superplastic creep. Grain size may be stabilized by either physical or kinetic inhibition of grain growth, thereby allowing linear superplastic creep in the mantle. Results suggest that superplastic creep can dominate in most of the upper mantle except possibly for the asthenosphere where homologous temperatures are maximal and hyperbolic sine law creep can dominate. Mantle diapirism is at least in part accomplished by superplastic flow above and along the margins of the rising diapir.     

Thermally induced brittle deformation in oceanic lithosphere and the spacing of fracture zones

Brittle deformation of oceanic lithosphere due to thermal stress is explored with a numerical model, with an emphasis on the spacing of fracture zones. Brittle deformation is represented by localized plastic strain within a material having an elasto-visco-plastic rheology with strain softening. We show that crustal thickness, creep strength, and the rule governing plastic flow control the formation of cracks. The spacing of primary crack decreases with crustal thickness as long as it is smaller than a threshold value. Creep strength shifts the threshold such that crust with strong creep strength develops primary cracks regardless of crustal thicknesses, while only a thin crust can have primary cracks if its creep strength is low. For a thin crust, the spacing of primary cracks is inversely proportional to the creep strength, suggesting that creep strength might independently contribute to the degree of brittle deformation. Through finite versus zero dilatation in plastic strain, associated and non-associated flow rule results in nearly vertical and V-shaped cracks, respectively. Changes in the tectonic environment of a ridge system can be reflected in variation in crustal thickness, and thus related to brittle deformation. The fracture zone-free Reykjanes ridge is known to have a uniformly thick crust. The Australian-Antarctic Discordance has multiple fracture zones and thin crust. These syntheses are consistent with enhanced brittle deformation of oceanic lithosphere when the crust is thin and vice versa.     

Dating the lower crust by ion microprobe

Ion microprobe UThPb ages of zircons from granulite facies lower crustal xenoliths from north Queensland, Australia, correlate well with the ages of major orogenic episodes manifest at the earth's surface. About half of the xenoliths contain Proterozoic zircons which are similar in age to the episodes of high-grade metamorphism of the older surface rocks. All the xenoliths contain late Paleozoic zircons which show a real 100 Ma range in²⁰⁶Pb²³⁸/U ages (from 320 to 220 Ma), which is attributed to granulite facies metamorphism followed by slow cooling in the deep crust. The Paleozoic zircon ages coincide in time with the prolonged episode of eruption of voluminous felsic ash-flows and intrusion of high-level granites in this region (320-270 Ma). Mineral and melt inclusions in the zircons provide clues to the origin of some of the xenoliths, and coupled with the age information, can be used to infer the geological processes operating in the lower crust. The zircons from two mafic xenoliths contain felsic and intermediate melt inclusions implying at least a two-stage history for these rocks, involving either partial melting of a more felsic protolith or crystal accumulation from an evolved melt. Some of the zircons from the felsic xenoliths contain CO₂-rich fluid inclusions, indicating that those zircons grew during high-grade metamorphism. The isotopic and chemical data for the whole rock xenoliths show that they originate from a segment of the lower crust which is a heterogeneous mixture of supracrustal and mafic, mantle-derived, lithologies. The major orogenic event responsible for the formation of that crust occurred in the late Paleozoic, when Proterozoic supracrustal rocks were emplaced into the lower crust, possibly along thin-skinned thrust slices. This was accompanied by intrusion of high-temperature, mantle-derived melts which caused partial melting of pre-existing crust. The most likely setting for such tectonism is a continental margin subduction zone.     

A preliminary model for 3-D rheological structure of the lithosphere in North China

3-D rheological structure is mainly the spatial distribution of lithospheric strength or viscos-ity, its strength and viscosity are indispensable parameters in quantitative study of the lithosphere deformation. Plate tectonics theory initially divided the…     

磁铁矿岩的流变与磁化率各向异性

在高温高压及慢应变率下使磁铁矿岩样品发生了流变。对此过程前后的磁性测试表明,产生了与流变过程相关的磁化率各向异性。进一步研究说明,这种“流变磁效应”,主要是由于岩样内磁性矿物颗粒在流变过程中,由大致等粒形且方向性不明显而转变为扁椭球形且具有了一定优选方位而造成的。地震孕育是在高温、高压及缓慢的时间过程中进行的,因此研究流变带来的物性变化对于地震前兆的认识至关重要。本实验结果从一个新的角度探索地震磁效应,具有理论和实际意义。