磁组构与构造变形

磁组构通常指磁化率各向异性,即AMS（Anisotropy of Magnetic Susceptibility）,是一种重要的岩石组构,是弱变形沉积岩地区灵敏的应变指示计.近年来,AMS在造山带及前陆地区的广泛应用为构造变形研究提供了极大的帮助,同时提升了该方法的理论认识.本文在研读最新相关文献与著作的基础上,结合笔者及研究团队在龙门山地区获得的磁组构研究成果,综述了磁组构在沉积岩地区构造变形研究中的应用进展,并基于现有的研究认识对关键问题进行讨论,提出以下几点认识：（1）磁性矿物分析是AMS研究的关键,应结合多种岩石磁学实验及光学与电子显微构造研究手段展开详细的磁性矿物学分析;（2）磁化率椭球与应变椭球的对应主轴在绝大多数情况下相互平行,但在不同期次、不同种类复杂的磁性矿物组成,或者多期次构造变形的影响下,AMS与应变的关系相对复杂,应比对高场和低温AMS及非磁滞剩磁各向异性（AARM）测试结果,获得不同矿物的优选定向特征,并对获得的组构进行分期;（3）AMS可以揭示造山带及其前陆地区的构造演化历史,并且是分析断层相关褶皱的有限应变特征和变形机制的重要方法,同时也是厘定断裂带变形性状和期次及运动学分析的有效手段;（4）磁组构形成于成岩作用早期或构造变形的最早阶段,能很好地记录褶皱和逆冲作用之前的平行层缩短变形,因此可以揭示同沉积阶段的古构造应力方向.后期足够强烈的构造变形能局部改造或彻底掩盖先存AMS记录,构造流体有关的同构造期结晶矿物或先存矿物的重结晶导致的再定向被认为是其根本原因;（5）斜交磁线理是一种特殊的磁组构类型,反映了区域构造叠加或多期构造变形作用或隐伏斜向逆冲等可能的构造过程,有必要结合多方面的地质证据对其成因作出合理解释.

     

岩浆岩磁组构的实验模拟研究

岩浆岩的模拟实验,由于其实验条件的可控性,能够较好地研究岩浆岩的影响因素及其机理．本文实验表明,岩浆岩的磁组构除与成岩时的流动有关外,还受地磁场及重力场的影响,当流动较强时,岩浆岩的磁线理与流动方向有很好的一致性;当流动较弱时,地磁场方向的影响更大．另外,由于岩浆岩较弱的磁各向异性,以及成岩后期各种因素的影响,使其机理呈现复杂性,主轴方位具有分散性．     

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的作用而发生了显著偏转, 因而不能直接用于构造演化和古地理重建等块体运动学研究.

     

岩浆岩磁组构的实验模拟研究

岩浆岩的模拟实验,由于其实验条件的可控性,能够较好地研究岩浆岩的影响因素及其机理．本文实验表明,岩浆岩的磁组构除与成岩时的流动有关外,还受地磁场及重力场的影响,当流动较强时,岩浆岩的磁线理与流动方向有很好的一致性;当流动较弱时,地磁场方向的影响更大．另外,由于岩浆岩较弱的磁各向异性,以及成岩后期各种因素的影响,使其机理呈现复杂性,主轴方位具有分散性．     

甘肃北山地区岩石磁组构特征及其构造意义

甘肃北山地区岩石磁组构特征是磁各向异性度P值大,反映该区总体韧性剪切变形强烈磁椭球扁率E＞0占优势,磁面理发育,部分样品磁线理发育,反映变形以压扁变形为主;但局部剪切拉伸变形明显,最小磁化率轴投影显示变形主压应力为近南北向为主.矿化蚀变的糜棱岩中P值与金品位呈反相关,说明矿化蚀变的热液作用削弱了岩石中已经形成的磁化率各向异性,反映出矿化蚀变晚于构造变形的时序关系.这与该区处于塔-哈两板块碰撞带的大地构造位置以及碰撞带的变形性质、变形样式、变形动力学和运动学,以及成矿作用的特点是相吻合的.     

岩石磁组构在构造混杂岩带和韧性剪切带研究中的应用——以西天山地区为例

本文以西天山地区为例,讨论了岩石磁组构在构造混杂岩带和韧性剪切带中的作用和功能．指出岩石磁组构的某些参数能反映混杂岩带的变形类型,磁化率椭球体三个轴的方位在混杂岩基质中分布有规律,而在岩块中没有规律．在韧性剪切带中,磁组构的各向异性度Ｐ值绝大部分大于１．１,并有变化．磁面理的产化变化可用来求出韧性剪切带的位移量和变形路径．磁组构的Ｌ—Ｆ参数图可确定韧性剪切带的类型,最小主磁化率的方位可用来确定韧性剪切带的剪切指向．     

用旋转式磁化率仪和卡帕桥对岩样磁组构的比测

磁化率各向异性(AMS)是岩石的普遍特性,它反映岩石磁性矿物的择优取向,即磁组构(Magnetic fabric).近三十年来,磁组构技术逐渐应用于地质和地球物理学,显示广阔的研究前景.测量磁组构的仪器有多种,原理不一,故有必要用同一样品在不同原理的仪器上进行比测,以便确认数据的一致性和可靠性.用黄土、变质岩、玄武岩及掺有铁粉的断层泥制成正方体或圆柱体样品,在卡帕桥(KLY-1和KLY-2)和旋转式磁化率仪(Minisep)上进行比测发现:(1)前者的精度(即重复性)在多数情况下优于后者;2)用旋转式磁化率仪测量之前,必须先测得样品z轴的体磁化率,其标定值取决于厂家在标准样品上标示的数值及被测样品与标准样品的相对体积.就旋转样品测量AMS而言,其标定值应为z轴体磁化率测量标定值的一半.      

大别山双河同构造花岗岩体显微构造与磁组构研究

双河岩体应变分析结果显示, 主应变面发生明显变形, XZ面应变轴比达1.59~2.18, 付林指数K在0.11~0.82. 磁化率各向异性结果显示, 磁面理主体倾向SE, 与岩体宏观面理一致; 磁线理主体向SE倾伏; 磁化率各向异性度P在1.109~1.639; 形态参数T在0.079~0.534; 应变分析与磁化率各向异性分析均表明岩体受到了强烈的挤压变形. 石英C轴组构以沿b轴的高极密为特点, 显示石英以高温的柱面(1010) 〈a〉 滑动为主, 这与赋存在岩体中的高压-超高压岩石及围岩的石英C轴组构明显不同. 综合分析表明, 双河岩体在稍晚于超高压岩石形成的区域挤压背景下侵位, 具有同构造花岗岩变形特点.     

勉略构造带康县区段磁组构特征及其构造意义

野外研究表明勉略构造带康县区段的两期主要构造变形特征:早期以较深层次的韧性变形为主,晚期为较浅层次的脆-韧性变形;表现为早期发育的紧闭褶皱、透入性面理,被晚期的褶皱以及脆性断层所截切、改造.磁组构主要反映晚期构造变形特征,样品磁化率各向异性度(PJ)总体较高,为强变形类型;T、E反映挤压、剪切为主的应变特征,与Flinn图解结果一致;两组磁面理均值为344°∠40°和179°∠61°,在平面上呈共轭状出露,锐角平分线近东西向,暗示了构造带内南北向挤压伴随着近东西向的走滑;磁线理倾角总体较小,近东西向展布;高角度磁面理以及占优势的低角度磁线理指示了挤压兼走滑的变形机制,部分高角度磁线理与逆冲作用有关.最小磁化率显示出晚期近南北向的应力方向.结合野外构造解析认为,晚期挤压兼右行走滑构造变形反映西秦岭逆冲推覆构造在碧口地块的阻挡下发生右行斜向逆冲,并暗示碧口地块相对向西逃逸.     

阿尔金断裂带西段磁组构特征及其构造意义

变形岩石的磁组构参数Kmax、Kint、Kmin、P、T、F、L、E等可以用来定量地表征构造变形的形状及期次.本文通过对阿尔金断裂带（郭扎错—空喀山口段）中岩石磁组构特征分析,认为该断裂带具多期活动性,变形性状由早到晚依次表现为韧性、韧-脆性及脆性变形,应力机制为剪切以及带有剪切性质的拉伸和压扁,主应力方向为NNE-SSW和近SN向.磁组构特征还表明该断裂带两侧断块相对差异运动在不同地区有所不同,而且它们所经历的构造期次以及各期活动的应力机制、影响程度也有明显区别.此外,磁组构数据显示阿尔金断裂带具有中间变形强、向两侧变形逐渐减弱的准对称特点,其早期变形具有由东往西逐渐减弱的变化规律.由磁组构揭示的应力应变特征与野外露头、显微构造和古应力测量结果一致.     

北京房山岩体的磁组构特征及其对岩体侵位的约束

岩浆岩的磁组构与岩浆流动、侵位时应力和冷凝后遭受后期构造改造作用有关.本文分析了北京房山岩体东山口—凤凰亭剖面岩石磁化率各向异性(AMS)变化.热磁曲线和磁滞回线分析显示,岩石的主要载磁矿物为多畴磁铁矿颗粒.AMS磁化率椭球体呈压扁状,磁面理发育且产状陡倾,磁化率各向异性度P值平均值高达1.189.自边缘相(东山口)到中心相(凤凰亭),岩石磁化率椭球的扁率E值和形状因子T值逐渐减小,说明磁组构主要反映岩浆流动和岩体侵位时近NNE-SSW方向的挤压作用,在侵位冷凝后并没有遭受到明显的构造改造.     

Magnetic fabric of Plio-Pleistocene sediments from the Crotone fore-arc basin: Insights on the recent tectonic evolution of the Calabrian Arc (Italy)

Low-field anisotropy of magnetic susceptibility (AMS) analyses were performed on 532 samples collected in 36 (mostly lower Pliocene to lower Pleistocene) marine clay sites from the Crotone basin, a fore-arc basin located on top of the external Calabrian accretionary wedge. The Crotone basin formed since mid-late Miocene under a predominant extensional tectonic regime, but it was influenced thereafter by complex interactions with NW–SE left-lateral strike-faults bounding the basin, which also yielded post-1.2 Ma ∼30° counterclockwise block rotations. The basin is filled by continental to marine sediments yielding one of the thickest and best-exposed Neogene succession available worldwide. The deep-marine facies – represented by blue-grey marly clays gave the best results, as they both preserved a clear magnetic fabric, and provided accurate chronology based on previously published magnetostratigraphy and calcareous plankton (i.e. foraminifers and nannofossils) biostratigraphy. Magnetic susceptibility range and rock magnetic analyses both indicate that AMS reflects paramagnetic clay matrix crystal arrangement. The fabric is predominantly oblate to triaxial, the anisotropy degree low (<1.06), and the magnetic foliation mostly subparallel to bedding. Magnetic lineation is defined in 30 out of 36 sites (where the e12 angle is <35°). By also considering local structural analysis data, we find that magnetic fabric was generally acquired during the first tectonic phases occurring after sediment deposition, thus validating its use as temporally dependent strain proxy. Although most of the magnetic lineations trend NW–SE and are orthogonal to normal faults (as observed elsewhere in Calabria), few NE–SW compressive lineations show that the Neogene extensional regime of the Crotone basin was punctuated by compressive episodes. Finally, compressive lineations (prolate magnetic fabric) documented along the strike-slip fault bounding the basin to the south support the significance of Pleistocene strike-slip tectonics. Thus the Crotone basin shows a markedly different tectonics with respect to other internal and western basins of Calabria, as it yields a magnetic fabric still dominated by extensional tectonics but also revealing arc-normal shortening episodes and recent strike-slip fault activity. The tectonics documented in the Crotone basin is compatible with a continuous upper crustal structural reorganization occurring during the SE-migration of the Calabria terrane above the Ionian subduction system.     

秦祁接合带造山缝合带磁组构特征及其构造意义

结合构造及磁化率各向异性研究详细解剖了秦祁接合带唐藏—关子镇—武山和新阳—元龙造山缝合带的应变及岩组特征.41个采点168个构造岩样品的平均磁化率全部较低,磁化率椭球形态分析表明其以平面和压扁应变为主,磁化率各向异性度普遍较高,属强变形岩石组构类型,结合野外观察认为其与变形强度明显正相关.此外,磁化率各向异性参数T、P′可能受岩石类型一定程度的影响.磁化率椭球主轴方位与变形密切相关,提供了丰富的岩组信息.两构造带具有类似的岩组特征,磁面理大致分为呈共轭形态的两组,暗示高应变剪切带在平面上可能以网格状形态出露;高倾伏角磁面理与占优势的低倾伏角、近水平磁线理表明了构造带明显的走滑特征,部分高角度磁线理可能与构造带的挤压和（或）转换挤压相关;磁组方法不能简单用于判别复杂强变形带的运动指向,糜棱面理的复杂变化及Kmin与构造带夹角过高使其判别结果意义不明,而野外及显微构造观察都表明了构造带的右行走滑特征.上述结果表明,沿缝合带大规模的右行转换挤压形成了秦祁接合带反“S”型的平面构造形态,暗示在南北板块拼合过程中,西秦岭诸中、小块体一定程度的向西挤逸.     

Microstructure and magnetic fabric in the Shuanghe pluton: A synkinematic granite in Eastern Dabie Mountains,China

Strain analyses for the Shuanghe pluton show that the main strain planes suffered distinct deformation. The main strain value (XZ) is up to 1.59-2.18, and the value of Flinn index (K) ranges from 0.11 to 0.82. Anisotropy of magnetic susceptibility (AMS) measurements reveal that the orientations of the magnetic foliation and lineation gently dip SE, consistent with the macroscopic foliation of the pluton. The value of anisotropy degree (P) ranges from 1.109 to 1.639, and the shape parameter (7) from 0.079 to 0.534. These studies prove that the pluton was deformed under strong compression. Quartz c-axis textures, defined by monoclinic or triclinic asymmetry, usually developed the high maxima paralleling the b-axis, which is defined by the developed in the high-ultrahigh pressure rocks (UHP) which were captured in the pluton or country rocks. It is concluded that the Shuanghe pluton emplaced under regional compression slightly after the formation of UHP, and it is characterized by synkinematic granitic deformation.     

川滇地块古新统宁蒗组磁组构特征及构造意义

对川滇地块程海断裂附近的宁蒗地区古新统宁蒗组地层进行了详细的磁组构研究,沿战河-西布河布置了22个采点（钻取287块样品）,综合分析表明研究区内存在四种磁组构类型,分别为初始变形磁组构和铅笔状磁组构以及介于上述两者之间的两种过渡型磁组构. 研究区西侧（采点1—9）K1轴优势方向为近NNE-SSW方向,东侧（采点13—22）K1轴优势方向则为近S-N向,K1轴方向的突然变化可能与研究区内的隐伏断层活动有关.另外,磁组构也可以很好判断断层所夹持块体之间的相对旋转运动.将两组K1轴优势方向经过旋转校正之后,发现研究区内中-晚始新世时古应力方向为近E-W向,该应力方向主要与新生代印欧碰撞有关.此外,E-W向的古应力场明显不同于现今的近S-N向的应力场方向,这可能与印欧碰撞后青藏高原从前期的挤压缩短阶段进入到后期的E-W向伸展阶段有关.     

Formation of elongated granite–migmatite domes as isostatic accommodation structures in collisional orogens

The mid-Carboniferous Pelhřimov core complex, Bohemian Massif, is a crustal-scale elongated granite–migmatite dome interpreted to have formed by gravity-driven diapiric upwelling of the metapelitic middle crust. The vertical diapiric flow is evidenced by outward-dipping foliation and lineation patterns, deformation coeval with the widespread presence of melt, rapid exhumation of the dome center from depths corresponding to pressure of about 0.6 GPa to shallow levels (pressure less than 0.2 GPa) within 2 M.y., and kinematic indicators of downward return flow of the mantling rocks. As compared to common diapirs, however, the Pelhřimov complex exhibits a more complicated inferred strain pattern with two perpendicular, irregularly alternating directions of horizontal extension in what is interpreted as the diapir head. Comparison of structural data from migmatites with anisotropy of magnetic susceptibility (AMS) data in granites also reveals that only final increments of strain are recorded in the granites. The map dimensions and gravity image of the complex suggest that the diapiric upwelling affected a large portion of the orogen's interior between two microplates brought together during continental collision. The northwesterly microplate (the upper-crustal Teplá–Barrandian unit) collapsed vertically as an ‘elevator’ at around 346–337 Ma whereas the easterly microplate (Brunia) was underthrust beneath the Moldanubian rocks during ∼346–330 Ma (the indentor). It is suggested that these microplates then acted as cold and rigid margins localizing mid-crustal diapirism and associated voluminous S-type granite plutons inbetween, parallel to the edge of the Brunia indentor.We conclude that bringing together soft metapelitic middle crust with two rigid lithospheric blocks during collision resulted in significant lateral temperature and strength variations across the orogen's interior. A general conclusion from these inferences is that granite–migmatite domes delineating margins of collided microplates may form as crustal-scale structures accommodating late-orogenic isostatic reequilibration.     

Magnetic fabric of bulgarian loess sediments derived by using various sampling techniques

Summary The anistropy of low field magnetic susceptibility has been studied for seven outcrops of loess sediments in North-Eastern Bulgaria. Different sampling methods were applied in order to choose the best technique for obtaining the primary magnetic fabric of such unconsolidated sediments. AMS results show significant changes in the petrofabric of samples collected by the first technique which disturbs the original sedimentary fabric. The second applied technique does not cause such a strong deformation but some disturbance of the magnetic fabric is probable. Typical sedimentary fabric is obtained from hand samples and it is therefore concluded that this represents the best method for obtaining reliable AMS results from soft sediments.     

龙门山南段山前天全-乐山剖面磁组构研究及其对新生代构造变形的指示意义

四川盆地西南部西侧为龙门山冲断带,南面紧挨川西南褶皱带,其新生代构造变形特征对于认识青藏高原东南缘的变形机制具有一定的指示意义.磁组构是一种灵敏的应变指示计,在变形微弱的沉积岩地区尤为适用.本文在雅安-乐山剖面选取12个采样点进行磁组构分析,结合已有的天全-雅安飞仙关剖面的27个采样点数据,综合讨论川西南地区的构造变形特征.所有采样点的磁组构测试结果显示出3种弱变形的磁组构类型:沉积磁组构、初始变形磁组构和铅笔状磁组构.雅安-乐山剖面采样点的磁线理绝大部分为北东-南西走向,和龙门山南段的整体延伸方向一致,表明四川盆地西南缘新生代构造变形主要受控于龙门山的构造作用.飞仙关剖面的磁组构测试结果显示44%的采样点表现出磁线理和地层走向斜交的特征,由初始变形磁组构演变而来,并且所有异常磁组构仅局限在断层上盘,本文认为这是雅安地区新生代期间局部逆时针旋转引起变形叠加的结果.