Magnetic anisotropy and fabric of some foliated rocks from S.E. Australia

Summary The magnetic anisotropy of foliated rocks of several types has been measured by the torque-meter method, and shows that the alignment of long axes of magnetic grains in rocks normally follows the pattern of foliation evident in field observations. In a sharp fold in a lit-par-lit formation the magnetic anisotropy indicated an otherwise undetected lineation independent of the bedding and superimposed upon the foliation determined by the layering. In two adamellites, each with two alignment patterns separated by an angle of 30° the magnetic data are shown to be consistent with two foliations but not with one foliation plus a lineation. Magnetic anisotropy data can be ambiguous for rocks in which two or more grain alignment processes have operated, but combined with other observations magnetic measurements can provide a valuable new tool in the study of rock fabrics.     

New insights into asymmetric folding by means of the anisotropy of magnetic susceptibility,Variscan and Pyrenean folds (SW Pyrenees)

Magnetic fabric allows to unravel the petrofabrics of sedimentary rocks and to assess their deformational history. The use of this technique, in addition to classical structural field observations in the limbs of seven asymmetric folds in the Pyrenees, helps to determine the differences of internal deformation as well as the folding kinematics. Three folds developed during the Variscan Orogeny in Ordovician and Devonian rocks, and four folds developed during the Pyrenean Orogeny in Eocene rocks, are studied. Folds show a variety of structural locations, in different thrust sheets of the Southern Central Pyrenees, different cleavage development, age, geometry and lithology. Sampling follows an equivalent lithological layer in the two limbs, except for one case, of the selected folds. Results show a modified tectonic magnetic fabric in most sites with the magnetic lineation on the tectonic foliation plane. A larger scattering of the magnetic lineation (maximum magnetic anisotropy axis) and a higher intensity of the preferred orientation of minerals (eccentricity of the anisotropy of magnetic susceptibility - AMS ellipsoid) is better observed in the overturned (short) limb of the asymmetric Variscan folds than in the normal (long) limb. On the other hand, the shape parameter in Alpine folds is generally larger in the overturned (short) limb then in the normal (long) one. A good clustering of the minimum magnetic anisotropy axes is observed in all limbs. The combination of the AMS data with the structural data helps to understand and better constrain the deformation degree in these asymmetric folds and to unravel the deformational history.     

Probing crustal anisotropy by receiver functions at the deep continental drilling site KTB in Southern Germany

Seismic anisotropy is a unique observational tool for remotely studying deformation and stress within the Earth. Effects of anisotropy can be seen in seismic data; they are due to mineral alignment, fractures or layering. Seismic anisotropy is linked to local stress and strain, allowing modern geophysics to derive geomechanical properties from seismic data for supporting well planning and fracking. For unravelling anisotropic properties of the crust, the teleseismic receiver functions methodology has started to be widely applied recently due to its ability in retrieving the three-dimensional characteristics of the media sampled by the waves. The applicability of this technique is tested here by a field test carried out around the Kontinental Tiefbohrung site in southeastern Germany. We compare our results to previous investigations of the metamorphic rock pile of the Zone Erbendorf-Vohenstrauss, drilled down to 9 km depth, which sampled an alternating sequence of paragneiss and amphibolite, in which a strong foliation has been produced by ductile deformation. The application of the receiver functions reveals the presence of two distinct anisotropic layers within the metamorphic rock pile at 0–4 km and below 6 km depth, with up to 8% anisotropy; the depth of these two layers corresponds to the location of mica-rich paragneiss which show intense foliation, and finally proves the relation between the signal in the receiver functions, rock texture and presence of cracks. We have now the capability of providing insights from passive seismic data on geomechanical properties of the rocks, useful for geological exploration and engineering purposes, which will help influencing expensive drilling decisions thanks to future application of this seismic technique.     

Anisotropy of magnetic susceptibility in the Scaglia Rossa pelagic limestone

During folding of the Scaglia Rossa limestone in Umbria, Italy, deformation was mainly accommodated by pressure solution cleavage. Fossils between the cleavage planes appear visibly undeformed, yet the limestone possesses a weak magnetic fabric. The maximum and intermediate principal axes of the magnetic anisotropy ellipsoid define a distinct magnetic foliation plane within which a weak concentration of the maximum axes forms a magnetic lineation. Neither of these features is of sedimentary origin. Results from a slumped outcrop, where bedding and a cleavage induced by overburden compaction have different attitudes, show that the magnetic foliation is caused by the compaction. Comparisons with field-derived structural data suggest that the magnetic lineation was produced tectonically during deformation of the Apennine fold belt.     

Magnetic anisotropy of rocks and its application in geology and geophysics

Magnetic anisotropy in sedimentary rocks is controlled by the processes of deposition and compaction, in volcanic rocks by the lava flow and in metamorphic and plutonic rocks by ductile deformation and mimetic crystallization. In massive ore it is due to processes associated with emplacement and consolidation of an ore body as well as to ductile deformation. Hence, it can be used as a tool of structural analysis for almost all rock types. Morcover, it can influence considerably the orientation of the remanent magnetization vector as well as the configuration of a magnetic anomaly over a magnetized body. For these reasons it should be investigated in palaeomagnetism and applied geophysics as well.     

Magnetic susceptibility anisotropy,strain, and progressive deformation in Permian sediments from the Maritime Alps (France)

The relationships among magnetic susceptibility anisotropy, finite strain, and progressive deformation have been studied in Permian red shales and slates of the Maritime Alps (southeastern France). These rocks contain deformed reduction spots which serve as finite strain indicators. The magnetic fabric of undeformed regions is modified during deformation to yield characteristic magnetic susceptibility anisotropy patterns and a magnetic equivalent of the deformation path derived from strain measurements. The magnetic fabric changes progressively from oblate to prolate, and back to oblate as deformation increases. The quantitative relationships between natural strain and magnetic anisotropy in these rocks have been determined. They differ between the less and more deformed areas, perhaps due to a change in deformation mechanism accompanying an increase in metamorphism. The relationships provide a rapid means of strain determination using magnetic measurements but their variation emphasizes the need for local structural control.     

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.     

Duplex of metamorphic units including ultramafic rocks in the central region of Tokunoshima,Southwest Japan

Geological observations in the central part of Tokunoshima in the Amami Islands, Southwest Japan, reveal that discrete layers of serpentinite, dioritic gneiss, and amphibolite are intercalated into pelitic schist and these rock bodies form a northwest‐dipping tectonic stack. A subhorizontal psammitic schist layer overlies them. These rocks underwent ductile deformation that is denoted by penetrative foliation and mineral lineation. Microstructures of the sheared metamorphic rocks and serpentinite indicate top‐to‐the‐east, ‐southeast or ‐south (hanging‐wall up) displacements. The en echelon array of rock bodies is interpreted as a duplex with the psammitic schist layer on its top and the pelitic schist layer on its bottom. It is inferred that the serpentinite‐bearing duplex was formed due to the tectonic erosion and the subsequent accretionary growth operated in a Cretaceous or older subduction zone. Tokunoshima has been considered to belong to the Shimanto Belt. However, regional low‐pressure and high‐temperature type amphibolite‐facies metamorphism and related ductile deformation have not been recognized in the other areas of the Shimanto Belt. There is no metamorphic rock occurrence comparable to that of Tokunoshima in the neighboring islands. The metamorphic rocks in Tokunoshima can be correlated to any of low‐pressure/temperature type metamorphic regions in Kyushu.     

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K_min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.     

韧性剪切带及其变形岩石

本文讨论了地壳和上地幔中韧性剪切带及其中的变形岩石。在大多数情况下,韧性剪切带中的变形岩石为糜棱岩,因为经受韧性剪切变形时,岩石的粒度显著减小并发育了强化的叶理(线理)。但是在某些情况下,例如,当隐晶质灰岩及富含长石的岩石经受韧性剪切变形时,剪切带中的变形岩石粒度局部增大或者没有发生明显减小,它们并不是典型的糜棱岩。由于变形环境、变形介质及变形机制的不同,韧性剪切带内岩石变形的产物是不同的     

Metamorphic,deformation, fluids and geological significance of low-temperature retrograde mylonites of Diancangshan metamorphic massif along Ailaoshan-Red River strike-slip fault zone,Yunnan, China

Diancangshan metamorphic massif is one of the four metamorphic massifs developed along the Ailaoshan-Red River strike-slip fault zone, Yunnan, China. It has experienced multi-stage metamorphism and deformation, especially since the late Oligocene it widely suffered high-temperature ductile shear deformation and exhumation of the metamorphic rocks from the deep crust to the shallow surface. Based on the previous research and geological field work, this paper presents a detailed study on deformation and metamorphism, and exhumation of deep metamorphic rocks within the Diancangshan metamorphic massif, especially focusing on the low-temperature overprinted retrogression metamorphism and deformation of mylonitic rocks. With the combinated experimental techniques of optical microscope, electron backscatter diffraction attachmented on field-emission scanning electron microscopy and cathodoluminescence, our contribution reports the microstructure, lattice preferred orientations of the deformed minerals, and the changes of mineral composition phases of the superposition low-temperature retrograde mylonites. All these results indicate that: (1) Diancangshan deep metamorphic rock has experienced early high-temperature left-lateral shear deformation and late extension with rapid exhumation, the low-temperature retrogression metamorphism and deformation overprinted the high-temperature metamorphism, and the high-temperature microstructure and texture are in part or entirely altered by subsequent low-temperature shearing; (2) the superposition of low-temperature deformation-metamorphism occurs at the ductile-brittle transition; and (3) the fluid is quite active during the syn-tectonic shearing overprinted low-temperature deformation and metamorphism. The dynamic recrystallization and/or fractures to micro-fractures result in the strongly fine-grained of the main minerals, and present strain localization in micro-domians, such as micro-shear zones in the mylonites. It is often accompanied by the decrease of rock strength and finally influences the rheology of the whole rock during further deformation and exhumation of the Diancangshan massif.     

Nano-texture of penetrative foliation in metamorphic rocks

By observing four samples obtained from Jiangxi Province, China, under the scanning electron mi-croscope (SEM), we discovered that nano-particle layers were commonly formed on sliding planes of the penetrative foliation in metamorphic rocks. We also successfully reproduced this phenomenon with a tri-axial pressure experiment. Having gone through the granulitization-alienation-partition in the shear sliding process, the nano-particles (40-95 nm in diameter) display different individual shapes and dis-tinct layered textures. This nano-confinement layer is essentially a frictional-viscous stripe with vis-cous-elastic deformation. In the micro-domain stripe, activities in structural stress field-rheological physical field-geochemical field are very dynamic, corresponding to the three stages (i.e., shear sliding strengthening-weakening-exfoliating) of the foliation development in metamorphism rocks. As such, the viscous-elastic deformation behavior helps shed light on the understanding of the micro-dynamic mechanism of the structural shearing.     

A comparison of cross-hole electrical and seismic data in fractured rock

Cross‐hole anisotropic electrical and seismic tomograms of fractured metamorphic rock have been obtained at a test site where extensive hydrological data were available. A strong correlation between electrical resistivity anisotropy and seismic compressional‐wave velocity anisotropy has been observed. Analysis of core samples from the site reveal that the shale‐rich rocks have fabric‐related average velocity anisotropy of between 10% and 30%. The cross‐hole seismic data are consistent with these values, indicating that observed anisotropy might be principally due to the inherent rock fabric rather than to the aligned sets of open fractures. One region with velocity anisotropy greater than 30% has been modelled as aligned open fractures within an anisotropic rock matrix and this model is consistent with available fracture density and hydraulic transmissivity data from the boreholes and the cross‐hole resistivity tomography data. However, in general the study highlights the uncertainties that can arise, due to the relative influence of rock fabric and fluid‐filled fractures, when using geophysical techniques for hydrological investigations.     

Dielectric anisotropy and fabric of rocks

Summary Measurements have been made of the dieletric anisotropies of a number of rocks for which magnetic anisotropy data have been obtained previously. The purpose was to examine the possible usefulness of dielectric anisotropy as a physical property indicative of rock fabrics. Its advantage over the magnetic method is that it measures an average alignment of crystals of the dominant minerals, whereas magnetic anisotropy is due only to the ferromagnetic grains. Disadvantages are an extreme sensitivity to specimen shape and difficulty in distinguishing the several types of alignment which can give rise to dielectric anisotropy. In a number of strongly foliated rocks the axes of dielectric anisotropy were found to coincide with the axes of magnetic anisotropy. Specimens from a magnesian-pyroxene rich layer in a Tasmanian dolerite sill and from the olivine rich layer of the Palisades dolerite sill, New York, were found to have no systematic anisotropy. The pyroxenes in the Tasmanian dolerite are elongated crystals (about 2: 1) so that the dielectric measurements show that they do not have a preferred horizontal alignment and therefore have probably not settled as individual crystals. Most of the olivines in the Palisades dolerite are more nearly equidimensional so that the absence of measurable anisotropy in this rock is less conclusive evidence against crystal settling.     

MAGNETIC SUSCEPTIBILITY ANISOTROPY OF VARIOUS ROCK TYPES AND ITS SIGNIFICANCE FOR GEOPHYSICS AND GEOLOGY*

In the interpretation of magnetic anomalies and in paleomagnetism, the anisotropy of magnetic susceptibility is commonly neglected. Nevertheless, this property has basic significance, because, owing to susceptibility anisotropy, the directions of the vectors of induced and remanent magnetization are deflected from the direction of the Earth's magnetic field. Almost all rock types investigated possess higher or lower degree of the susceptibility anisotropy. Effusive and sedimentary rocks have the lowest degree of anisotropy. For the latter, the “masking effect” of the paramagnetic mineral components has some influence on the anisotropy degree due to the low mean susceptibility of sedimentary rocks. Metamorphic and plutonic rocks usually exhibit a considerable degree of anisotropy. The highest degree of anisotropy has been found in the rocks containing ferromagnetic minerals with mimetic fabric. The dependence of the degree of the susceptibility anisotropy on the degree of metamorphism proved to be very complicated; of the rock sequence from slates to gneisses, the transient rocks (roofing slates and mica-schist-gneisses) showed the highest degree of anisotropy. This result can be used in geology for reliable determination of these rock types.     

The magnetic fabric of some dolerite intrusions,northeast Spitsbergen; implications for their mode of emplacement

The titanomagnetite fabric of some dolerites from two areas of northeast Spitsbergen, Lomfjorden and the Bastian, and Rønnbeck islands have been studied by means of magnetic susceptibility anisotropy measurements. The shape and orientation of the susceptibility ellipsoids were determined with an a.c. bridge. Both areas showed a dominating magnetic foliation in the horizontal plane. A weakly developed preferred orientation of the maximum (K^I) and intermediate (K^II) susceptibility axis within the magnetic foliation plane was found, the K^I-axis striking predominantly E-W and the K^II-axis striking N-S. The magnetic fabric is interpreted as resulting from horizontal movement of magma. It is concluded that the Bastian and Rønnbeck islands are the relics of one or more larger sills.     

利用含可控裂缝人工岩样研究裂缝密度对各向异性的影响

利用新方法制作出含可控裂缝的双孔隙人工砂岩物理模型,具有与天然岩石更为接近的矿物成分、孔隙结构和胶结方式,其中裂缝密度、裂缝尺寸和裂缝张开度等裂缝参数可以控制以得到实验所需要的裂缝参数,岩样具有真实的孔隙和裂缝空间并可以在不同饱和流体状态下研究流体性质对于裂缝介质性质的影响.本次实验制作出一组具有不同裂缝密度的含裂缝人工岩样,对岩样利用SEM扫描电镜分析可以看到真实的孔隙结构和符合我们要求的裂缝参数,岩样被加工成八面棱柱以测量不同方向上弹性波传播的速度,用0.5 MHz的换能器使用透射法测量在饱和空气和饱和水条件下各个样品不同方向上的纵横波速度,并得出纵横波速度、横波分裂系数和纵横波各向异性强度受裂缝密度和饱和流体的影响.研究发现流体对于纵波速度和纵波各向异性强度的影响较强,而横波速度、横波分裂系数和横波各向异性强度受饱和流体的影响不大,但是对裂缝密度的变化更敏感.     

Track of fluid paleocirculation in dolomite host rock at regional scale by the Anisotropy of Magnetic Susceptibility (AMS): An example from Aptian carbonates of La Florida,Northern Spain

The present study aims to apply the AMS method (Anisotropy of Magnetic Susceptibility) at a regional scale to track the fluid circulation direction that has produced an iron metasomatism within pre-existing dolomite host rock. The Urgonian formations hosting the Zn–Pb mineralizations in La Florida (Cantabria, northern Spain) have been taken as target for this purpose. Sampling was carried out, in addition to ferroan dolomite host rock enclosing the Zn–Pb mineralizations, in dolomite host rock and limestone to make the comparison possible between magnetic signals from mineralized rocks, where fluid circulation occurred, and their surrounding formations. AMS study was coupled with petrofabric analysis carried out by texture goniometry, Scanning Electron Microscopy (SEM) observations and also Shape Preferred Orientation (SPO) statistics. SEM observations of ferroan dolomite host rock illustrate both bright and dark grey ribbons corresponding respectively to Fe enriched and pure dolomites. SPO statistics applied on four images from ferroan dolomite host rock give a well-defined orientation of ribbons related to the intermediate axis of magnetic susceptibility K₂. For AMS data, two magnetic fabrics are observed. The first one is observed in ferroan dolomite host rock and characterized by a prolate ellipsoid of magnetic susceptibility with a vertical magnetic lineation. The magnetic susceptibility carrier is Fe-rich dolomite. These features are probably acquired during metasomatic fluid circulations. In Fe-rich dolomite host rock, ?c? axes are vertical. As a rule, (0001) planes (i.e. planes perpendicular to ?c? axes) are isotropic with respect to crystallographic properties. So, the magnetic anisotropy measured in this plane should reflect crystallographic modification due to fluid circulation. This is confirmed by the texture observed using the SEM. Consequently, AMS results show a dominant NE–SW elongation interpreted as the global circulation direction and a NW–SE secondary elongation that we have considered as sinuosities of the fluid trajectory. The second type of magnetic fabric is essentially observed in the limestone and characterized by an oblate form of the ellipsoid of magnetic susceptibility, a horizontal magnetic foliation and mixed magnetic susceptibility carriers. It is interpreted as a sedimentary fabric.     

从差异形变导至发震来探讨我国大陆强震机理

从密云变质岩的矿物组合和变质相律中,本文揭示出多震层的某些主要特征。在地壳10—25公里深处的不同物性、成分的岩石,彼此具有不同程度的形变。差异形变的机理是按应变椭球体的形成过程来解释的。其中,不同岩石相对的流劈理和破劈理是在同一受力系统下发育形成。当达到差异形变的极限状态时,破劈理可导致产生压剪逆掩断层,激发强震,故差异形变力即是激发强震的驱动力。本文并对缓慢形变和快速发震、震源体构造和岩石作了某些初步探讨     

南海北部陆区岩石磁化率的矿物学研究

基于2517套现场测量资料,245块岩石样品的体积磁化率测量和详细的岩矿鉴定及硅酸盐全分析结果,结合单矿物磁化率特征及各岩石之间的对比研究,发现岩石磁化率主要受组成岩石的矿物磁化率控制.即岩石磁化率(κr)与组成岩石各个矿物磁化率(κ1)及其体积含量(C2)成正比.例如侵入岩磁化率,κr=-5.68×102Cq+2.8...