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1.
Paul F. Williams 《Australian Journal of Earth Sciences》2013,60(3):215-228
The Ordovician rocks exposed along the N.S.W. coast, near Bermagui, comprise a sequence of alternating greywacke and shale and a less abundant sequence of alternating chert and detrital beds. The only lithological boundary that can be mapped is the contact between the two sequences and it sheds little light on the large scale structure. However, due to continuity of outcrop, well‐defined vergence zones and abundant younging evidence it is possible to interpret the regional structure. Two generations of folds (B1 and B2) are recognized and the regional folds, a N/S trending anticlinorium to the east and synclinorium to the west, are interpreted as second generation structures (B2). First generation folds (B1) are refolded by B2 on the limbs of the large B2 structures and are commonly recumbent. In the hinges of the regional B2 folds, B1 axial planes are steeply dipping and the folds instead of being refolded by B2, are more tightly appressed than elsewhere. A model is described to explain these observations. 相似文献
2.
K.C. Burmeister M.J. Harrison S. Marshak E.C. Ferr R.A. Bannister K.P. Kodama 《Journal of Structural Geology》2009,31(9):1028
In moderately to highly strained sandstones, both the long axis of the bedding-parallel finite-strain ellipse, as calculated by the normalized Fry method, and the projection of the long axis of the AMS ellipsoid on the plane of bedding, align well with local “structural grain” (trends of cleavage, folds, and faults). This relationship implies that results of both 2D Fry and AMS analyses represent the local layer-parallel tectonic strain component. Do both methods provide comparable results for very low-strain sandstone (e.g., <5%)? To address this question, Fry and AMS analyses were conducted in very low-strain sandstone from two localities in the Appalachian foreland fold–thrust belt: near Rosendale in New York and the Lackawanna synclinorium of Pennsylvania. We compared the map projections of both bedding-parallel Fry ellipses and AMS ellipsoids to the local structural grain. In both study areas, projections of the long axis of Fry strain ellipses do not cluster in a direction parallel to structural grain, whereas the projection of the long axes of AMS ellipsoids do cluster closely to structural grain. This observation implies that in very low-strain sandstone, AMS analysis provides a more sensitive “quick” indicator of tectonic fabric than does normalized Fry analysis. 相似文献
3.
岩石组构记录了地壳形成与演化的关键信息,提取这些信息对分析和恢复地球动力学过程具有重要意义。磁化率各向异性(AMS)是一种重要的岩石组构方法,可以有效地揭示岩石的应变特征,分析其地球动力学过程,是研究构造变形性质以及应力作用方式的有效手段。本文在梳理AMS的研究历史、主要成果和最新进展的基础上,系统阐述了AMS的基本原理以及在剪切带的应用:①岩石组构具有复杂性,AMS作为一种间接组构手段受控于矿物的物理特性、含量以及变形变质等多方面因素;②AMS可以提供剪切带的运动学以及不同部位应变状态的信息;③对于剪切带,AMS主要受控于磁性矿物(矿物成分和粒度的变化导致全岩磁化率各向异性的变化)、构造变形强度(决定磁线理发展的重要因素)以及流体的作用(流体导致磁性矿物的类型与定向性的变化)。 相似文献
4.
Abstract Two varieties of charnockites are recognized in the Dharwar craton of southern India. The style and sequence of structures in one charnockite variety, and related intermediate to basic granulites, are similar to those in the supracrustal rocks of the Dharwar Supergroup and the adjacent Peninsular Gneiss. This style has isoclinal folds with long limbs and sharp hinges with an axial planar fabric in some instances. Additional evidence of flattening is provided by pinch-and-swell and boudinage structures, with basic granulites forming boudins in the more ductile charnockites/enderbites in the limbs of isoclinal folds. These folds are involved in near-coaxial upright folding resulting in the bending of the axial planes of the isoclinal folds and the associated boudins. All these structures are overprinted by non-coaxial upright folds with axial planes striking nearly N–S. The map pattern of charnockites suggests that this sequence of structures is present not only on a mesoscopic scale, but also on a macroscopic scale. Charnockites of this variety provide, in some instances, evidence of having been migmatized to give rise to hornblende–biotite gneiss and biotite gneiss, which form a part of the Peninsular Gneiss terrane.
The second variety comprises charnockite sensu stricto with an entirely different structural style. This type occurs in the tensional domains of the hinge zones of the later buckle folds, in the necks of foliation boudinage, in shear zones and in release joints parallel to the axial planes of the later folds in the Peninsular Gneiss. Because the non-coaxial later folds are associated with a strain pattern different from, and later than, that of the isoclinal folds of the first generation, it follows that charnockites of the Dharwar craton have evolved in at least two distinct phases, separate both in time and in process. 相似文献
The second variety comprises charnockite sensu stricto with an entirely different structural style. This type occurs in the tensional domains of the hinge zones of the later buckle folds, in the necks of foliation boudinage, in shear zones and in release joints parallel to the axial planes of the later folds in the Peninsular Gneiss. Because the non-coaxial later folds are associated with a strain pattern different from, and later than, that of the isoclinal folds of the first generation, it follows that charnockites of the Dharwar craton have evolved in at least two distinct phases, separate both in time and in process. 相似文献
5.
通过野外观察、室内显微构造分析和磁组构测量方法,在桂北四堡地区浅变质地层中厘定出一条NE30°走向,南东倾,倾角约40°的大型左旋斜冲韧性剪切带——四堡韧性剪切带;该韧性剪切带内发育糜棱岩系列、糜棱面理、拉伸线理、A型褶皱、S-C组构、亚颗粒、显微分层及石英条带等宏观和微观构造特征;磁各向异性度测量结果显示四堡韧性剪切带由一宽约4 km的强应变带及边缘弱带组成,全带宽达10 km,长度超30 km;在对韧性剪切带运动学、构造年代学研究的基础上,结合区域地质资料,认为四堡韧性剪切带是华南加里东晚期华夏地块由南东向北西作低角度斜冲到扬子地块的产物。这一发现揭示了扬子地块与华夏地块碰撞拼合的方式,为深化华南构造演化提供了新资料。 相似文献
6.
The Melechov pluton, Bohemian Massif, is interpreted as a mid-crustal nested granitic diapir with an apical part exposed at the present-day erosion level. The diapir head exhibits a concentric structure defined by lithologic zoning and by the anisotropy of magnetic susceptibility (AMS). In concert with theoretical models, outward-dipping margin-parallel magnetic foliations are associated with oblate shapes of the susceptibility ellipsoids and higher degree of anisotropy, passing inward into weaker triaxial to prolate fabric. By contrast, magnetic fabric in an inner granite unit is in places oriented at a high angle to internal contacts and is interpreted as recording an internal diapir circulation. We use inverse modeling to calculate strain variations across the diapir from the AMS data. The magnetic fabric parameters and calculated strains are in agreement with strain distribution in heads of model Newtonian diapirs traveling a distance of two body radii and suggest granitic magma ascent as a crystal-poor suspension followed by crystallization of fabric markers and their response to strain near the final emplacement level. The intrusive fabric thus formed late but, though generally weak, was still capable of recording incremental strain gradient in the granite diapir. 相似文献
7.
龙门山飞仙关断层传播褶皱磁组构特征及构造意义 总被引：3，自引：0，他引：3
沿龙门山南段冲断前锋带飞仙关断层传播褶铍剖面钻取了270个磁组构定向岩芯样品,对其进行了磁性矿物与磁组构分析。通过等温剩磁和三轴热退磁实验确定了样品中的主要载磁矿物为赤铁矿。磁组构测试结果显示27个采样点的磁组构为中间组构与构造组构两种类型。通过对各点磁组构特征及各项磁组构参数进行详细分析,再结合断层传播褶皱运动学模型,得出断层传播褶皱形成过程中岩石应变及磁组构演化：断层扩展前的平行层缩短作用把原始的沉积组构改造成为中问组构;在断层扩展过程中,两翼地层的旋转抬升产生的简单剪切作用对地层磁化率各向异性产生影响,使得校正的磁化率各向异性度Pj值局部升高,以及在剪切变形强烈的区域形成构造磁组构。 相似文献
8.
Marco Maffione Stefano Pucci Leonardo Sagnotti Fabio Speranza 《International Journal of Earth Sciences》2012,101(3):849-861
Anisotropy of magnetic susceptibility (AMS) represents a valuable proxy able to detect subtle strain effects in very weakly
deformed sediments. In compressive tectonic settings, the magnetic lineation is commonly parallel to fold axes, thrust faults,
and local bedding strike, while in extensional regimes, it is perpendicular to normal faults and parallel to bedding dip directions.
The Altotiberina Fault (ATF) in the northern Apennines (Italy) is a Plio-Quaternary NNW–SSE low-angle normal fault; the sedimentary
basin (Tiber basin) at its hanging-wall is infilled with a syn-tectonic, sandy-clayey continental succession. We measured
the AMS of apparently undeformed sandy clays sampled at 12 sites within the Tiber basin. The anisotropy parameters suggest
that a primary sedimentary fabric has been overprinted by an incipient tectonic fabric. The magnetic lineation is well developed
at all sites, and at the sites from the western sector of the basin it is oriented sub-perpendicular to the trend of the ATF,
suggesting that it may be related to extensional strain. Conversely, the magnetic lineation of the sites from the eastern
sector has a prevailing N–S direction. The occurrence of triaxial to prolate AMS ellipsoids and sub-horizontal magnetic lineations
suggests that a maximum horizontal shortening along an E–W direction occurred at these sites. The presence of compressive
AMS features at the hanging-wall of the ATF can be explained by the presence of gently N–S-trending local folds (hardly visible
in the field) formed by either passive accommodation above an undulated fault plane, or rollover mechanism along antithetic
faults. The long-lasting debate on the extensional versus compressive Plio-Quaternary tectonics of the Apennines orogenic
belt should now be revised taking into account the importance of compressive structures related to local effects. 相似文献
9.
Edward J. Fleming Carl T.E. Stevenson Michael S. Petronis 《Proceedings of the Geologists' Association. Geologists' Association》2013
North Norfolk is a classic area for the study of glacial sediments with a complex glaciotectonic deformational history, but the processes leading to the formation of some structures can be ambiguous. Anisotropy of magnetic susceptibility (AMS) analyses, providing quantitative fabric data, have been combined with the analysis of visible structures and applied to the Bacton Green Till Member, exposed at Bacton, Norfolk. Thermomagnetic curves, low temperature susceptibility and acquisition of isothermal remanent magnetism (IRM) reveal that the magnetic mineralogy is dominated by paramagnetic phases. The magnetic foliation is parallel to fold axial planes and weakly inclined to bedding, whilst the magnetic lineation is orientated parallel to stretching, indicated by the presence of stretching lineations and the trend of sheath folds. Variations in the orientation of the magnetic lineation suggest that the Bacton section has been subject to polyphase deformation. After subaqueous deposition, the sequence was overridden by ice and glaciotectonically deformed which involved stretching initially north–south, then east–west. These results show that AMS can be used to detect strain in three dimensions through a glaciotectonite where paramagnetic mineralogy is dominant. This approach therefore provides further support to the use of AMS as a fast, objective and accurate method of examining strain within deformed glacial sediments. 相似文献
10.
11.
Evaluating magnetic lineations (AMS) in deformed rocks 总被引：3，自引：0，他引：3
Magnetic lineation in rocks is given by a cluster of the principal axes of maximum susceptibility (Kmax) of the Anisotropy of Magnetic Susceptibility (AMS) tensor. In deformed rocks, magnetic lineations are generally considered to be the result of either bedding and cleavage intersection or they parallel the tectonic extension direction in high strain zones. Our AMS determinations, based on a variety of samples that were taken from mudstones, slates and schists from the Pyrenees and Appalachians, show that strain is not the only factor controlling the development of magnetic lineation. We find that the development and extent to which the magnetic lineation parallels the tectonic extension direction depends on both the original AMS tensor, which in turn depends on the lithology, and the deformation intensity. Rocks having a weak pre-deformational fabric will develop magnetic lineations that more readily will track the tectonic extension. 相似文献
12.
Analysis of anisotropy of magnetic susceptibility (AMS) and brittle mesostructures (hydroplastic synsedimentary faults and tension gashes) is applied in this study in order to characterize the Mesozoic tectonic events in the Cameros basin (NW Iberian Range), formed between Tithonian and Albian times. Low-field AMS at room and low temperature (LF-AMS at RT and LF-AMS at LT, respectively) together with high-field AMS (HF-AMS) measurements allow separating ferro- and paramagnetic fabrics. The combination of LF-AMS at LT and HF-AMS torque measurements confirms the reliability of both procedures in terms of isolating the paramagnetic contribution to the AMS. Magnetic fabric results combined with the analyses of synsedimentary faults indicate a NW–SE extension direction during Aptian (and probably Barremian) times. This extension direction is perpendicular to the main extension direction (NE–SW) prevailing during early and late stages of basin evolution. It is also consistent with extension direction deduced from large-scale bending folds and tension gashes, developed after partial lithification. Cleavage development during Albian enhanced the orientation of the magnetic fabric in lithologies where the previous extensional magnetic lineation is coaxial with the expected one for compression. 相似文献
13.
磁组构是由岩石中磁性矿物定向分布而产生的组构特征,因此磁组构分析是研究岩石组构常用的技术手段,它具有
见效快、灵敏度高、无损样品等特点,近年来得到广泛应用。然而在磁组构研究工作中钻取定向样品或定向手标本时,常
常会遇到天然样品含有裂缝且裂缝中充填大量近地表沉积物和自生矿物。本文针对辉绿岩裂缝充填物对其磁组构结果的影
响程度做了详细研究,选用160个含裂缝的辉绿岩岩芯定向样品,通过对比去除裂缝充填物前后的磁组构变化,发现实验
前后所有样品磁化率大小变化率平均值<1%,磁组构方向变化平均值<1°,因此认为辉绿岩裂缝物质对其磁组构影响甚微。
矿物学和岩石磁学分析表明裂隙充填物主要为石英、长石,以及少量赤铁矿、黄铁矿和绿泥石等表生矿物。而辉绿岩的携
磁矿物主要是亚铁磁性的磁铁矿,其磁化率强度约为同等质量裂缝填充物的30倍,致使辉绿岩中的裂隙充填物对磁组构的
影响很小。 相似文献
14.
川西盐井沟断层传播褶皱的三维构造建模与磁组构分析 总被引：2，自引：0，他引：2
当前断层相关褶皱研究的发展方向是从二维向三维的转换。文中基于Arcgis、Discovery以及Gocad等三维软件平台,对川西盐井沟地区地震资料进行精细的解析,得出盐井沟背斜是一个典型的三剪断层传播褶皱,并建立了它的三维模型。同时考虑到单纯符合几何约束的构造解释普遍存在着多解性和不确定性,结合几何学的三维建模和动力学的有限应变分析研究断层相关褶皱。在川西盐井沟地区18个采样点钻取了184块定向岩心样品,通过磁组构的分析结果发现,盐井沟地区的磁组构基本上都是弱应变的初始变形组构,褶皱前翼应变强度比后翼略强。断层传播褶皱三剪带是有限应变最为集中的区域,在模型预测的三剪带内,磁组构反映的有限应变也较为强烈。磁组构所指示的构造应力场大致为NW-SE向挤压缩短,与断层相关褶皱的几何学与运动学模型的预测相一致。 相似文献
15.
The magnetic lineation observed in “undeformed” sedimentary units has been interpreted either as an indication of paleoflow direction, or as a result of tectonic overprint which progressively modifies the original sedimentary fabric related to compactional processes. Distinguishing between the two processes is not always easy. In fact, most studies of the Anistropy of Magnetic Susceptibility (AMS) of “undeformed” sequences have been carried out in fine-grained sediments from foredeep sequences, which are characterized by sedimentary flow directions which are almost parallel to the main deformation structures, like thrust faults and folds. In the Alborz Mts., the Upper Triassic–Lower Jurassic Shemshak Group was deposited in a foreland to molassic basin of the Eo-Cimmerian orogen and now outcrops in several folds which are oriented parallel to the curved chain. Paleoflow directions are generally oblique to the main tectonic structures, being directed SSW to SSE and showing negligible changes in their orientation along the Alborz Mountains. We have, therefore, the opportunity to distinguish between tectonic- or sedimentary-related origins of the magnetic lineation. The AMS results show that magnetic lineations of the Shemshak Group are oriented almost parallel to the main fold axes and thrust structures, which follow the Alborz Mts. curved trend, suggesting that magnetic lineation is of tectonic origin in fine to medium grained, mostly massive sandstones, and confirming that AMS is a valuable tool to study deformation processes in sedimentary rocks. 相似文献
16.
We resolve the anisotropy of magnetic susceptibility (AMS) axes along fault planes, cores and damage zones in rocks that crop out next to the Dead Sea Transform (DST) plate boundary. We measured 261 samples of mainly diamagnetic dolostones that were collected from 15 stations. To test the possible effect of the iron content on the AMS we analyzed the Fe concentrations of the samples in different rock phases. Dolostones with mean magnetic susceptibility value lower than −4 × 10−6 SI and iron content less than ∼1000 ppm are suitable for diamagnetic AMS-based strain analysis. The dolostones along fault planes display AMS fabrics that significantly deviate from the primary “sedimentary fabric”. The characteristics of these fabrics include well-grouped, sub-horizontal, minimum principal AMS axes (k3) and sub-vertical magnetic foliations commonly defined by maximum and intermediate principal AMS axes (k1 and k2 axes, respectively). These fabrics are distinctive along fault planes located tens of kilometers apart, with strikes ranging between NNW-SSE and NNE-SSW and different senses of motion. The obtained magnetic foliations (k1–k2) are sub-parallel (within ∼20°) to the fault planes. Based on rock magnetic and geochemical analyses, we interpret the AMS fabrics as the product of both shape and crystallographic anisotropy of the dolostones. Preferred shape alignment evolves due to mechanical rotation of subordinate particles and rock fragments at the fault core. Preferred crystallographic orientation results from elevated frictional heating (>300 °C) during faulting, which enhances c-axes alignment in the cement-supported dolomite breccia due to crystal-plastic processes. The penetrative deformation within fault zones resulted from the local, fault-related strain field and does not reflect the regional strain field. The analyzed AMS fabrics together with fault-plane kinematics provide valuable information on faulting characteristics in the uppermost crust. 相似文献
17.
During the ascent, emplacement and post-emplacement deformation of igneous rocks, two or more phases of deformation that overprint
each other are often depicted. These overprints, when magnetic minerals are present, are recorded in magnetic fabric. In this
contribution, overprints are studied by means of numerical modeling, following several basic scenarios common to igneous rocks.
Biotite and amphibole that occur often together in igneous rocks are considered as carriers of the anisotropy of magnetic
susceptibility. Modeling shows that (1) a constrictional fabric with a low degree of anisotropy as commonly recorded in magmatic
rocks may result from a deformation overprint and not necessarily from an extensional/transtensional regime, and (2) that
the constrictional AMS fabrics originates from orthogonal superimposition of a deformation event on an AMS fabric inherited
from earlier magma emplacement history. Therefore, the interpretation of a constrictional fabric must be performed with caution.
Numerical modeling may provide a suitable help in strengthening the interpretation of real magnetic fabric data. 相似文献
18.
Hematite is a very abundant mineral in natural rock samples. Despite being one of the most important carriers of remanent
magnetization, its magnetic anisotropy is not well understood partially due to its high coercivity and complex behavior. In
particular, the field intensity beyond which the Rayleigh relation no longer holds varies from one crystal to another. This
field threshold is usually less than the field used in most commercial instruments. The nonlinear behavior of low-field susceptibility
may thus hinder the magnetic fabric analysis. We have carried out an intensive study of the low-field bulk susceptibility
and anisotropy of magnetic susceptibility (AMS) at increasing low fields in the range of 2–450 A/m (effective value) in a
collection of hematite natural crystals. Standard rock magnetic properties, X-ray diffraction, and mass spectrometry have
also been determined in order to discover the parameters influencing the low-field susceptibility variations with field. The
AMS principal directions, the shape of the AMS ellipsoid, and the degree of anisotropy are the parameters that can vary with
different applied fields. It has been found that there is no correlation between magnetic properties like coercivity or saturation
magnetization and the range in which the Rayleigh approximation is valid. However, there seems to be a correlation with the
peak width determined from X-ray diffraction, suggesting that the Rayleigh region in hematite crystals is related to the spatial
orientation of the physical domains within the basal plane. 相似文献
19.
Maria Irene Bartolomeu Raposo Alexandre O. Chaves Paulo Lojkasek-Lima Manoel Souza D'Agrella-Filho Wilson Teixeira 《Tectonophysics》2004,378(1-2):43-63
Magnetic fabric and rock magnetism studies were performed on 32 mafic dikes of a Proterozoic dike swarm from the southern São Francisco Craton (SFC; Minas Gerais State, SE Brazil). Magnetic anisotropies were determined by applying anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of remanent magnetization (ARM). The latter was performed imposing both anhysteretic (total (AAR) and partial
pAAR)) and isothermal remanence magnetizations (AIRM). Partial anhysteretic remanence anisotropy was performed based on remanent coercivity spectra from a pilot specimen of each site. In most sites, AMS is dominantly carried by ferromagnetic minerals, however, in some sites, the paramagnetic contribution exceeds 70% of bulk susceptibility. Rock magnetism and thin section analysis allow classifying the dikes as non-hydrothermalized and hydrothermalized. Magnetic measurement shows that the mean magnetic susceptibility is usually lower than 5×10−3 (SI). Ti-poor titanomagnetites up to pure magnetite pseudo-single-domain (PSD) grain sizes carry the majority of magnetic fabrics for non-hydrothermalized dikes whereas coarse to fine grained Ti-poor titanomagnetites carry the majority of magnetic fabrics for hydrothermalized dikes.Three primary AMS fabrics are recognized which are coaxial with ARM fabric, except for two dikes, from both non-hydrothermalized and hydrothermalized dikes. Normal AMS fabric surprisingly is not dominant (31%). The parallelism between AMS, pAAR0–30, pAAR30–60 and pAAR60–90 fabrics in the hydrothermalized dikes indicates that magnetic grains formed due to late-stage crystallization or to remobilization of iron oxides due to hydrothermal alteration after dike emplacement have acquired a mimetic fabric coaxial with the primary fabric given by coarse-grained early crystallized Ti-poor titanomagnetites. This fabric is interpreted as magma flow in which the analysis of Kmax inclination permitted the inference that the dikes were fed by horizontal or subhorizontal fluxes (Kmax<30°). Intermediate AMS fabric is the most important (41%) in the investigated swarm. It is interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike strike. ARM determinations for these sites also remained intermediate except for two dikes. In one of them, AIRM fabric resulted in normal AMS fabric while for the other AAR fabric resulted in inverse AMS fabric. A combination of AMS and ARM fabrics suggest that magmatic fabric for both dikes were overprinted by some late local event, probably related to Brasiliano orogenic processes after dike emplacement. InverseInverse AMS fabric is a minority (four dikes). ARM determinations also remained inverse suggesting a primary origin for inverse AMS fabric. 相似文献
20.
通过与粉尘堆积,以及玄武岩和片麻杂岩风化剖面磁组构的对比,探讨了以贵州平坝白云岩之上红色粘土剖面磁组构特征及其成因指示意义。红色粘土的各向异性较弱,并且不具有明显的沉积磁组构;母岩的组构特征对红色粘土磁化率各向异性没有明显的控制“痕迹” ;红色粘土与粉尘堆积物之间磁组构特征的明显差异,表明它们在物源和形成机制方面是不同的;红色粘土磁化率及磁组构特征反映出红色粘土中磁性矿物的物质来源和成因具有一定的特殊性。 相似文献