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1.
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. 相似文献
2.
Summary The high field torque curves of hematite bearing rocks are not caused by directional differences in the energy of magnetization to saturation, but rather by the couple between the ferromagnetic moment and the applied field. An expression, derived for the high field torque curve of a single crystal of hematite, whose basal plane makes an arbitrary angle with the plane of rotation of the applied field, is found to be in excellent agreement with experiment. Furthermore it is shown that the Fourier spectrum of hematite bearing rocks should in general contain significant higher harmonics and that therefore the high field method is not particularly suitable for determining the preferred crystalline alignment of hematite bearing rocks. 相似文献
3.
塔里木盆地库车坳陷克拉苏河沿岸发育良好的新生界地层.我们对克拉苏河沿岸新生界剖面分A、B两段进行系统采样,获得定向岩芯样品1700余块. 岩石磁学研究表明,新生代沉积岩中磁性矿物以赤铁矿为主,含有少量磁铁矿;磁组构测试结果显示,两段剖面磁面理均较磁线理发育,最小磁化率主轴近于直立,显示原生沉积组构. A剖面磁化率主轴к1的偏角指示古流向. 库姆格列木群沉积时的古水流方向为NEE-SWW向,至苏维依组沉积时,流向变为NNE-SSW向. 河流古流向在苏维依组形成时发生急剧变化,说明南天山此时可能发生了明显的隆升,且以北西部隆升为主;B剖面为吉迪克组、康村组和库车组粗粒碎屑岩,磁组构显示к1-к2构成的磁面理与层理面小角度相交,к3呈叠瓦状分布,可以用来指示古水流方向.从吉迪克组至库车组下段,古流向均为NNW-SSE,但逐渐偏东,暗示天山在此期间处于缓慢的隆升期或东西部处于同步隆升,且西部隆升速度快于东部. 相似文献
4.
Brooks B. Ellwood 《Earth and Planetary Science Letters》1984,67(3):367-376
Magnetic fabric measurements have been performed on three suites of sediment samples to contrast biologically disturbed and undisturbed sediments. The first of these analyses includes samples from laminated, relatively undisturbed and bioturbated, totally disturbed horizons found in an Athabasca Oil Sand core. The second includes samples from biologically undisturbed sediments from Mammoth Cave in Kentucky. The third includes evaluation of the evolution of a magnetite horizon which was established by the author in December of 1982 in a biologically active tidal flat on Sapelo Island, Georgia. Changes in this magnetite horizon were monitored by periodic subsampling.Results from these suites of samples indicate that the measured magnetic fabric in bioturbated sediments may exhibit primary sedimentary characteristics similar to undisturbed sediments. Such primary indicators include the anisotropy of magnetic susceptibility (AMS) parametersV used by Graham, always greater than 45°,Q used by Rees and Hamilton, always less than 0.69, and a subhorizontal magnetic foliation plane. I infer that the presence of apparently primary fabric distributions in these bioturbated sediments indicates that physical, not biological factors are responsible for the observed fabric. These factors include dewatering and simple, low magnitude compaction. The data indicate that it cannot be assumed, a priori, that poor fabric distributions are the result of bioturbation. On the contrary, for the Sapelo Island experiments, the magnetic fabric actually improved with bioturbation and the Athabasca bioturbated magnetic fabric is very similar to that from the Athabasca laminated sediment.Data from the Sapelo Island, Georgia, tidal flat experiment are interpreted to indicate that long-axis AMS alignments develop within the sediment. Such alignments are the result of water movement through sediment which exhibits increased porosity due to high rates of burrowing. It is also observed that RM precision improves during the experiment in spite of bioturbation. 相似文献
5.
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. 相似文献
6.
Belén Oliva-Urcia Inma Gil-Peña Ruth Soto José María Samsó Borja Antolín Emilio L. Pueyo 《Studia Geophysica et Geodaetica》2018,62(2):291-322
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. 相似文献
7.
To further evaluate the potential of magnetic anisotropy techniques for determining the origin of the natural remanent magnetization (NRM) in sedimentary rocks, several new remanence anisotropy measurement techniques were explored. An accurate separation of the remanence anisotropy of magnetite and hematite in the same sedimentary rock sample was the goal.In one technique, Tertiary red and grey sedimentary rock samples from the Orera section (Spain) were exposed to 13 T fields in 9 different orientations. In each orientation, alternating field (af) demagnetization was used to separate the magnetite and hematite contributions of the high field isothermal remanent magnetization (IRM). Tensor subtraction was used to calculate the magnetite and hematite anisotropy tensors. Geologically interpretable fabrics did not result, probably because of the presence of goethite which contributes to the IRM. In the second technique, also applied to samples from Orera, an anisotropy of anhysteretic remanence (AAR) was applied in af fields up to 240 mT to directly measure the fabric of the magnetite in the sample. IRMs applied in 2 T fields followed by 240 mT af demagnetization, and thermal demagnetization at 90°C to remove the goethite contribution, were used to independently measure the hematite fabric in the same samples. This approach gave geologically interpretable results with minimum principal axes perpendicular to bedding, suggesting that the hematite and magnetite grains in the Orera samples both carry a depositional remanent magnetization (DRM). In a third experiment, IRMs applied in 13 T fields were used to measure the magnetic fabric of samples from the Dome de Barrot area (France). These samples had been demonstrated to have hematite as their only magnetic mineral. The fabrics that resulted were geologically interpretable, showing a strong NW-SE horizontal lineation consistent with AMS fabrics measured in the same samples. These fabrics suggest that the rock's remanence may have been affected by strain and could have originated as a DRM or a CRM.Our work shows that it is important to account for the presence of goethite when using high field IRMs to measure the remanence anisotropy of hematite-bearing sedimentary rocks. It also shows that very high magnetic fields (>10 T) may be used to measure the magnetic fabric of sedimentary rocks with highly coercive magnetic minerals without complete demagnetization between each position, provided that the field magnetically saturates the rock. 相似文献
8.
Abstract The 1995 Hyogo-ken Nanbu Earthquake (Mw 6.9) occurred in the region around Kobe City and Awaji Island in south-west Japan. Co-seismic liquefaction caused subsidence of the land and damage to sea wall caissons on the man-made Port Island at Kobe City. A zone 2–3 m wide behind the caissons of the northern wharf on the island subsided into the intertidal zone and a sandy deposit settled into this subsided zone. The depos-it consists of upward-fining sequences that are subdivided into three parts, in ascending order: graded coarse- to medium-grained sand, parallel-laminated fine- to very-fine-grained sand, and massive mud. Grain fabric analysis (employing the anisotropy of magnetic susceptibility method and microscopic measurement) of these sequences shows that there is a remarkable contrast in grain fabric between the lowest portion of the graded sand division and the laminated sand division. The former has a high q -value (magnetic lineation/foliation) and a unimodal orientation of elongate grains in the horizontal plane, but random orientation in the vertical plane. Conversely, the latter is characterized by a low q -value and a grain fabric in which the long axes of the grains have random orientations and are nearly parallel to the plane of deposition. This result shows that the main depositional processes changed from a combination of flow and allied processes to the force of gravity. As still water is essential for gravity to be the dominant factor in deposition, this deposit is regarded as subaqueous sand blow deposits. If this interpretation is correct, the grain fabric produced by gravity alone is a useful criterion for distinguishing between subaqueous sand blow deposits and other liquefaction-induced deposits. 相似文献
9.
Erik Halvorsen 《Earth and Planetary Science Letters》1974,21(2):127-133
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 (KI) and intermediate (KII) susceptibility axis within the magnetic foliation plane was found, the KI-axis striking predominantly E-W and the KII-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. 相似文献
10.
The present study demonstrates how the Paleo-Proterozoic Wangtu Gneissic Complex (WGC) of the Lesser Himalayan Crystalline sequence experienced superposed folding and doming prior to its exhumation, with the help of integrated field, microstructural, magnetic fabric anisotropy and geochronological studies. The WGC forms the basement of the Lesser Himalaya and is bounded by Vaikrita Thrust (VT) to the northeast and Munsiari Thrust (MT) to the southwest. The regional structure consists of upright large scale early folds (D1) trending NW–SE. The mesoscopic fabric is related to axial plane foliation of the D1 folds and, to a lesser extent, late D2 folds. The axis of maximum compression for D1 and D2 folds are mutually orthogonal. The D1 folds have formed simultaneously with the major Himalayan thrusts whereas the D2 folds have developed during a later deformation event. The magnetic lineation at the hangingwall of the VT is sub-horizontal indicating stretching along the strike of the thrust. In the interior parts of the WGC, the magnetic fabric is of two types: (i) magnetic lineation demarks the intersection of mesoscopic and magnetic foliation indicating superposed deformation and (ii) scattered distribution of magnetic lineations due to D2 folding on initially curved and non-cylindrical D1 surface. 40Ar–39Ar dating of biotite from one site from the core of WGC gives an age of 9.3 ± 0.3 (2σ) Ma. It is inferred that the doming of the WGC took place at ∼9 Ma and, instead of large scale thrusting, it is characterized by superposed folding and strike-parallel stretching along the VT zone. It is suggested that the effect of superposed folding and ductile deformation of the Himalayan basement rocks has to be taken into account before cross-section balancing or any estimation of crustal shortening is attempted. 相似文献
11.
12.
Bo A. Sjöberg 《Studia Geophysica et Geodaetica》1992,36(1):42-50
Summary The magnetic fabric of the Limhamn limestones is characterized by a low initial magnetic susceptibility. Of 68 specimens with a positive susceptibility, 10 show a distinct anisotropy. In nine of these, the axes of minimum susceptibility are practically normal to the magnetic foliation plane. The observed anisotropy pattern has some resemblance to a magnetic fabric acquired by deposition under influence of currents. A post-depositional origin due to gravitational compaction seems, however, more probable.Presented at 2nd conference on New Trends in Geomagnetrsm, Castle of Bechyn, Czechoslovakia, September 24–29, 1990. 相似文献
13.
在地层剖面中,洪泛沉积物与正常的河道沉积物的沉积学特征非常相似,仅靠沉积学特征很难将两者区分. 为了识别古洪灾事件,因而从环境磁学角度建立两种沉积物的识别标志是非常必要和有意义的. 本文通过对1998年长江黣洲湾溃口扇及其附近的长江现代河漫滩沉积物进行磁组构参数测量和对比分析,并结合长江下游河漫滩沉积物的磁组构参数特征,揭示出长江中游正常河流沉积物与河流溃口沉积物的磁组构参数特征具有明显的不同,并且它们与沉积时的沉积环境有着密切的关系. 因此,对长江中游地区沉积物磁组构参数的测试及磁化率各向异性量值椭球体形态的研究,是目前分辨洪泛沉积物和正常的河道沉积物的一个较有效的方法. 相似文献
14.
The Taishu Group is a folded, Eocene–Lower Miocene, thick sedimentary package exposed widely on Tsushima Island between the Japan Sea and East China Sea. This location makes the strata important to understand tectonics and paleo-environments in the Far East, but the timing of the folding is controversial. We studied the styles of brittle deformations of the strata. It was found that flextural-slip folds were dominant. Mesoscale faults were classified into two groups: NE–SW trending reverse faults and NW–SE trending strike-slip faults. Members of both the groups showed movements largely perpendicular to the fold axes. The latter group consisted of sinistral and dextral faults. Accordingly, we interpreted that they were transfer faults activated during the folding. Consequently, mesoscale faults and flexural-slip faults evidence the map-scale plane strain of the Taishu Group in the plane perpendicular to the NE-trending fold axes. There were few transpressional deformations in the group. This is inconsistent with the transpression hypothesis for explaining the simultaneous folding and Japan Sea opening. Another hypothesis in which the folds in Tsushima are regarded as an onshore part of the Taiwan-Shinji fold belt is inconsistent with the timing of folding suggested by mining geologists to be consistent with and contemporaneous with this deformation. On the other hand, we found that dolerite dikes and sills were involved in the folding. Therefore, we conclude that the folding began during the late Early Miocene time and climaxed during the ore mineralization at around 15 Ma. We suggest that the folding in Tsushima was the easternmost manifestation of the compressional regime around the Yellow Sea and East China Sea in the Early to early Middle Miocene, and that the compression was brought about by the arrival of the Philippine Sea plate to initiate buoyant subduction under Kyushu. 相似文献
15.
猴石顶太古界重褶皱位于阜平太古宙穹状复合褶皱群中一个钩状褶皱的“钩子”部位。其中一层斜长角闪岩标志层的岩石磁性组构的分布情况为:1.在翼部的磁化率量值椭球是压扁形的,磁性叶理面的方向与变质层面及片麻理的方向基本一致,表现为向北西倾斜的倒转褶皱;2.在太古宙再次变形阶段所形成的转折端部位,磁性叶理面的走向变化较大,并出现拉长形的磁化率量值椭球,反映了经受过拉张作用的变形和多于一次的不同方向应力的作用;3.西翼的磁性线理值与磁性叶理值以及磁化率各向异性度都大,反映它经受过较强的剪切应力与压应力的作用;4.磁性线理主要倾向北半球,而且部分磁性线理近南北方向分布。 推测猴石顶重褶皱可能处于向北倾伏的主体构造的东侧。 上述岩石磁性是由自行设计的静态测量方法,用国产的数字式无定向磁力仪测定的。 相似文献
16.
一般认为磁组构能有效地反映岩石所经历的应变特征.为了研究不同类型的磁组构和不同期次应变之间的关系,对来自华南地块两个地区的早三叠世灰岩样品进行了岩石磁学、磁组构以及应变特征的对比分析.来自湖北通山县的样品经历了三期构造变形,这为解析磁组构和多期次应变提供了理想的机会.岩石磁学结果显示携磁矿物主要为磁铁矿.磁化率各向异性（AMS）和非磁滞剩磁各向异性（AAR）结果显示其最小轴与层面垂直,最大轴和中间轴分布于层面内,反映了沉积和压实作用产生的应变,而后期构造应变在磁组构中没有体现.来自广东连县的样品发育有渗透性压溶缝面理和方解石脉,说明经历了构造应变.AMS结果没有显示占优势的组构方向.AAR结果显示三轴组构,其最大轴分布于最大应力方位,与构造应变特征吻合,最初的压实组构被构造应变组构所代替.上述结果表明：（1）AAR可以很好地反映渗透性应变的特征,而AMS有时会失效;（2）应变的尺度要小于样品的尺度,磁组构才能有效地反映应变. 相似文献
17.
《Acta Geochimica》2021,(2)
Iron ore deposits hosted by Precambrian banded iron formation(BIF) are the most important source of mineable iron.In Cameroon,they are located in the southern part of the country.This study reports the petrological and geochemical data of iron ores collected from a weathering profile in the Mamelles BIF deposit,SW Cameroon.The profile is composed of three levels which are from the bottom to the top:the saprock,the ferruginous horizon,and the loose horizon.Eight representative iron ore samples(rock fragments and loose clayey material)were collected along the profile and were subjected to petrographic and geochemical analyses.Their mineralogy consists of martite,goethite,quartz,and lesser amounts of hematite,magnetite,kaolinite,and halloysite.The presence of minerals such as kaolinite and goethite in the Mamelles iron ores suggests their supergene origin.Geochemically,the saprock is characterized by high iron content(70.25 wt% Fe_2 O_(3 t)),and low silica(26.38 wt% SiO_2) and alumina(1.14 wt% Al_2 O_3).The rock fragments collected from the ferruginous horizon display higher Fe_2 O_(3 t)(72-76.40 wt%),Al_2 O_3(2.80-5.43 wt%),and lower SiO_2(16.70-18.35 wt%) contents,suggesting the leaching of silica during the enrichment process.The loose clayey samples collected from both the ferruginous horizon and the upper loose horizon show lower iron and higher silica contents.When normalized to the underlying BIF saprock,both rock fragments and loose clayey ores display LREE enrichment,suggesting that they formed through supergene processes.Economically,most of the Mamelles iron ores are classified as medium-grade ores and a few display acceptable contents in contaminants.Overall,this petrological and geochemical study of the Mamelles iron ores revealed encouraging results.Given its strategic location near the deep seaport,the deposit should be investigated in more detail for its mining potential. 相似文献
18.
Neli Jordanova Diana Jordanova Vassil Karloukovski 《Studia Geophysica et Geodaetica》1996,40(1):36-49
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. 相似文献
19.
行口铁矿位于沁阳市北部太行山山脚下,是上世纪80年代发现的一个中型贫磁铁矿床。矿体成群出现,延长900～1200m,宽度变化70～290m,厚度变化5～270m;呈似层状、透镜状,控制延深700m,有用矿物主要为磁铁矿和赤铁矿。矿体的品位在走向上和倾向上基本稳定,矿石化学成分和矿物组成简单,有害物质含量低,矿体群中夹层较多,属于火山沉积变质铁矿床。 相似文献
20.
Richard Weaver Andrew P. Roberts Andrew J. Barker 《Earth and Planetary Science Letters》2002,200(3-4):371-386
Paleomagnetic, rock magnetic, and sedimentary micro-textural data from an early Miocene mudstone sequence exposed in Okhta River, Sakhalin, Russia, indicate the presence of pyrrhotite and magnetite at different stratigraphic levels. Sites that contain only magnetite have a reversed polarity characteristic remanent magnetization (ChRM) with a low-coercivity overprint, which coincides with the present-day geomagnetic field direction. Pyrrhotite-bearing sites have stable normal polarity ChRMs that are significantly different from the present-day field direction. After correction for bedding tilt, the ChRM data fail a reversals test. However, the normal polarity pyrrhotite ChRM directions become antipodal to the tilt-corrected magnetite ChRM directions and are consistent with the expected geocentric axial dipole field direction at the site latitude after 40% partial unfolding. These data suggest that the pyrrhotite magnetization was acquired during folding and after lock-in of the magnetite remanences. Electron microscope observations of polished sections indicate that fluid-associated halos surround iron sulphide nodules. Pyrrhotite is present in randomly oriented laths in and around the nodules, and the nodules do not appear to have been deformed by sediment compaction. This observation is consistent with a late diagenetic origin of pyrrhotite. Documentation of a late diagenetic magnetization in pyrrhotite-bearing sediments here, and in recent studies of greigite-bearing sediments, suggests that care should be taken to preclude a late origin of magnetic iron sulphides before using such sediments for geomagnetic studies where it is usually crucial to establish a syn-depositional magnetization. 相似文献