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1.
A detailed paleomagnetic and rock-magnetic investigation spanning loess L7 to paleosol S8 has been carried out at the Baoji and Xifeng sections. Results of anisotropy of magnetic susceptibility confirm that the studied loess–paleosol sediments retain primary sedimentary fabrics. Stepwise thermal demagnetization shows that two well-defined magnetization components can be isolated from both loess and paleosol specimens. A low-temperature component, isolated between 100°C and 200°C, is consistent with the present geomagnetic field direction, and a high-temperature component, isolated between 200–300°C and 620–680°C, includes clearly normal and reversed polarities. Isothermal remanent magnetization and thermomagnetic analyses indicate that characteristic remanent magnetization is mainly carried by magnetite and hematite. The Day plot, together with the stratigraphic variations of rock-magnetic parameters, shows that the uniformity of magnetic mineralogy and grain size fulfills the criteria for relative paleointensity (RPI) studies. RPI records have been constructed using natural remanent magnetization (NRM) intensity after thermal demagnetization at 300°C normalized by low-frequency magnetic susceptibility (NRM300/χ). The results show that the RPI record from the Baoji section, where pedogenesis is quite weak, is compatible with the stacked PISO-1500 paleointensity record, suggesting that it might reflect the paleointensity variation of the geomagnetic field. The RPI record from the Xifeng section, where pedogenesis is rather strong, indicates a clear dissimilarity with the stacked PISO-1500 paleointensity record, implying that it does not reflect the paleointensity variation of the geomagnetic field. Our new results show that the NRM300/χ from the strongly pedogenetic paleosols does not completely eliminate the pedogenetic (climatic) influence, so it might be unsuitable for a reliable paleointensity study.  相似文献   

2.
本文对藏东昌都地区侏罗纪汪布组、东大桥组和小索卡组红层共71个采点开展了磁组构(AMS)研究。磁组构测试结果表明,早侏罗世汪布组岩石磁线理较磁面理发育,磁化率各向异性度较高,磁化率椭球最小轴K3散布于层面缩短方向,代表了与构造成因相关的磁组构;中侏罗世东大桥组和晚侏罗世小索卡组岩石则磁面理较磁线理发育,磁化率各向异性度较低,磁化率椭球最小轴K3与层面近垂直,指示了原生沉积磁组构。早侏罗世汪布组地层的磁组构揭示了其构造应力场方向为NE-SW向。中侏罗世东大桥组的磁组构指示了其沉积时的古水流方向为SE向(138.3°),而晚侏罗世小索卡组磁组构指示了其沉积时的古水流方向为NNW向(328.3°)。古水流方向的明显变化揭示了昌都地区从中侏罗世到晚侏罗世沉积物物源发生了相应的转变,表明昌都地区南早北晚的隆升过程。  相似文献   

3.
Granulite from 66 sites along the Além-Paraiba dextral shear zone were collected for magnetic analyses. The rocks were affected by the Braziliano orogeny, which was responsible for the present structural pattern. Magnetic fabrics were determined applying anisotropy of low—field magnetic susceptibility (AMS, all sites) and anisotropy of remanence magnetization (ARM, in 21 sites). The ferromagnetic minerals are magnetite, titanohematite, and in some samples, minor pyrrhotite. Hysteresis curves show that both para— and ferromagnetic minerals are the carriers of AMS. Thus AMS is due to the preferred crystallographic orientation of paramagnetic matrix minerals and titanohematite, to the shape anisotropy of magnetite grains, or to a combination of all three. ARM was performed imposing both anhysteretic remanence (AAR) and isothermal remanence (AIRM). The AMS, AAR, and AIRM fabrics are coaxial and are tectonic in origin. Their parallelism indicates that both ferromagnetic and paramagnetic minerals recorded the same metamorphic event. A passive—marker model is suggested for ferromagnetic minerals at the outcrop scale. The magnetic foliation is very close to the strike of the Além Paraíba shear zone, suggesting that this generated the local rock fabrics during the Braziliano orogeny.  相似文献   

4.
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   

5.
对六盘山以西132 m水洛红黏土剖面磁化率各向异性特征的研究表明,水洛红黏土剖面记录的14.8~8.7 Ma沉积序列受到了同沉积时期不同程度的应力作用,磁化率各向异性变化特征对应于应力作用强度较弱的初期和应力作用强度有所加强的中期。进一步分析发现,14.8~11.0 Ma之间显示出3次应力增强和后续应力减弱交替变化的特点,这一应力作用事件可能与青藏高原东北缘在此阶段强烈构造活动的区域应力传递相关,而11.0~8.7 Ma期间存在一个相对较强的应力作用事件,可能是对局部地区应力增强事件的响应。  相似文献   

6.
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   

7.
We ask the question whether petrofabric data from anisotropy of magnetic susceptibility (AMS) analysis of deformed quartzites gives information about shape preferred orientation (SPO) or crystallographic preferred orientation (CPO) of quartz. Since quartz is diamagnetic and has a negative magnetic susceptibility, 11 samples of nearly pure quartzites with a negative magnetic susceptibility were chosen for this study. After performing AMS analysis, electron backscatter diffraction (EBSD) analysis was done in thin sections prepared parallel to the K1K3 plane of the AMS ellipsoid. Results show that in all the samples quartz SPO is sub-parallel to the orientation of the magnetic foliation. However, in most samples no clear correspondance is observed between quartz CPO and K1 (magnetic lineation) direction. This is contrary to the parallelism observed between K1 direction and orientation of quartz c-axis in the case of undeformed single quartz crystal. Pole figures of quartz indicate that quartz c-axis tends to be parallel to K1 direction only in the case where intracrystalline deformation of quartz is accommodated by prism <c> slip. It is therefore established that AMS investigation of quartz from deformed rocks gives information of SPO. Thus, it is concluded that petrofabric information of quartzite obtained from AMS is a manifestation of its shape anisotropy and not crystallographic preferred orientation.  相似文献   

8.
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   

9.
Phase change of dielectric magnesiowustite in the lower mantle may leave signatures in geomagnetic records of the globally distributed array of observatories. The related features appear in EM induction responses of lower mantle, which are studied theoretically. The surface EM field corresponding to a response of the earth with conductivity anisotropy in a mantle spherical layer is presented as the sum of the magnetic and electric modes. Equations for the fields of both modes and their relationship in a weakly anisotropic earth are obtained by the perturbation method. The two field modes are analyzed jointly and separately to characterize the conductivity tensor of the anisotropic lower mantle. The tensor elements corresponding to the tangential components of the field can be estimated from the magnetic mode alone recorded currently by the global network of geomagnetic observatories. For the tensor data to be complete, observatory data on lateral variations of the electric field are required in addition to three-component geomagnetic records.  相似文献   

10.
In this work we analyse and check the results of anisotropy of magnetic susceptibility (AMS) by means of a comparison with palaeostress orientations obtained from the analysis of brittle mesostructures in the Cabuérniga Cretaceous basin, located in the western end of the Basque–Cantabrian basin, North Spain. The AMS data refer to 23 sites including Triassic red beds, Jurassic and Lower Cretaceous limestones, sandstones and shales. These deposits are weakly deformed, and represent the syn-rift sequence linked to basins formed during the Mesozoic and later inverted during the Pyrenean compression. The observed magnetic fabrics are typical of early stages of deformation, and show oblate, triaxial and prolate magnetic ellipsoids. The magnetic fabric seems to be related to a tectonic overprint of an original, compaction, sedimentary fabric. Most sites display a NE–SW magnetic lineation that is interpreted to represent the stretching direction of the Early Cretaceous extensional stage of the basin, without recording of the Tertiary compressional events, except for sites with compression-related cleavage.Brittle mesostructures include normal faults, calcite and quartz tension gashes and joints, related to the extensional stage. The results obtained from joints and tension gashes show a dominant N–S to NE–SW, and secondary NW–SE, extension direction. Paleostresses obtained from fault analysis (Right Dihedra and stress inversion methods) indicate NW–SE to E–W, and N–S extension direction. The results obtained from brittle mesostructures show a complex pattern resulting from the superposition of several tectonic processes during the Mesozoic, linked to the tectonic activity related to the opening of the Bay of Biscay during the Early Cretaceous. This work shows the potential in using AMS analysis in inverted basins to unravel its previous extensional history when the magnetic fabric is not expected to be modified by subsequent deformational events. Brittle mesostructure analysis seems to be more sensitive to far-field stress conditions and record longer time spans, whereas AMS records deformation on the near distance, during shorter intervals of time.  相似文献   

11.
古地磁学是研究地质历史上地球磁场强度及其变化的一门学科,其核心是岩石中富含若干的磁性矿物。通过测定其保存的剩磁强度和方向等,可计算获得岩石形成时的初始磁性强度和方向,从而为约束岩石形成时的古磁极位置、形成时代和地球动力学背景等研究提供证据。矿石作为地球演化过程中形成的一类特殊的岩石,对其进行古地磁研究可为矿床的"源—运—储—保—变"等过程提供强有力的证据。现有关于矿床古地磁学的研究主要集中在两方面,一是通过矿物的退磁参数限定其形成时的磁化年龄（矿床定年）,二是通过磁组构信息获取矿床形成后的构造应力场变化（矿床保存变化）。需要指出的是,矿床古地磁学的研究一定要立足于地质事实。由于地质产状和围岩蚀变对磁性参数的影响极大,在进行矿床地磁数据处理时一定要慎重,并与磁性地层学互相印证,才有可能获得有价值的磁性参数,从而为约束矿床成因提供有效证据。  相似文献   

12.
Gentoso, M. J., Evenson, E. B., Kodama, K. P., Iverson, N. R., Alley, R. B., Berti, C. & Kozlowski, A. 2012 (January): Exploring till bed kinematics using AMS magnetic fabrics and pebble fabrics: the Weedsport drumlin field, New York State, USA. Boreas, Vol. 41, pp. 31–41. 10.1111/j.1502‐3885.2011.00221.x. ISSN 0300‐9483. Thick, relatively homogeneous basal tills exposed in the drumlins and flutes of the Weedsport drumlin and flute field in New York State exhibit anisotropy of magnetic susceptibility (AMS) and pebble fabrics that are consistently oriented parallel to the streamlined bedforms. The pebble fabrics and AMS fabrics are concordant. In this study, six drumlins and five flutes were sampled. Thermally induced, incremental reduction of isothermal remanent magnetization indicates that AMS is caused by primarily elongate maghaemite grains. The orientations of principal axes of maximum susceptibility (k1) are generally parallel to pebble long‐axis orientations, and tend to plunge mildly up‐glacier. Fabric directions are generally parallel to drumlin long‐axis orientations, but deviate by 12°–23° from flute directions. Fabrics of the flutes are stronger and more unidirectional than those of the drumlins. These results support the use of AMS as a fast and objective method for characterizing fabrics in tills, and suggest hypotheses about basal processes linked to glacially streamlined landforms.  相似文献   

13.
航磁异常的构造解释和华北-塔南-扬子超陆块演化   总被引：11，自引：0，他引：11  
张家声  劳秋元  李燕 《地学前缘》1999,6(4):379-390
在现代地球磁场中,总体感磁效应与岩石的磁化率无关。典型地区大比例尺构造组构填图结果与岩石磁组构的对照分析,证明磁线理和构造线理具有一致的矢量特征。强烈韧性剪切导致岩石中区域一致的矿物定向和磁化率各向异性,是干扰现代地球磁场、产生特定航磁异常分布格局的主要因素。对五种基本的区域尺度构造 航磁异常样式的成因进行了分析,证明华北地区的航磁异常的分布样式与结晶基底的基本构造格架基本一致。并据此对塔里木、扬子地体基底岩石的构造性质进行了解释,提出存在前寒武纪华北－ 塔南－ 扬子超陆块。结合显生宙构造演化的地质证据,划分出新太古代克拉通化、古元古代左行韧性剪切改造和活动带发育、中－ 新元古代裂解和活动大陆边缘、古生代原地改造和中生代再裂解五个演化历史阶段  相似文献   

14.
Anisotropy of electrical conductivity in the lower mantle, presumably caused by phase change of dielectric magnesiowustite, can be estimated from geomagnetic records of the globally distributed geomagnetic array. The components of the conductivity tensor corresponding to the tangential components of the geomagnetic field are obtained from the magnetic mode alone recorded on the surface. Inversion by the perturbation method is applied to monthly means of geomagnetic variations along the X, Y, and Z components for the period from 1920 to 2009, borrowed from the World Monthly Means Database. The anisotropy contribution is expressed as corrections to the conductivity tensor components. The contribution of anisotropy to induction responses becomes notable for geomagnetic variations with at least decadal periods and increases with the period length.  相似文献   

15.
Magnetic fabric and rock-magnetism studies were performed on the four units of the 578 ± 3-Ma-old Piracaia pluton (NW of São Paulo State, southern Brazil). This intrusion is roughly elliptical (~32 km2), composed of (i) coarse-grained monzodiorite (MZD-c), (ii) fine-grained monzodiorite (MZD-f), which is predominant in the pluton, (iii) monzonite heterogeneous (MZN-het), and (iv) quartz syenite (Qz-Sy). Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). The two fabrics are coaxial. The parallelism between AMS and AARM tensors excludes the presence of a single domain (SD) effect on the AMS fabric of the units. Several rock-magnetism experiments performed in one specimen from each sampled units show that for all of them, the magnetic susceptibility and magnetic fabrics are carried by magnetite grains, which was also observed in the thin sections. Foliations and lineations in the units were successfully determined by applying magnetic methods. Most of the magnetic foliations are steeply dipping or vertical in all units and are roughly parallel to the foliation measured in the field and in the country rocks. In contrast, the magnetic lineations present mostly low plunges for the whole pluton. However, for eight sites, they are steep up to vertical. Thin-section analyses show that rocks from the Piracaia pluton were affected by the regional strain during and after emplacement since magmatic foliation evolves to solid-state fabric in the north of the pluton, indicating that magnetic fabrics in this area of the pluton are related to this strain. Otherwise, the lack of solid-state deformation at outcrop scale and in thin sections precludes deformation in the SW of the pluton. This evidence allows us to interpret the observed magnetic fabrics as primary in origin (magmatic) acquired when the rocks were solidified as a result of magma flow, in which steeply plunging magnetic lineation suggests that a feeder zone could underlie this area.  相似文献   

16.
Oriented samples of sediments from Ariyalur Group, Cauvery Basin, south India, were studied for low field anisotropy of magnetic susceptibility (AMS) measurements to unravel the magnetic fabrics and paleocurrent directions. The results of AMS parameters of the sediments indicate primary depositional fabrics for Sillakkudi, Ottakovil and Kallamedu sandstone formations and secondary fabric for Kallankurichchi limestone formation. The obtained low degree of anisotropy (P j ), oblate shape AMS ellipsoid and distribution of maximum (K 1) and minimum (K 3) susceptibility axes on equal area projection confirm the primary sedimentary fabric for Sillakkudi, Ottakovil and Kallamedu Formations. In the case of ferruginous, lower arenaceous, Gryphaea limestone and upper arenaceous limestone beds of Kallankurichchi Formation have recorded more than one fabric. The observed AMS parameters like shape factor (T) (prolate to oblate), q value and random distribution of minimum (K 3) and maximum (K 1) susceptibility axes are supported for secondary fabrics in Kallankurichchi Formation as a result of post-depositional processes. Based on petrographic studies, it can be established that K 1 AMS axis of biotite mineral could represent the flow direction. The established paleocurrent direction for Sillakkudi is NW–SE direction while Ottakovil and Kallamedu Formations recorded NE–SW direction. Overall the paleoflow directions observed for Ariyalur Group is NE–SW to NW–SE.  相似文献   

17.
Studies of the anisotropy of magnetic susceptibility (AMS) were carried out in order to define the directions and strength of palaeowind during the sedimentation of the youngest loesses in Poland and western Ukraine. These sediments, like the Chinese and Alaskan loesses, appear suitable for application of the AMS method. The inclined mean minimum axes were considered as reflecting the prevailing palaeowind direction at the time of loess sedimentation. The majority of the sections studied indicate a palaeowind direction from W-SW to E-NE, with a mean azimuth of 258°. This direction corresponds to the strike of the Weichselian ice-sheet margin and is parallel to the axis of the lowland between the ice-sheet margin to the north and the Carpathian Mountains and the Podole Upland to the south. Further AMS studies of the oldest parts of loess sequences in Poland and western Ukraine may help in reconstruction of the atmospheric circulation in this part of Europe since c. 900 kyr BP.  相似文献   

18.
The anisotropy of magnetic susceptibility (AMS) of single crystals of biotite, muscovite and chlorite has been measured in order to provide accurate values of the magnetic anisotropy properties for these common rock-forming minerals. The low-field AMS and the high-field paramagnetic susceptibility are defined. For the high-field values, it is necessary to combine the paramagnetic deviatoric tensor obtained from the high-field torque magnetometer with the paramagnetic bulk susceptibility measured from magnetization curves of the crystals. This leads to the full paramagnetic susceptibility ellipsoid due to the anisotropic distribution of iron cations in the silicate lattice. The ellipsoid of paramagnetic susceptibility, which was obtained for the three phyllosilicates, is highly oblate in shape and the minimum susceptibility direction is subparallel to the crystallographic c-axes. The anisotropy of the susceptibility within the basal plane of the biotite has been evaluated and found to be isotropic within the accuracy of the instrumental measurements. The degree of anisotropy of biotite and chlorite is compatible with previously reported values while for muscovite the smaller than previously published values. The shape of the chlorite AMS ellipsoid for all the samples is near-perfect oblate in contrast with a wide distribution of oblate and prolate values reported in earlier studies. Reliable values are important for deriving models of the magnetic anisotropy where it reflects mineral fabrics and deformation of rocks.  相似文献   

19.
柬埔寨王国位于低纬度地区,地质勘查程度较低,地面高精度磁测技术能够快速获取磁异常信息,进行找矿评价。在低纬度地区磁化方向主要以水平磁化为主,磁性体产生的磁异常以负磁异常为主。本文通过对柬埔寨王国Mesam金矿磁测数据进行倒相180°、化赤和低纬度化极处理对比,认为三种处理方法得到的结果都能够完整地反映磁场分布特点,在不能忽略剩磁影响的低纬度地区对实测数据进行低纬度化极后再进行解释更加贴近真实且细节更加丰富。通过对低纬度化极后的磁异常进行水平总梯度模和小波多尺度分解计算等位场异常处理,认为磁异常水平总梯度模计算能够较好地反映深部地质体的边界范围,小波多尺度分解计算能够提取特定深度地质体的空间信息,在该区选取的磁异常处理技术是有效的,为进一步扩大该金矿区远景储量提供了新的思路。  相似文献   

20.
Chuquicamata, in northern Chile, is one of the largest porphyry copper deposits in the world; the western side of its orebody is bounded by a major longitudinal fault, the West fault. We report paleomagnetic results from surface sites and drill cores from different geological units at Chuquicamata, especially within the late Eocene Fiesta granodiorite of the western block of the West fault. Characteristic remanent magnetizations (ChRM) were determined after detailed thermal or alternating field demagnetization. Soft components carried by multidomain magnetite crystals in the Fiesta granodiorite were removed by AF demagnetization at 10–20 mT. The ChRMs, not demagnetized by alternating fields up to 100 mT, have unblocking temperatures above 580 °C with ~ 75% of the magnetization removed in the temperature range of 580–590 °C. Optical and SEM mineralogical observations, and microprobe data indicate the occurrence of multidomain magnetite formed during a late magmatic stage of alteration coeval with strong oxidation of primary titanomagnetite and formation of ilmenite, hematite, pseudobrookite, and rutile. The characteristic directions have negative inclinations and declinations (330° to 230°); strongly deflected from the expected Eocene direction. Anisotropy of magnetic susceptibility (AMS), with degree up to 1.4, is carried by multidomain magnetite. AMS ellipsoids have subvertical foliations with azimuth varying strongly from N280° to N20°. We show that both the ChRMs and the AMS fabrics record the same apparent relative rotations between sites. Although the AMS anisotropy is high, there is no evidence for a solid-state deformation and the apparent rotation of the magnetic fabric is interpreted to be the consequence of small-block rotation. The apparent large (> 100°) counterclockwise rotations of small blocks within the Fiesta granodiorite suggest a wide damaged zone related to sinistral displacement along the West fault. This interpretation is consistent with previous models indicating that the Fiesta granodiorite was sinistrally translated and brought in front of the early Oligocene porphyry copper deposit during the Oligocene–early Miocene. This study shows that paleomagnetic markers are useful for improving the quantification and understanding of small-scale deformation within plutons adjacent to major fault zones.  相似文献   

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