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1.
A series of discontinuous sediment sequences, of Plio/Pleistocene age, occur onshore around the southern North Sea margins, notably in the East Anglian region of Britain. Intensive lithological and palaeontological analyses of these sediments have shown that they record both major and minor oscillations in climate, sea level and environmental conditions. However, significant uncertainties exist regarding the absolute and relative chronostratigraphies of many of these sequences, hindering understanding of the relative impacts of climatic, eustatic and tectonic changes on the palaeogeographic development of the southern North Sea basin. Here, a number of key East Anglian Plio/Pleistocene sites are subjected to robust palaeomagnetic and mineralogical examination, in order to determine those sediments which display reliable, syn‐depositional magnetic polarities, which are thus of use in ascribing a palaeomagnetically determined age from comparison with the Geomagnetic Polarity Timescale (GPTS). Based on a range of palaeomagnetic and complementary mineralogical methods, reliable palaeomagnetic directions were obtained from eight sites, with reversed polarities displayed by sediments from three sites. These polarity determinations can be used to infer absolute ages and test published, between‐site correlations. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

2.
A detailed palaeomagnetic investigation of early Silurian volcanics of the Lawrenceton Formation in the Central Mobile Belt (CMB) of Newfoundland reveals primary remanence that places the sampled part of the CMB on the margin of Laurentia in low southerly latitudes in the early Silurian. The palaeomagnetic data indicate large vertical axis (net clockwise) rotations within the CMB and between the CMB and cratonic Laurentia. The volcanics were folded, faulted and in places cleaved in Silurian to Early Devonian times. Anisotropy of magnetic susceptibility measurements on lava flow samples verify field observations of a primary flow-parallel foliation and confirm field measurements of the palaeo-horizontal. A well-developed tectonic fabric was detected at one sampling site near the Reach Fault.Silurian redbeds of the Big Indian Pond Formation were also sampled for palaeomagnetic study. The sediments are folded, faulted and cleaved. A statistically sound syn-folding remanence was identified which, in a tectonically corrected state, is consistent in direction with the remanence in the Silurian volcanics. The sampled sediments are more intensely deformed than the volcanics and magnetic anisotropy measurements indicate both primary layering- and secondary cleavage-related fabrics in varying degrees from site to site. It is possible that cleavage development resulted in remanence reorientation leading to the illusion of syn-folding remanence when, in fact, the remanence was acquired prior to folding.Previously published palaeomagnetic investigations of Silurian rocks in the CMB revealed a discrepancy in palaeofield inclination between volcanic and sedimentary rock types. The sedimentary rocks yielded shallow inclinations and the volcanics intermediate inclinations. Inclination shallowing in the sediments was put forward as an explanation, as well as contamination of the sedimentary results with a shallow overprint. Others argued that the results from the volcanics were in error. The present investigation demonstrates that key volcanics carry a shallower remanence than previously thought, bringing them into approximate agreement with the majority of the results from sediments. Steep results from the Springdale volcanics and very shallow results from the King George IV lake sediments are at variance with the rest of the data. The Springdale result has large error limits that overlap with the other data. The anomalous King George IV result remains an outlier.  相似文献   

3.
A high-resolution record of a palaeomagnetic reversal is documented in Miocene alluvial rocks of Pakistan. We examined lateral variability of lithostratigraphy and palaeomagnetic stratigraphy through the same palaeomagnetic reversal in six correlated sections. Each section contains one or more palaeomagnetic sites with directions between fully reversed and fully normal. The position of the reversal illustrates local relief in the study area and the time-transgressive nature of certain stratigraphic units. Variability in the thickness of the transitional interval indicates contemporaneous variability in sediment accumulation rates. Different characteristics of palaeomagnetic remanence are associated with the depositional and post-depositional history of these sediments. Variability in patterns of remanence behaviour is the basis for inferences about post-depositional processes. We discuss two magnetic parameters that express coherency of palaeomagnetic samples—the maximum angle of deviation and the circular standard deviation. Of particular interest are samples with incoherent palaeomagnetic signals. The incoherency of samples is inversely correlated with the thickness of the transitional interval. A low rate of sediment accumulation, suggested by a thin transitional interval, may facilitate a prolonged period of remagnetization through pedogenic or hydrological processes. Alternatively, transition intervals, denned by coherent magnetization, may be thin as a result of pedogenically induced incoherency.  相似文献   

4.
PETER TURNER 《Sedimentology》1975,22(4):563-581
Carboniferous Limestones in the Craven Basin of northern England carry a stable natural remanent magnetization (NRM) the intensity of which is facies dependent. Dark argillaceous limestones are most strongly magnetized and pure, pale coloured limestones most weakly magnetized. Partial thermal and alternating field demagnetization suggest that magnetite is the principal carrier of the remanence although some haematite is present in the limestone. The presence of magnetite is confirmed by the low temperature transition, isothermal remanent magnetization (IRM build-up curves) and microprobe analysis. Partial demagnetization of IRM and ARM suggest that the magnetite is relatively coarse grained and in the multidomain state. There are no indications of pseudo-single domain behaviour but magnetite of this type cannot be excluded as a possible remanence carrier. A grain size estimate of 10–20 μm based on coercive force and remanent coercive force is compatible with the theoretical consideration of grain size. The limestones show a weak but marked magnetic susceptibility anisotropy. This anisotropy defines a depositional fabric which indicates that the magnetization is a depositional remanent magnetization (DRM). A DRM was acquired by each specimen before compaction and cementation and was preserved because of the reducing conditions which prevailed in the early diagenetic environments of the limestones.  相似文献   

5.
Measurements of palaeomagnetic horizontal remanent intensity, saturation isothermal remanence, coercivity of remanence, high field remanence and magnetic susceptibility, have been made on lateglacial and postglacial sediments from two small, inter-drumlin hollow lakes from central Northern Ireland. These magnetic measurements have been compared with pollen and chemical analyses from the same profiles and with complementary mineral magnetic data from local soils. Representative sediment samples have also been divided into a range of particle size fractions and each fraction has been subjected to magnetic investigation. Five distinctive stratigraphic horizons have been identified on the basis of the magnetic mineralogy. Two horizons relate to phases of pronounced erosion. One of these is connected with solifluction processes in the lateglacial and the other to an intensive period of farming activity which started in Medieval times. Two horizons are associated with periods of very high iron and manganese deposition and are possibly related to the precipitation of magnetic minerals within the lake. Hydrological changes during periods of local anthropogenic activity appear to produce magnetic mineral assemblages indicative of stream bed/bank substrate sources. The particle size data also support the suggestion that downcore changes in the mineral magnetic record are mainly caused by changes in the sediment source rather than particle size or sedimentological effects.  相似文献   

6.
This paper reports on the complex relation between rock emplacement and remanence acquisition in tuffs deposited by pyroclastic density currents, disclosed by systematic measurements of the anisotropy of magnetic susceptibility and natural remanent magnetization (NRM). Thermal demagnetization shows that the NRM consists of two components with different blocking-temperature spectra. The direction of the low-temperature component is consistent with the geocentric axial dipole value, whereas the high-temperature component has dispersed directions. The magnetic fabric is oblate, the magnetic foliation is close to the bedding and the lineations are generally dispersed along a girdle within the foliation plane. The directions of the magnetic lineation and the high-temperature remanence component of individual specimens are close to each other. This correspondence suggests that the high blocking-temperature grains acquired a remanence aligned to their long dimension before deposition, while cooling within the explosive cloud and the moving pyroclastic current. Thereafter, during deposition, the traction processes at the base of the current oriented the grains along the flow direction and affected both fabric and high-temperature remanence. This NRM component results from mechanical orientation of previously magnetized grains and is thus detrital in origin. A second, thermal component was then acquired during the cooling of the low blocking-temperature grains after deposition. These results show that NRM in fine-grained pyroclastic rocks is affected by the Earth’s magnetic field as well as the emplacement processes and that magnetic fabric data are essential to unravel its complex nature.  相似文献   

7.
The Chilean Iron Belt is an important location for Fe and Cu(Fe)-Au deposits and includes the recently developed Candelaria deposit, which is located some 20 km south of the city of Copiapó in northern Chile. This mine is now a major Cu producer in Chile but its discovery in the late 1980s was relatively fortuitous. The exploration programme included a ground magnetic survey from which lessons can be learnt in the search for further such deposits. Palaeomagnetic studies throughout the northern part of the Chilean Iron Belt indicate major crustal rotation of the region, probably, related to oblique convergence and transpression at the Andean Margin. The application of palaeomagnetic techniques to a magnetite-apatite deposit, Mina Fresia, indicates that magnetite-rich ores in this area are capable of maintaining a significant remanence component which will contribute to their magnetic anomaly. As with the volcanics, intrusives and sediments of the region this, remanence is clearly rotated clockwise. Using 2.5D and 3D magnetic modelling, it is demonstrated that the magnetic anomaly associated with the Candelaria deposit is also dominated by a remanence component which is significantly rotated but of reversed polarity. Recognition of clockwise-rotated, remanence-dominated anomalies should provide a new key to the search for deposits in this part of Chile and elsewhere. An example of an unexplored anomaly showing this key feature of a clockwise-rotated remanence component dominating over the induced component is presented. Modelling of this anomaly indicates a pair of sources which are either ten times as big or ten times as magnetic as the Candelaria deposit. It is suggested that the low cost of palaeomagnetic study of an exploration target should be a prerequisite to magnetic anomaly interpretation of targets in tectonic areas where vertical axis crustal rotations may be a significant element of the overall deformation pattern. Received: 9 April 1999 / Accepted: 5 October 1999  相似文献   

8.
张惠民  赵凤清 《地质论评》1994,40(4):312-321
本文从变质作用与岩石矿物获得剩磁的关系和剩磁获得时间与同位素记年的相关性，岩石的形变对磁化方向的影响等方面讨论了前寒武纪变质岩古地磁研究的可行性；并列举部分国外前寒武纪早期岩石的例证；同时重点以闽北地区前寒武纪变质岩的古地磁结果为例，讨论了获得磁性可信性及其在地质构造方面的意义。  相似文献   

9.
An Early Devonian age for the continental, red‐bed succession of the Peel Sandstone Group can be defined on the basis of: (1) a derived marine fauna of late Wenlock (Homerian) age, (2) a Scoyenia ichnofacies assemblage (including Beaconites and Diplichnites) characteristic of latest Silurian to Early Devonian (Lower Old Red Sandstone magnafacies) sediments in the British Isles, (3) a microflora of late Lochkovian to Pragian age, (4) a detrital palaeomagnetic remanence that pre‐dates local, Acadian palaeomagnetic directions and coincides with a prominent, southerly, Late Silurian to Early Devonian excursion in the local apparent polar wander path, and (5) a mid‐Devonian palaeomagnetic remanence that overprints (?)Acadian, thrust‐related folding. Data presented in this study confirm previous suggestions (Allen and Crowley 1983) that the Peel Sandstone Group represents a rare example of Early Devonian sedimentation preserved on the northern margin of the former Eastern Avalonia microcontinent. Potential correlations and linkages with other Lower Old Red Sandstone successions exposed in the Anglo‐Welsh Basin are developed and discussed. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

10.
郑洪波 Rolph  T 《第四纪研究》1996,16(4):329-337
环县剖面的古地磁研究结果表明，在第一层古土壤S1的上部和下部分别存在两个地磁场异常。通过剖面磁化率曲线与深海沉积物氧同位素曲线的对比，建立了地磁场变化的时间序列。S1上部记录的地磁场异常，年龄与挪威-格陵兰海及大西洋中发现的地磁异常年龄相当。S1下部的地磁异常，以磁偏角方向强烈摆动，偶极磁场强度明显下降为特征，其年龄与报道的布莱克事件的年龄相当，也与索马里海和地中海所记录的大幅度偶极磁场减弱相吻合，但与西宁剖面的记录不同的是，环县剖面记录的地磁场从没有达到完全的倒转，VGP曲线表现为围绕采样点旋转的经过南半球低纬地区的大环。这些特征表明，在偶极磁场减弱期，具有强烈区域性的非偶极磁场的变化可能对地磁场起着重要的作用。  相似文献   

11.
Magnetic properties, namely intensity of natural remanent magnetisation (NRM) and magnetic susceptibility, of mafic and ultramafic formations of Eastern Ghats belt in the Godavari Districts, Andhra Pradesh are studied. The formations sampled for the study are, iron-ore, chromite-ore, amphibolite, gabbro, charnockite and khondalite gneiss. Mineralogical differences in iron-ore and the degree of martitisation are reflected in magnetic susceptibility. NRM and susceptibility of chromite-ore are feeble. The gabbro exhibits strong magnetic properties because of its contiguity with iron formation. The Koenigsberger ratio is found to indicate relative palaeomagnetic stability.  相似文献   

12.
红层是古地磁学的重要研究对象之一。以往对河湖相红层的古地磁研究较多,而对于风成红层的研究较少。因 此,对于风成红层剩磁记录是否可靠等基本问题仍然缺乏清晰的认识。文章对江西信江盆地上白垩统圭峰群塘边组风成 红层和河口组河流相红层开展了古地磁研究,并通过对比风成红层与河流相红层的古地磁结果,探究风成红层剩磁记录 的可靠性及不同沉积过程对古地磁记录的影响。逐步热退磁实验结果显示仅有19% 的塘边组风成红层分离出稳定的特征 剩磁,而且其强度衰减曲线为凸形,表明特征剩磁为碎屑赤铁矿携带的原生剩磁。其平均方向为Ds=15.6 °, Is=28.9 °, n= 25, κ=13.0, α95=8.3 °;对应的古地磁极为Latitude=70.7 °, Longitude=245.6 °, A95=6.8 °。该古地磁极与赣州地区河湖相红层 的古地磁极及华南晚白垩世的古地磁极位置一致,表明风成红层的剩磁记录是可靠的。河口组河流相红层绝大部分样品 未能分离出稳定的特征剩磁。磁化率各向异性结果显示塘边组和河口组为沉积组构。岩石磁学结果表明,载磁矿物为赤 铁矿和磁铁矿。通过对塘边组风成红层的薄片观察和红度值比较等进一步研究表明,颗粒粒度和胶结程度可能对红层剩 磁记录的稳定性有一定影响。  相似文献   

13.
红层是古地磁学的重要研究对象之一。以往对河湖相红层的古地磁研究较多，而对于风成红层的研究较少。因 此，对于风成红层剩磁记录是否可靠等基本问题仍然缺乏清晰的认识。文章对江西信江盆地上白垩统圭峰群塘边组风成 红层和河口组河流相红层开展了古地磁研究，并通过对比风成红层与河流相红层的古地磁结果，探究风成红层剩磁记录 的可靠性及不同沉积过程对古地磁记录的影响。逐步热退磁实验结果显示仅有19% 的塘边组风成红层分离出稳定的特征 剩磁，而且其强度衰减曲线为凸形，表明特征剩磁为碎屑赤铁矿携带的原生剩磁。其平均方向为Ds=15.6 °, Is=28.9 °, n= 25, κ=13.0, α95=8.3 °；对应的古地磁极为Latitude=70.7 °, Longitude=245.6 °, A95=6.8 °。该古地磁极与赣州地区河湖相红层 的古地磁极及华南晚白垩世的古地磁极位置一致，表明风成红层的剩磁记录是可靠的。河口组河流相红层绝大部分样品 未能分离出稳定的特征剩磁。磁化率各向异性结果显示塘边组和河口组为沉积组构。岩石磁学结果表明，载磁矿物为赤 铁矿和磁铁矿。通过对塘边组风成红层的薄片观察和红度值比较等进一步研究表明，颗粒粒度和胶结程度可能对红层剩 磁记录的稳定性有一定影响。  相似文献   

14.
INTRODUCTIONTheUpperProterozoicglacigrnousrocksarewidelydistributedinthesoutheasternmarginoftheYangtzeplatformandaflpreservedinHunan,Hubei,GulhouandGuanghProvince.PreviousbarkontheSestudhasbeendonebyLuetal.(1985)andLiuetal.(1991).TheysUggeStedthatthesegladgenousrockscanbeusedasbarkersforhighresolutionstratigl'aphiccoagulationfromcontinenttocontinent,andadoptedtheconceptofPrecambrianglobaliceagepearland,1900.ReCentresea~showthattheSestudhaveah~useaschronostratigraphicmarkers.InrejeC…  相似文献   

15.
Palaeomagnetic results from 40 dykes sampled on Eyre Peninsula and Yorke Peninsula, South Australia, are inconsistent with the clockwise rotation of the ‘foot’ of Yorke Peninsula by fault‐drag during the Early Ordovician. A reinterpretation of earlier data from Tasmania also indicates that the east‐west structural trends in the Mount Reed volcanic arc and Dundas Trough are primary. The effects of shearing were probably restricted to linear displacements without causing rotation of adjacent structural elements. Late Precambrian and Cambrian palaeomagnetic data from Fleurieu Peninsula and Kangaroo Island have also been analysed. They do not clarify the pattern of structural evolution in the southern part of the Adelaide Orogen because the magnetic remanence was probably acquired syntectonically.  相似文献   

16.
根据福建九龙江河口ZK1孔及ZK7孔沉积物AMS(Accelerator Mass Spectrometry,加速器质谱测年技术) 14C测年、古地磁和剖面特征分析,进行该地区的第四纪地层划分,依据粒度特征,结合磁化率、微体古生物和Sr/Ba等环境代用指标进行沉积相分析,重建了该区晚第四纪的古环境演变历史。结果表明:(1)九龙江河口第四纪沉积时代为晚更新世中期至全新世晚期。(2)上更新统沉积可划分为下部龙海组洪冲积相和上部东山组河漫滩相沉积,中间有10 ka的沉积间断,末次冰期又再遭受风化剥蚀作用,推测有5～6 ka的风化剥蚀期。(3)全新统最早沉积年代约为14 ka BP,依次出现河口湾相、河口砂坝—分汊河道相、潮滩相沉积。(4)根据年代学数据和沉积相变化,重塑了该区晚第四纪沉积环境变化过程,为该区的第四纪研究和工程地质工作提供了参考依据。  相似文献   

17.
碳酸盐岩中磁性矿物及其在古地理研究中的作用   总被引：2，自引：0，他引：2       下载免费PDF全文
磁性矿物及其转化现象在碳酸盐岩中十分常见。在其沉积演化各个阶段形成的磁性矿物,分别携带了它们形成时的古环境信息,是沉积古地理学和环境磁学的直接研究对象。磁性矿物在其形成过程中还受到地磁场的磁化作用,致使其获得了原生剩磁。鉴于对这类磁性矿物的准确鉴定及其所携带的原生剩磁成分的准确辨识,是获得可靠古地磁资料和建立地块极移曲线、编制非当今界限的古地理图与进行古地理重建的前提,从而成为古地磁学和大地构造古地理学的主要研究对象。在后生阶段形成的磁性矿物,通常由原有的磁性矿物转化而成,这种转化作用常起因于构造运动及与其密切相关的岩浆活动和流体活动。由于在其转化过程中,地磁场也相应发生了变化,所以新生磁性矿物就携带上新的剩磁（重磁化剩磁）,因而它们也应该是古地磁学和大地构造古地理学的重要研究对象。如果在进行古地磁研究时,增加与构造事件和热事件相关样品的采集,加强磁性矿物的成因矿物学研究,那么就可以为进行古地理重建,提供更加可靠和更加丰富的古地磁资料。由此表明,在古地理学这一门综合性很强的地球科学,发展到巳经形成许多分支学科的今天,有必要关注磁性矿物与古地理研究之间的密切联系。  相似文献   

18.
Several examples of fault-related pseudotachylites display a significantly higher initial magnetic susceptibility than their granitic host rock (10:1 to 20:1). These higher values are attributed to the presence of fine magnetic particles formed during melt quenching. The hysteresis properties of the particles indicate a single domain (SD) to pseudo single domain (PSD) magnetic grain size. The Curie temperature (Tc) of the magnetic particles is close to 580 °C.The natural remanent magnetization (NRM) of these pseudotachylites is also significantly higher than that of the host rock (up to 300:1). Such anomalously high remanence cannot be explained by a magnetization acquired in the Earth's magnetic field, regardless of pseudotachylite age.Ground lightning and other strong electric pulses can cause anomalously high NRM intensities. A ground lightning explanation seems unlikely to explain the systematically high NRM intensities, particularly in the case of recently exposed samples that have been collected from active quarries. Alternatively, high NRM intensities could be explained by earthquake lightning (EQL), a seismic phenomenon occasionally reported in connection with large magnitude earthquakes (M > 6.0).The coseismic electrical properties of the pseudotachylite vein–host rock system are characterized by (1) a core of molten material (high conductivity), (2) vapor-rich margins of thermally and mechanically fractured host rocks (low conductivity) and (3) moderately fractured to undeformed host rock (normal conductivity). Such a core conductor bordered by insulating margins is potentially responsible for the propagation of EQL pulses.The coseismic thermal history of pseudotachylite veins has been modeled in 2-D using conductive heat transfer equations. It shows that EQL can be recorded only during a brief time interval (less than 1 min) for a given vein thickness and host-rock temperatures. If the vein is too thick or if the host rock is too hot, the pseudotachylite remains above Tc after the electric pulse has lapsed.  相似文献   

19.
This study has investigated magnetic remanence, rock magnetism and anisotropy of magnetic susceptibility (AMS) in granulite and amphibolite grade metamorphic terranes of the Huabei Shield between Inner Mongolia in the west and the Bohai Sea in the east. Rock magnetic studies identify annealed metamorphic magnetite grains with multidomain properties as the remanence carriers; a widely recorded stable remanence was probably fixed by grain shape effects. Granulite facies terranes are typically between one and two orders more strongly magnetised than amphibolite terranes and AMS fabrics correlate mostly with metamorphic mineral fabrics observed in the country rocks. Progressive thermal demagnetisation identifies a range of two and three component structures resident in magnetite. An important component recognised as a partial or complete remagnetisation by Late Mesozoic–Tertiary tectonic/magmatic activity is present in basement at the southern margin of the outcrop (Miyun terrane) and where extensive granite plutonism has occurred (Zhunhua terrane). These components have directions corresponding to remanence in the Yunmeng Shan Granite (119–114 Ma, D/I=33/58°, 39 samples, a95=3.5°, palaeopole at 201°E, 64°N). Most remanence elsewhere was probably acquired during post-tectonic uplift and cooling of the basement between ∼2200 and 1850 Ma because palaeomagnetic directions are removed from the Phanerozoic palaeofield path and they are distinct from the palaeomagnetic record in the overlying Jixian Supergroup deposited at ∼1840–900 Ma. These latter magnetisations are considered reliable indicators of the palaeofield during Late Palaeoproterozoic times because deformation of overlying supracrustal rocks is mostly slight and no prominent deflection of magnetic remanence by magnetic fabrics is observed. Palaeofield directions and poles attributed to the time of uplift-related cooling are: Qian’an Terrane (D/I=215/71°, a95=9°, 17 samples, pole at 99°E, 10°N) and North Qianxi Terrane (D/I=44/−45°, a95=4°, 41 samples, pole at 79°E, 11°S). In addition, a more widely-preserved shallow northerly component correlates with a NW→E swathe of components recorded by uplift-related cooling within the Datong–Huan’an granulite terrane in the west of the shield. A preliminary Palaeo-Mesoproterozoic apparent polar wander path for the Huabei Shield is defined from the Palaeoproterozoic record in the metamorphic basement rocks and the Meso-Neoproterozoic record in the overlying Jixian Supergroup. It incorporates a loop between ∼2200 and 1850 Ma and exhibits a general east to west trend in subsequent times.  相似文献   

20.
Although geological comparisons between Australia and North America have provided a basis for various Neoproterozoic Rodinia reconstructions, quantitative support from precisely dated palaeomagnetic poles has so far been lacking. We report U–Pb ages and palaeomagnetic results for two suites of mafic sills within the intracratonic Bangemall Basin of Western Australia, one of which is dated at 1070 ± 6 Ma and carries a high‐stability palaeomagnetic remanence. Comparison of the Bangemall palaeopole with Laurentian data suggests that previous reconstructions of eastern Australia against either western Canada (SWEAT) or the western United States (AUSWUS) are not viable at 1070 Ma. This implies that the Pacific Ocean did not form by separation of Australia–Antarctica from Laurentia, and that up to 10 000 km of late Neoproterozoic passive margins need to be matched with other continental blocks within any proposed Rodinia supercontinent. Our results permit a reconstruction (AUSMEX) that closely aligns late Mesoproterozoic orogenic belts in north‐east Australia and southernmost Laurentia.  相似文献   

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