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1.
Rodionov V.P. Dekkers M.J. Khramov A.N. Gurevich E.L. Krijgsman W. Duermeijer C.E. Heslop D. 《Studia Geophysica et Geodaetica》2003,47(2):255-274
The Middle Ordovician Volginsky and Kirensky fossil zones were sampled in the Krivaya Luka section (Krivolutsky suite) that outcrops along the Lena river in Siberia. The Volginsky and Kirensky zones are coeval to the Llandeilo in the global geologic time scale. The Krivaya Luka section consists of siltstones, clays, sandstones, and limestones, and displays a remarkably distinct sedimentary cyclicity, especially in its reddish middle part.Stepwise thermal demagnetization yields three NRM components. Component A, isolated in the 100—250°C interval can be either normal or reversed. The normal A-component has a direction close to recent local magnetic field. The reversed A-component directions are scattered around a direction close to that of the lower Triassic Siberian traps. Component B has unblocking temperatures that range from 400 to 500°C and is represented mainly by normal polarity directions. The B-component, isolated from rocks of the middle part of the section is of a normal polarity with D = 176.5°, I = 30.0° and a North pole position at 16.2°S, 111.3°E. The other parts of the section are characterized by intermediate B-directions, which resulted possibly by partially overlapping A- and C-components. The highest temperature dual-polarity component C was isolated in the 550—670°C interval, resulting in the detection of two complete polarity zones and three magnetic reversals. The C-component is characterized by the following mean directions: for the reversed component D = 335.7°, I = 6.9°, and for the normal component D = 188.6°, I = 28.0°, which is very close to the normal polarity directions of the B-component. The corresponding paleomagnetic North pole for reversed polarity rocks is 32.6°S, 137°E, which is typical of Middle Ordovician rocks from Siberia – the mean pole for Llanvirn-Llandeilo is 30°S, 136°E (cf. Smethurst et al., 1998) – whereas for normal polarity rocks the pole position 17.2°S, 99.1°E is markedly different. Nevertheless, we assume that the C-component records the ancient geomagnetic field of Ordovician times, even though it does not pass the reversals test. This could be explained by overlapping NRM unblocking temperature spectra for the B and C components. In this case, the paleomagnetic pole positions should be interpreted with some caution.In addition, the section was logged and sampled in detail for cyclostratigraphic purposes. Spectral analysis in the depth domain using the high-field susceptibility as input parameter showed that the observed cyclicity is most likely orbitally forced. Detected spectral peaks (significant at the 95% confidence level) were close to the expected positions of the periodicities of precession, obliquity and eccentricity for the Ordovician. Consequently, the average sediment accumulation rate is estimated at 3.5 cm/kyr. Extrapolating this sedimentation rate yields a total duration of at least 1 Myr for the Volginsky fossil zone and 1.2 Myr for the entire Krivaya Luka section. 相似文献
2.
本文报道塔里木地块阿克苏—柯坪—巴楚地区奥陶纪古地磁研究新结果.对采自44个采点的灰岩、泥灰岩及泥质砂岩样品的系统岩石磁学和古地磁学研究表明,所有样品可分成两组：第一类样品以赤铁矿和少量磁铁矿为主要载磁矿物,该类样品通常可分离出特征剩磁组分A;第二类样品以磁铁矿为主要载磁矿物,系统退磁揭示出这类样品中存在特征剩磁组分B.特征剩磁组分A分布于绝大多数奥陶纪样品中,具有双极性,但褶皱检验结果为负,推测其可能为新生代重磁化.特征剩磁组分B仅能从少部分中晚奥陶世样品中分离出,但褶皱检验结果为正,且其所对应古地磁极位置（40.7°S,183.3°E,dp/dm=4.8°/6.9°）与塔里木地块古生代中期以来的古地磁极位置显著差别,表明其很可能为岩石形成时期所获得的原生剩磁.古地磁结果表明塔里木地块中晚奥陶世位于南半球中低纬度地区,很可能与扬子地块一起位于冈瓦纳古大陆的边缘;中晚奥陶世之后,塔里木地块通过大幅度北向漂移和顺时针旋转,逐步与冈瓦纳大陆分离、并越过古赤道;至晚石炭世,塔里木地块已到达古亚洲洋构造域的南缘. 相似文献
3.
TianShui Yang Masayuki Hyodo ZhenYu Yang ShiHong Zhang Toshiaki Mishima HuaiChun Wu HaiYan Li Yi Li XingAn Shi Kan Wang YiMing Ma 《中国科学:地球科学(英文版)》2014,57(8):1929-1943
Paleomagnetic records of the Gauss-Matuyama reversal were obtained from two loess sections at Baoji on the Chinese Loess Plateau. Stepwise thermal demagnetization shows two obvious magnetization components. A low-temperature component isolated between 100 and 200–250°C is close to the present geomagnetic field direction, and a high-temperature component isolated above 200–250°C reveals clearly normal, reversed, and transitional polarities. Magnetostratigraphic results of both sections indicated that the Gauss-Matuyama reversal consists of a high-frequency polarity fluctuation zone, but the characteristic remanent magnetization directions during the reversal are clearly inconsistent. Rock magnetic experiments demonstrated that for all the specimens with normal, reversed, and transitional polarities magnetite and hematite are the main magnetic carriers. Anisotropy of magnetic susceptibility indicates that the studied loess sediments have a primary sedimentary fabric. Based on virtual geomagnetic pole latitudes, the Gauss-Matuyama reversal records in the two sections are accompanied by 14 short-lived geomagnetic episodes (15 rapid polarity swings) and 12 short-lived geomagnetic episodes (13 rapid polarity swings), respectively. Our new records, together with previous ones from lacustrine, marine, and aeolian deposits, suggest that high-frequency polarity swings coexist with the Gauss-Matuyama reversal, and that the Gauss-Matuyama reversal may have taken more than 11 kyr to complete. However, we need more detailed analyses of sections across polarity swings during reversals as well as more high-resolution reversal records to understand geomagnetic behavior and inconsistent characteristic remanent magnetization directions during polarity reversals. 相似文献
4.
The International Geomagnetic Reference Field models (IGRF) for 1900–2000 are used to calculate the geomagnetic field distribution
in the Earth’ interior from the ground surface to the core-mantle boundary (CMB) under the assumption of insulated mantle.
Four reversed polarity patches, as one of the most important features of the CMB field, are revealed. Two patches with +Z
polarity (downward) at the southern African and the southern American regions stand out against the background of ™Z polarity
(upward) in the southern hemisphere, and two patches of ™Z polarity at the North Polar and the northern Pacific regions stand
out against the +Z background in the northern hemisphere. During the 1900–2000 period the southern African (SAF) patch has
quickly drifted westward at a speed of 0.20–.3° /a; meanwhile its area has expanded 5 times, and the magnetic flux crossing
the area has intensified 30 times. On the other hand, other three patches show little if any change during this 100-year period.
Extending upward, each of the reversed polarity patches at the CMB forms a chimney-shaped “reversed polarity column” in the
mantle with the bottom at the CMB. The height of the SAF column has grown rapidly from 200km in 1900 to 900km in 2000. If
the column grows steadily at the same rate in the future, its top will reach to the ground surface in 600–700 years. And then
a reversed polarity patch will be observed at the Earth’s surface, which will be an indicator of the beginning of a magnetic
field reversal. On the basis of this study, one can describe the process of a geomagnetic polarity reversal, the polarity
reversal may be observed firstly in one or several local regions; then the areas of these regions expand, and at the same
time, other new reversed polarity regions may appear. Thus several poles may exist during a polarity reversal. 相似文献
5.
D. V. Kovalenko 《Izvestiya Physics of the Solid Earth》2017,53(3):410-428
Most of the studied Early Phanerozoic rocks of West Mongolia have undergone repeated remagnetization. Secondary magnetization components with normal and reversed polarity are isolated. The magnetization components with normal polarity are associated with the Mesozoic remagnetization of the rocks. The components with reversed polarity were probably formed during the Carboniferous–Permian superchron of reversed polarity. The analysis of the distribution of the reversed-polarity magnetization component in the structure of Mongolia permits some zonation. Within Mongolia, the regions with insignificant post- Permian deformations and complicated post-Permian deformations are identified; also the area of rotations of large geological blocks about the horizontal axis (Khan-Khukhei Ridge) is distinguished. It is hypothesized that in the Ordovician rocks of West Mongolia, the magnetization component that is close to primary was identified. If this is the case, the paleolatitude calculated from this magnetization direction corresponds to the interval 14°–17°–20° (minimal–mean–maximal) of probably northern latitude 相似文献
6.
7.
The results of remanent magnetic studies on eight of the nine Deccan Trap flows in the vicinity of Sagar (23°56′ N: 78°38′ E) are presented. It is found that the lower four flows in the sequence are of ‘reversed’ magnetic polarity. Of the upper four flows, the top and the bottom ones show ‘intermediate’ directions while the two flows sandwiched between these are ‘normal’. These results suggest a transitional stage between the polarity inversion of the geomagnetic field from ‘reversed’ to ‘normal’ during the eruption of these Deccan Trap flows. The remanent magnetic directions of these ‘reversed’ and ‘normal’ flows show fairly shallow inclinations and are comparable to the remanent magnetic directions of the Pavagarh basalts. 相似文献
8.
Palaeomagnetic and oxygen isotope studies on samples taken from five cores in the Mediterranean area are described. Directions of the stable remanent magnetisation show the presence of an anomalous (reversed) zone during the transition between the oxygen isotope substages 5e and 5d. This position corresponds to the age of about 117 kyr B.P. and thus to the Blake polarity episode. The duration of this episode is estimated to be about 6000 years. The study of the anisotropy of the magnetic susceptibility shows no indication of sedimentological disturbances in recorded sections. The Blake episode therefore represents a stratigraphic marker which may be used to link the continental and marine palaeoclimatic and sedimentary records. 相似文献
9.
Friedrich Heller William Lowrie Li Huamei Wang Junda 《Earth and Planetary Science Letters》1988,88(3-4)
Continous marine sedimentation characterizes many Late Permian to Early Triassic sections on the Yangtze terrane in South China. The Permo-Triassic (P/Tr) boundary section at Shangsi (Sichuan Province) consists of limestones intercalated with clays and mudstones which belong to the Wuchiapingian and Changxingian (Upper Permian) and the Griesbachian and Dienerian (Lower Triassic) stages. The P/Tr boundary is formed by a clay horizon with an unusually high iridium concentration. The intensity of natural remanent magnetization is very low with a mean of 0.15 mA m−1. About 40% of the samples contain secondary or unstable magnetization components only, whereas the remaining samples carry a characteristic remanent magnetization thought to reflect the polarity of the geomagnetic field during deposition with sufficient accuracy. Normal and reversed polarity of the characteristic magnetization constitute a pattern of at least six polarity zones, the P/Tr boundary being situated very close to the transition from a reversed to a normal polarity zone. The Shangsi polarity sequence represents part of the Illawarra interval of mixed polarity, the exact beginning of which has still to be determined. 相似文献
10.
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
11.
Superposition of paleomagnetic polarity logs of seven chronologically overlapping piston cores from the central equatorial Pacific, using the established tropical radiolarian zonation as a stratigraphic reference, produced a nearly continuous correlation of magnetic and radiolarian events ranging from late Pleistocene to earliest Miocene. Twenty magnetic polarity epochs, and possibly as many as 30 polarity events, occur during this time span. Epoch 16 (reversed polarity) appears to be the longest interval (～ 14.8–17.6m.y. B.P.) among these Neogene magnetostratigraphic units. The middle/late Miocene boundary is shown to fall within latest Epoch 11 (normal) and its approximate age is between 10.5 and 11 m.y. B.P. The early/middle Miocene boundary occurs within the top of Epoch 16 at a suggested age of about 15 m.y. B.P. 相似文献
12.
A paleomagnetic record of the geomagnetic field during its change of polarity from the reversed Matuyama epoch to the normal Brunhes epoch has been obtained from sediments of ancient Lake Tecopa in southeastern California. The polarity switch occurs in siltstone of uniform composition, and anhysteretic magnetization experiments indicate that the magnetic mineralogy does not change markedly across the transition. Within the transition interval, intensity of the magnetization drops to a minimum of 10% of the intensity after the transition. The interval of low field intensity preceded and lasted longer than the interval during which the field direction reversed, the latter being shorter than the interval of low intensity by a factor of at least 2.5. The VGP's make a smooth transit from reversed to normal polarity, with the path lying in the sector of longitude between 30°E and 60°W. Pole paths for the Brunhes-Matuyama transition recorded in California and Japan are completely different, indicating that the dipole field decayed. The transition field appears to be nondipolar, and there is no evidence for an equatorial component. Since there is little dispersion of the VGP's about a great circle path, it is possible that large-scale drift of the nondipole field ceased during this polarity transition. 相似文献
13.
Marie-Gabrielle Moreau Hugo BucherAnne-Marie Bodergat Jean Guex 《Earth and Planetary Science Letters》2002,203(2):755-767
Sampling of an industrial drill string from the northeastern Paris Basin (Montcornet, France) provides early Jurassic magnetostratigraphic data coupled with biochronological control. About 375 paleomagnetic samples were obtained from a 145 m thick series of Pliensbachian rocks. A composite demagnetization thermal up to 300°C and an alternating field up to 80 mT were used to separate the magnetic components. A low unblocking temperature component (<250°C) with an inclination of about 64° is interpreted as a present-day field overprint. The characteristic remanent component with both normal and reversed antipodal directions was isolated between 5 and 50 mT. Twenty-nine polarity intervals were recognized. Correlation of these new results from the Paris Basin with data from the Breggia Gorge section (Ticino, southern Alps, Switzerland), which is generally considered as the reference section for Pliensbachian magnetostratigraphy, reveals almost identical patterns of magnetic polarity reversals. However, the correlation implies significant paleontological age discrepancies. Revised age assignments of biostratigraphic data of Breggia as well as an objective evaluation of the uncertainties on zonal boundaries in both Breggia and Moncornet resolve the initial discrepancies between magnetostratigraphic correlations and biostratigraphic ages. Hence, the sequence of magnetic reversals is significantly strengthened and the age calibration is notably improved for the Pliensbachian, a stage for which sections combining adequate magnetic signal and biostratigraphic constraints are still very few. 相似文献
14.
Magnetostratigraphic study of the Toarcian type sections of Thouars and Airvault (Deux-Sèvres, France) has yielded two reliable magnetic polarity sequences. Most samples were treated by mixed cleaning: thermal demagnetization (250°, 300° or 350°C) and subsequent alternating field demagnetization. Polarity intervals are easily identified and correlate well between the two sections using the biostratigraphic data provided by the detailed standard ammonite zonation of the Toarcian stage. The polarity sequence extends from ammonite horizon V (Pseudoserpentinum horizon,Serpentinus zone) to horizon XXV (Subcompta horizon,Aalensis zone); it shows 5 reversed and 5 normal polarity magnetozones. 相似文献
15.
The Padul-15-05 sediment core provides an exceptional perspective of the paleoenvironmental and climate change in the Western Mediterranean region for the last ca. 200 kyr. However, even though a robust chronology mainly relying on radiometric dating is available for the last 50 ka, the chronology for the older sediments is not yet fully resolved. Ages for the bottom part of the core (>21 m) were previously inferred from amino-acid racemization dating and sediment accumulation rates. In this work, we provide a more accurate chronology for the older part (>100 kyr) of the Padul-15-05 sediment core record based on the recognition of past Earth's magnetic excursions. We identify an interval prone of reversed polarity samples close to MIS-5e/5 d transition that we correlate to the Blake geomagnetic excursion (116.5 kyr–112 kyr). In addition, we identify an interval of low inclinations and two reversed samples that we interpret as the Iceland Basin geomagnetic excursion (192.7 kyr–187.7 kyr: wide scenario of VGP <40°). Our new results, which include IRM acquisition curves that contribute to understand the magnetic mineralogy, enhances the robustness of the age model for the Padul-15-05 sedimentary sequence by adding an independent age dataset with new accurate tie-points. Our refined age control together with the available paleoenvironmental and paleoclimate multiproxy data provide insightful information to unveil the response of the western Mediterranean environments to regional environmental and climate change. 相似文献
16.
The results of palacomagnetic studies made on the Deccan Traps by various workers are reviewed in the light of the recent palaeomagnetic data on these rocks and the general geological information. It is suggested that: (a) the earlier altitude-polarity classification of the Deccan Traps, suggesting that the flows below the general elevation of 2000±200 feet above mean sea level are of reversed magnetic polarity while those above this horizon are normal, is not without exceptions; (b) the geomagnetic field reversed its polarity several times during the eruption of these lavas; (c) the Deccan Trap eruptions probably consisted of several phases of volcanicity over a protracted period; and (d) the phases of Deccan Trap volcanism, the phases of Himalayan upheaval, and the northward drift of the Indian landmass were rather concrescent events. 相似文献
17.
Maurice K. -Seguin 《Physics of the Earth and Planetary Interiors》1977,15(4):363-373
This palaeomagnetic study is centered on agglomerates and volcanic rocks from the western margin of the Appalachian belt in the Drummondville-Actonvale-Granby area, Quebec (long.: 72°30′W, lat.: 46°00′N). It involves a total of 36 oriented samples (111 speciments) distributed over eleven sites. Both thermal and AF cleaning techniques were used to isolate residual remanent components. The dispersion of the directions is slightly reduced after AF cleaning and thermal treatment.The palaeopole position obtained is 191°E, 6°N (dm = 14°, dp = 7°) after thermal treatment and 164°E, 19°N (dm = 11°, dp = 6°) after AF cleaning. The polarity of most of the sites (two exceptions) are reversed. The thermal-treated data appear to be relatively stable and an approximate value of the primary magnetization is extracted from them. The palaeopole obtained does not lie close to the tentatively defined position of the Cambrian and Ordovician poles from rocks of the North American plate; it is located near the Upper Cambrian and Lower Ordovician poles from eastern Newfoundland and the Lower Ordovician pole from the Caledonides in Europe. 相似文献
18.
Paleomagnetic studies of the basalt samples of Mid-Atlantic Ridge recovered during DSDP Leg 45 and the FAMOUS Project have led to a revision of our view of the oceanic igneous crust as a recorder of geomagnetic field reversals. The discovery of several magnetic polarity reversals with depth in the crust has indicated that oceanic igneous basement should not necessarily be considered magnetized uniformly in direction, or even polarity, in a given vertical cross section. Statistical arguments, based on the ratio of the average time of crustal formation to the average length of a magnetic polarity interval, indicate that magnetic reversals with depth are to be expected in typical ocean crust, but also that this does not conflict with current theories of plate tectonics or exclude the upper layers of the crust from making a major contribution to the overlying linear magnetic anomalies. Certain ratios of average crustal formation time to average polarity interval do, however, result in an effective zero magnetization for the oceanic crust and these conditions may be responsible for the reduced amplitude of magnetic anomalies in some areas. 相似文献
19.
Manuel Calvo-Rathert Bertha Aguilar Reyes Avto Goguitchaichvili José Rosas Elguera Héctor Franco Juan Morales Ruth Soto Ángel Carrancho Hugo Delgado 《Studia Geophysica et Geodaetica》2013,57(2):309-331
A rock-magnetic and paleomagnetic investigation was carried out on eleven Pleistocene and Pliocene 40Ar/39Ar dated lava flows from the Tepic-Zacoalco rift region in the western sector of the Trans-Mexican Volcanic Belt (TMVB) with the aim of obtaining new paleomagnetic data from the study region and information about the Earth’s magnetic field recorded in these rocks. Rock-magnetic experiments including measurement of thermomagnetic curves, hysteresis parameters and isothermal remanence acquisition curves were carried out to find out the carriers of remanent magnetisation and to determine their domain structure. Although some samples were characterised by the presence of a single ferromagnetic phase (magnetite), in most cases more phases were observed. Analysis of hysteresis parameters showed a mixture of single domain and multidomain particles, the fraction of the latter varying between 40% and 80%. Paleomagnetic results were obtained in all sites, although in 7 sites characteristic remanence directions and remagnetisation circles had to be combined in order to calculate site means. The six Pliocene sites not showing intermediate polarity yielded a paleomagnetic pole (latitude ? = 81.1°, longitude λ = 94.3°) which roughly agrees with the expected one. Paleomagnetic directions do not indicate significant vertical-axis block rotations in the western TMVB area. Reversed polarities observed can be correlated to the Gilbert chron, normal polarities to the Gauss chron or the Brunhes chron and intermediate polarities to the Cochiti-Gilbert or the Gilbert-Gauss transition. The reversed or intermediate polarity magnetisation recorded in one of the sites (542 ± 24 ka) corresponds either to the West Eifel 4 or the West Eifel 5 excursion, while the reversed polarity observed in the other site (220 ± 36 ka) very likely provides new evidence for the Pringle Falls excursion or the event recorded in the Mamaku ignimbrite. 相似文献
20.
Günther Kletetschka 《Studia Geophysica et Geodaetica》1992,36(3):230-239
Summary The paper deals with the statistical approach to processing the polarity durations of the Earth's magnetic dipole over the past 170 million years. Partial elimination of the random component enables the mutual relation between normal and reversed polarities to be determined. The polarity time sequence has further been divided into several intervals. The frequency distributions for normal and reversed polarities have been established for each of these intervals. It has been found that the random component of the reversal triggering mechanism hides a pattern of behaviour which displays a certain amount of logic. 相似文献