共查询到20条相似文献，搜索用时 15 毫秒
1.
V. E. Pavlov A. M. Pasenko A. V. Shatsillo V. I. Powerman V. V. Shcherbakova S. V. Malyshev 《Izvestiya Physics of the Solid Earth》2018,54(5):782-805
Representative paleomagnetic collections of Lower Cambrian rocks from the northern and eastern regions of the Siberian platform are studied. New evidence demonstrating the anomalous character of the paleomagnetic record in these rocks is obtained. These data confidently support the hypothesis (Pavlov et al., 2004) that in the substantial part of the Lower Cambrian section of the Siberian platform there are two stable high-temperature magnetization components having significantly different directions, each of which is eligible for being a primary component that was formed, at the latest, in the Early Cambrian. The analysis of the world’s paleomagnetic data for this interval of the geological history shows that the peculiarities observed in Siberia in the paleomagnetic record for the Precambrian–Phanerozoic boundary are global, inconsistent with the traditional notion of a paleomagnetic record as reflecting the predominant axial dipole component of the geomagnetic field, and necessitates the assumption that the geomagnetic field at the Proterozoic–Phanerozoic boundary (Ediacaran–Lower Cambrian) substantially differed from the field of most of the other geological epochs. In order to explain the observed paleomagnetic record, we propose a hypothesis suggesting that the geomagnetic field at the Precambrian–Cambrian boundary had an anomalous character. This field was characterized by the presence of two alternating quasi-stable generation regimes. According to our hypothesis, the magnetic field at the Precambrian–Cambrian boundary can be described by the alternation of long periods dominated by an axial, mainly monopolar dipole field and relatively short epochs, lasting a few hundred kA, with the prevalence of the near-equatorial or midlatitude dipole. The proposed hypothesis agrees with the data obtained from studies of the transitional fields of Paleozoic reversals (Khramov and Iosifidi, 2012) and with the results of geodynamo numerical simulations (Aubert and Wicht, 2004; Glatzmayer and Olson, 2005; Gissinger et al., 2012). 相似文献
2.
A. M. Fetisova R. V. Veselovskiy F. Scholze Yu. P. Balabanov 《Izvestiya Physics of the Solid Earth》2018,54(1):150-162
The results of detailed paleomagnetic studies in seven Upper Permian and Lower Triassic reference sections of East Europe (Middle Volga and Orenburg region) and Central Germany are presented. For each section, the coefficient of inclination shallowing f (King, 1955) is estimated by the Elongation–Inclination (E–I) method (Tauxe and Kent, 2004) and is found to vary from 0.4 to 0.9. The paleomagnetic directions, corrected for the inclination shallowing, are used to calculate the new Late Permian–Early Triassic paleomagnetic pole for the East European Platform (N = 7, PLat = 52.1°, PLong = 155.8°, A95 = 6.6°). Based on this pole, the geocentric axial dipole hypothesis close to the Paleozoic/Mesozoic boundary is tested by the single plate method. The absence of the statistically significant distinction between the obtained pole and the average Permian–Triassic (P–Tr) paleomagnetic pole of the Siberian Platform and the coeval pole of the North American Platform corrected for the opening of the Atlantic (Shatsillo et al., 2006) is interpreted by us as evidence that ~250 Ma the configuration of the magnetic field of the Earth was predominantly dipolar; i.e., the contribution of nondipole components was at most 10% of the main magnetic field. In our opinion, the hypothesis of the nondipolity of the geomagnetic field at the P–Tr boundary, which has been repeatedly discussed in recent decades (Van der Voo and Torsvik, 2001; Bazhenov and Shatsillo, 2010; Veselovskiy and Pavlov, 2006), resulted from disregarding the effect of inclination shallowing in the paleomagnetic determinations from sedimentary rocks of “stable” Europe (the East European platform and West European plate). 相似文献
3.
Catherine L. Johnson Jan R. Wijbrans Catherine G. Constable Jeff Gee Hubert Staudigel Lisa Tauxe Victor-H. Forjaz Mrio Salgueiro 《Earth and Planetary Science Letters》1998,160(3-4):637-649
We present new 40Ar/39Ar ages and paleomagnetic data for São Miguel island, Azores. Paleomagnetic samples were obtained for 34 flows and one dike; successful mean paleomagnetic directions were obtained for 28 of these 35 sites. 40Ar/39Ar age determinations on 12 flows from the Nordeste complex were attempted successfully: ages obtained are between 0.78 Ma and 0.88 Ma, in contrast to published K–Ar ages of 1 Ma to 4 Ma. Our radiometric ages are consistent with the reverse polarity paleomagnetic field directions, and indicate that the entire exposed part of the Nordeste complex is of a late Matuyama age. The duration of volcanism across São Miguel is significantly less than previously believed, which has important implications for regional melt generation processes, and temporal sampling of the geomagnetic field. Observed stable isotope and trace element trends across the island can be explained, at least in part, by communication between different magma source regions at depth. The 40Ar/39Ar ages indicate that our normal polarity paleomagnetic data sample at least 0.1 Myr (0–0.1 Ma) and up to 0.78 Myr (0–0.78 Ma) of paleosecular variation and our reverse polarity data sample approximately 0.1 Myr (0.78–0.88 Ma) of paleosecular variation. Our results demonstrate that precise radiometric dating of numerous flows sampled is essential to accurate inferences of long-term geomagnetic field behavior. Negative inclination anomalies are observed for both the normal and reverse polarity time-averaged field. Within the data uncertainties, normal and reverse polarity field directions are antipodal, but the reverse polarity field shows a significant deviation from a geocentric axial dipole direction. 相似文献
4.
Miguel Cervantes-Solano Lisa Kapper Avto Goguitchaichvili Vicente Carlos Ruiz-Martínez José Rosas-Elguera Juan Morales Rafael Maciel-Peña Rubén Cejudo-Ruiz 《Studia Geophysica et Geodaetica》2017,61(2):233-248
We present a detailed rock-magnetic and paleomagnetic survey from Autlan volcanic succession in western Mexico. The principal aim of this study is to extend paleomagnetic data from Autlan lavas in order to confirm vertical-axis rotation observed in reconnaissance study and to evaluate long-term variation of the geomagnetic field strength based on existing and global data. The mean inclination (44.7°) is in agreement with the expected inclination for 60 and 70 Ma, as derived from available reference poles for the North American craton. The declination (333.6°), however, is significantly different from those expected, which suggests a statistically significant counterclockwise tectonic rotation ranging between 10° ± 6° and 14° ± 7°. As a measure of paleosecular variation (PSV), we obtained a geomagnetic field dispersion of 9.6° (upper and lower limits: 7.2°–11.9°) in perfect agreement with the previously published PSV compilation of selected Cretaceous data from lavas. The mean virtual dipole moments available for Autlan lavas are about 65% of the present geomagnetic axial dipole but are in reasonably good agreement with other comparable quality determinations between 5 and 90 Ma. This reinforces the hypothesis that low geomagnetic field strengths persisted for the entire Jurassic extending into the Upper Cretaceous. 相似文献
5.
A. N. Khramov 《Izvestiya Physics of the Solid Earth》2007,43(10):800-810
The paper analyzes previously published results of studies of detailed records of geomagnetic reversals in sedimentary and volcanic sequences of the Paleozoic in the Siberian and Eastern European platforms. It is shown that the processes of geomagnetic reversals, both in the Early Paleozoic and at the end of this era, are well described by a model in which the transitional field is controlled by an equatorial dipole. During a reversal, this dipole maintained a magnetic field at the Earth’s surface whose intensity amounted to about 20% of the intensity before and after the reversal. The equatorial dipole existed before and during the reversal and was responsible for the deviation from antipodality of paleomagnetic poles of adjacent polarity chrons (the so-called reversal bias). The position of the equatorial dipole axis during the Paleozoic correlates with the supposed geometry of convective motions in the mantle at that time. 相似文献
6.
A variety of paleomagnetic data from the Mediterranean region show a strong bias toward shallow inclinations. This pattern of shallow inclinations has been interpreted to be the result of (1) major northward terrane displacement, (2) large nondipole components in the Earth's magnetic field, and (3) systematic inclination flattening of the paleomagnetic directions. Here, we use the observation that, in addition to the well-known variation of magnetic inclination with latitude, the N-S elongation of directional dispersion also varies, being most elongate at the equator and nearly symmetric at the poles. Assuming that inclination shallowing follows the relationship long known from experiment, we invert the inclinations using a range of “flattening factors” to find the elongation/inclination pair consistent with a statistical model for the paleosecular variation. We apply the so-called “elongation/inclination” method to the extensive paleomagnetic data sets from the Miocene sediments of the Calatayud basin (Spain) and the island of Crete (Greece). After correction, the Spanish data are in good agreement with the expected middle Miocene latitude of the region. The data from Crete suggest that it occupied a position in the late Miocene about 275 km north of the predicted location. This is in agreement with the geological and geodynamical models for the east Mediterranean region, which indicate that slab rollback processes in combination with Anatolian push generated southward migration of Crete. The 7.5 million year average displacement rate of Crete estimated by the E/I method is 37 mm/yr to the south, which closely coincides with present-day rates based on global positioning system (GPS) and model measurements. We also show that inappropriate tilt corrections lead to a shallow inclination bias as well, explaining that observed in studies of lava flows of the region. We conclude that the east Mediterranean inclination anomaly is caused by sedimentary inclination error and not by a persistent octupolar contribution to the geomagnetic field, or northward transport. 相似文献
7.
Pacific plate equatorial sediment facies provide estimates of the northward motion of the Pacific plate that are independent of paleomagnetic data and hotspot tracks. Analyses of equatorial sediment facies consistently indicate less northward motion than analyses of the dated volcanic edifices of the Hawaiian-Emperor chain. The discrepancy is largest 60–70 Ma B.P.; the 60- to 70-Ma equatorial sediment facies data agree with recent paleomagnetic results from deep-sea drilling on Suiko seamount [1] and from a northern Pacific piston core [2]. Equatorial sediment facies data and paleomagnetic data, combined with K-Ar age dates along the Emperor chain [3], indicate a position of the spin axis at 65 Ma B.P. of 82°N, 205°E in the reference frame in which the Pacific Ocean hotspots are fixed. This pole agrees well with the position of the spin axis in the reference frame in which the Atlantic Ocean hotspots and the Indian Ocean hotspots are fixed [4,5], supporting the joint hypotheses that (1) the Pacific Ocean hotspots are fixed with respect to the hotspots in other oceans, (2) the hotspots have shifted coherently with respect to the spin axis, and (3) the time average of the earth's magnetic field 65 Ma B.P. was an axial geocentric dipole. Global Neogene paleomagnetic data suggest that a shift of the mantle relative to the spin axis has been occurring during the Neogene in the same direction as the shift between 65 Ma B.P. and the present. All data are consistent with a model in which the hotspots (and by inference the mantle) have shifted with respect to the spin axis about a fixed Euler pole at a constant rate of rotation for the last 65 Ma. 相似文献
8.
Some problems of the conventional “minimum polar distance” approach to Precambrian pole path construction are discussed. An alternative technique, based on a less restricted approach, is proposed and assessed by using all presently-available Precambrian and Paleozoic data from the world. In the construction of any pole path, assumptions are always made on the shape of the path through regions of apparently missing data. Many different assumptions are possible. Assuming that the Precambrian tectono-stratigraphic record conforms to repeated sequences of geological events suggests we should observe cycles in the paleomagnetic data. Empirically, it is found that the pole paths for the periods Cambrian-Devonian, and 750–1000 Ma, define cycles of pole motion from equator, up to and over the pole, and back down to the equator. Similar cycles of 250-Ma periodicity can be identified in paleomagnetic data back to 2500-Ma. Global wide changes in the phase of the cycles appears to be coincident with the major subdivisions of geological time i.e., Phanerozoic + Hadrynian, Helikian, Aphebian, and Archean. Finally, the geological implications of these cyclic changes in Precambrian pole paths are briefly discussed. 相似文献
9.
10.
One of the reasons for performing paleomagnetic studies is to determine whether the geomagnetic field remains dipolar during a polarity transition. Data on 23 field reversals of Recent, Tertiary and Upper Mesozoic age are examined with regard to the longitudinal and latitudinal distribution of paleomagnetic poles during a polarity change. Both frequency distributions of the transitional pole positions are not random. The results suggest that some field reversals are characterized by the rotation of the dipole axis in the meridional plane and show that two preferential meridional bands of polarity transitions exist centered on planes through 40°E–140°W and 120°E–60°W respectively. The latitudinal distribution of transitional paleopoles shows that there is a decrease in the number of observed poles with decreasing latitude. This is interpreted as the result of an acceleration in the motion of the dipole axis when it approaches the equator. Comparison of transitional velocities and paleointensity magnitudes reveals that the dipole moment is very weak only for a short part of the transitional period when the paleopole position lies within the latitudes of 10°N and 10°S. The overall conclusion is that the geomagnetic field retains its dipolar character during polarity changes. 相似文献
11.
O. V. Pilipenko N. Abrahamsen V. M. Trubikhin 《Izvestiya Physics of the Solid Earth》2007,43(8):691-701
Petro-and paleomagnetic methods are applied to the study of the lower part of the Early Pleistocene Tuzla section on the Black Sea coast of the Taman Peninsula, This part of the section is composed of marine and lagoonal sediments deposited over the time interval 120–70 ka. The measured curves of the variation in the geomagnetic field inclination reveal an anomalous direction dated at ～110 ka that coincides with a similar anomalous direction in the Eltigen section (Ukraine) correlating with the Blake paleomagnetic event. The significant correlation between the time series NRM0.015/SIRM0.015 (Tuzla section) and the world composite Sint-800 curve indicates that the curve NRM0.015/SIRM0.015 in the interval 110–70 ka actually reflects the variation in the relative paleointensity of the geomagnetic field. 相似文献
12.
Summary A statistical model of the geomagnetic field is derived, based on the assumption of an axial geocentric dipole field of strengthH
e at the equator perturbed by randomly directed components of constant magnitudeh. The model fits the dispersions found from an analysis of the 1945 field, and the ratioh/H
e obtained for this field and from the palaeomagnetic data both average to about 0.4. The model predicts that during reversal of the dipole field, the field intensity falls to between 0.2 and 0.4 of the steady field intensity, and this agrees with estimates made from the palaeomagnetic observations. 相似文献
13.
14.
Sediments spanning the last 9000 y from two sites in lake Vatnsdalsvatn (Lat. 66°N; Long. 23°E) in northwest Iceland hold repeatable palaeomagnetic direction records. The Vatnsdalsvatn sediments have mean palaeomagnetic inclinations of 76° close to that expected for a geocentric axial dipole field, and direction fluctuations of around 20° from the mean. The palaeomagnetic directions are stable under alternating field partial demagnetization experiments. A time scale for the Vatnsdalsvatn sediments has been estimated from 14C dating. The pattern of palaeomagnetic secular change shows few similarities with British records 2000 km distant and a central North American record 5000 km distant. 相似文献
15.
The paper presents the results of analyzing the set of dual-polarity paleomagnetic results the Global Paleomagnetic Database
(GPMDB). The dataset was expanded by the results from the Paleomagnetic Data Catalogue for the USSR and with new data published
after 2005. Some results were rejected to avoid the influence of overprints of ancient and recent magnetization. Overall,
59 dual-polarity results for the lithospheric plates of Baltica, Laurentia, and Siberia with their immediate framing were
selected for the analysis in the interval of ages 207–359 Ma. The new data confirmed the model of the paleomagnetic field,
according to which the field contains a long-lived component corresponding to the equatorial dipole which is responsible for
the non-antipodal paleomagnetic directions in the zones of normal and reverse polarity in sedimentary and volcanic rock sequences.
Retaining its value of 5–8% of the central axial dipole, the equatorial dipole changed its polarity a few times during the
interval 359–207 Ma. The northern poles of the dipole formed two antipodal groups on the Earth’s surface, which lie within
or near the subduction zones on the periphery of the Pangaea Supercontinent. Such localization of the equatorial dipole is
suggested to be related to the ascending branches of the mantle convection and to the topography of both boundaries of the
outer Earth’s core. 相似文献
16.
O. V. Pilipenko Z. V. Sharonova V. M. Trubikhin 《Izvestiya Physics of the Solid Earth》2007,43(5):430-441
The lower part of the Roxolany section (Ukraine) is studied, and paleomagnetic and petromagnetic characteristics of rock samples are obtained for the time interval 300-75 ka. Detailed curves of variations in angular parameters of the geomagnetic field are constructed in the entire time interval, and curves of variations in the relative paleointensity are obtained for the interval 300-180 ka. Using the values of the angular parameters and the deviations of the virtual geomagnetic pole from the position of the stationary field, anomalous directions compatible with the Jamaica excursion are identified in the intervals 250-249 and 221-220 ka. The geomagnetic field evolution is studied by methods of wavelet analysis, and the field generation process is shown to vary in the interval 300-180 ka. 相似文献
17.
Kenneth A. Hoffman 《Physics of the Earth and Planetary Interiors》1981,24(4):229-235
A model of the reversing geodynamo based on the assumptions (1) that reversals start in a localized region of the core and (2) that upon its onset this reversed region extends, or “floods”, both north-south and east-west until the entire core is affected, has recently been shown to provide a generally successful simulation of existing paleomagnetic records of the Matuyama-Brunhes transition (Hoffman, 1979). In this paper the modelled solution is analyzed so as to reveal the behavior of the dominant Gauss coefficients during the transition. At the time of total axial dipole decay the controlling components are found to be a zonal octupole (g30) and a non-axisymmetric quadrupole (g21, h21). Given the distribution of sites corresponding to the available records of the Matuyama-Brunhes, the existence of a significant zonal quadrupole field component cannot be ruled out; however, the role of any equatorial dipole component can be neglected.Due to the presence of a significant low-order non-axisymmetric term in the analyzed transition field, the predicted minimum intensity experienced during the Matuyama-Brunhes is found to be dependent on both site latitude and longitude. In particular, over a mid-northern circle of latitude, the predicted minimum intensity is found to vary by more than a factor of three, averaging about 10% of the full polarity field strength.Although not a unique solution, the applicability of the findings from this analysis is not tied to the phenomenological model from which they were derived. More specifically, whether the above two-component non-dipole transitional field arises from assumed configurational changes of the reversing geodynamo (as is the case for the flooding model) or, alternatively, is considered to be a stationary (non-reversing) portion of the field during axial dipole decay and regeneration, has little effect on either the calculated path locality of the virtual geomagnetic pole or the minimum intensity experienced at a given site. These two possible situations, in principle, should be distinguishable given the future attainment of detailed paleomagnetic data corresponding to back-to-back (R → N and N → R) polarity transitions. 相似文献
18.
The reversed paleomagnetic direction of the Laschamp and Olby flows represents a specific feature of the geomagnetic field. This is supported by paleomagnetic evidence, showing that the same anomalous direction was recorded at several distinct sites, including scoria of the Laschamp volcano. To examine this anomalous geomagnetic fluctuation, we studied the paleointensity of the Laschamp and Olby flows, using the Thellier method. Twenty-five samples were selected for the paleointensity experiments, and from seven we obtained reliable results. Because the paleointensity results of the Olby and Laschamp flows as well as Laschamp scoria are very similar, they can be represented by a single mean paleointensity,F = 7.7 μT. Considering that this low paleointensity is less than 1/6 of the present geomagnetic field and is more characteristic of transitional behavior, our results suggest that the paleomagnetic directions of the Laschamp and Olby flows were not acquired during a stable reversed polarity interval. A more likely explanation is that the Laschamp excursion represents an unsuccessful or aborted reversal. 相似文献
19.
The late Pleistocene geomagnetic field as recorded by sediments from Fargher Lake,Washington, U.S.A.
Measurement of the remanent magnetization of a 6.88-m oriented core of soft sediments and tephras from Fargher Lake near Mount St. Helens in southwestern Washington State shows that no significant geomagnetic reversals were recorded in the sediments of the lake. Radiocarbon and palynological dating of the tephra layers from the lake bed indicates deposition during the interval 17, 000–34, 000 years B.P. although geochemical correlation of a prominent tephra layer in the core with tephra set C of Mount St. Helens could mean that the maximum age of the sediments may be at least 36, 000 years B.P. The core was divided into specimens 0.02 m long, each representing approximately 55 years of deposition assuming a constant rate of sedimentation. Pilot alternating field demagnetization studies of every tenth specimen indicated a strong, stable remanence with median destructive field of 15 mT, and the remaining specimens were subsequently demagnetized in fields of this strength. The mean inclination for all specimens exclusive of the unstably magnetized muck and peat from near the surface is 56.1° which is 8° shallower than the present axial dipole field at this site, perhaps because of inclination error in the detrital remanent magnetization of the sediments, although because of the variability in the data, this departure from the axial dipole field may not be significant. The ranges of inclination and declination are comparable to those of normal secular variation at northern latitudes. Although three isolated specimens have remanence with negative inclination, these anomalous directions are due to sampling and depositional effects. Measurement of a second core of 6.86 m length also revealed only normal magnetic polarity, but this result is of little stratigraphic value as this core failed to penetrate the distinctive tephra found near the base of the former core.Studies of a concentrate of the magnetic minerals in the sediments by optical microscopy and X-ray diffraction indicate that the primary magnetic constituent is an essentially pure magnetite of detrital origin. The magnetite occurs in a wide range of grain sizes with much of it of sub-multidomain size (< 15 μm).As a whole, this study provides substantial evidence against the existence of large-scale worldwide geomagnetic reversals during the time interval of Fargher Lake sedimentation, a segment of geological time for which many excursions and reversals have been reported elsewhere. 相似文献
20.
O. V. Pilipenko N. Abrahamsen V. M. Trubikhin 《Izvestiya Physics of the Solid Earth》2006,42(4):344-356
Petro-and paleomagnetic methods are applied to the study of the upper part of the Late Pleistocene Tuzla section (Azov coast of the Taman Peninsula) composed of continental sediments and dated at 50–10 ka. The detailed curves of the angular components of the geomagnetic field obtained in this study display an anomalous direction coinciding in time (～25–35 ka) with an anomalous horizon discovered in rocks of the Roxolany section (Ukraine). According to the world time scale of geomagnetic excursions, the anomalous direction correlates with the Mono Lake excursion. A significant correlation between the time series NRM0.015/SIRM (Tuzla section) and NRM250/KB (Roxolany section) in the interval 50–10 ka and the world composite curves VADM-21 and Sint-800 implies that, in this time interval, the curve NRM0.015/SIRM reflects the variation in the relative paleointensity of the geomagnetic field. 相似文献