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1.
An updated analysis of the global paleomagnetic database shows that the frequency distributions of paleomagnetic inclinations for the Cenozoic and Mesozoic eras (younger than 250 Ma) are compatible with a random geographical sampling of a time-averaged geomagnetic field that closely resembles that of a geocentric axial dipole. In contrast, the frequency distributions of paleomagnetic inclinations for the Paleozoic and Precambrian eras (prior to 250 Ma) are over-represented by shallow inclinations. After discounting obvious secondary causes for the bias, such as from data averaging, sedimentary inclination error, inhomogeneous lithological distributions, and tropical remagnetization, we show that the anomalous inclination distributions for the Paleozoic and Precambrian can be explained by a geomagnetic field source model which includes a relatively modest (25%) contribution to the axial dipole from a zonal octupole field and an arbitrary zonal quadrupolar contribution. The apparent change by around 250 Ma to a much more axial dipolar field geometry might be due to the stabilization of the geodynamo from growth of the inner core to some critical threshold size, a gross speculation which would imply that either the threshold size was rather large or the inner core nucleated rather late in Earth history. Alternatively, if a geocentric axial dipole model is assumed or can eventually be demonstrated independently, the anomalous inclination distributions for the Paleozoic and Precambrian may reflect a tendency of continental lithosphere to be cycled into the equatorial belt, perhaps because geoid highs associated with long-term continental aggregates influence true polar wander. 相似文献
2.
《Earth and Planetary Science Letters》2007,253(1-2):172-195
The Earth's magnetic field changed its polarity from the last reversed into today's normal state approximately 780 000 years ago. While before and after this so called Matuyama/Brunhes reversal, the Earth magnetic field was essentially an axial dipole, the details of its transitional structure are still largely unknown. Here, a Bayesian inversion method is developed to reconstruct the spherical harmonic expansion of this transitional field from paleomagnetic data. This is achieved by minimizing the total variational power at the core–mantle boundary during the transition under paleomagnetic constraints. The validity of the inversion technique is proved in two ways. First by inverting synthetic data sets from a modeled reversal. Here it is possible to reliably reconstruct the Gauss coefficients even from noisy records. Second by iteratively combining four geographically distributed high quality paleomagnetic records of the Matuyama/Brunhes reversal into a single geometric reversal scenario without assuming an a priori common age model. The obtained spatio-temporal reversal scenario successfully predicts most independent Matuyama/Brunhes transitional records. Therefore, the obtained global reconstruction based on paleomagnetic data invites to compare the inferred transitional field structure with results from numerical geodynamo models regarding the morphology of the transitional field. It is found that radial magnetic flux patches form at the equator and move polewards during the transition. Our model indicates an increase of non-dipolar energy prior to the last reversal and a non-dipolar dominance during the transition. Thus, the character and information of surface geomagnetic field records is strongly site dependent. The reconstruction also offers new answers to the question of existence of preferred longitudinal bands during the transition and to the problem of reversal duration. Different types of directional variations of the surface geomagnetic field, continuous or abrupt, are found during the transition. Two preferred longitudinal bands along the Americas and East Asia are not predicted for uniformly distributed sampling locations on the globe. Similar to geodynamo models with CMB heatflux derived from present day lower mantle heterogeneities, a preference of transitional VGPs for the Pacific hemisphere is found. The paleomagnetic duration of reversals shows not only a latitudinal, but also a longitudinal variation. Even the paleomagnetically determined age of the reversal varies significantly between different sites on the globe. The described Bayesian inversion technique can easily be applied to other high quality full vector reversal records. Also its extension to inversion of secular variation and excursion data is straightforward. 相似文献
3.
4.
Eighty-nine basaltic lava flows from the northwest wall of Haleakala caldera preserve a concatenated paleomagnetic record of portions of the Matuyama-Brunhes (M-B) reversal and the preceding Kamikatsura event as well as secular variation of the full-polarity reversed and normal geomagnetic field. They provide the most detailed volcanic record to date of the M-B transition. The 24 flows in the transition zone show for the first time transitional virtual geomagnetic poles (VGPs) that move from reverse to normal along the Americas, concluding with an oscillation in the Pacific Ocean to a cluster of VGPs east of New Zealand and back finally to stable polarity in the north polar region. All but one of the 16 Kamikatsura VGPs cluster in central South America. The full-polarity flows, with 40Ar/39Ar ages spanning a total of 680 kyr, pass a reversal test and give an average VGP insignificantly different from the rotation axis, with standard deviation consistent with that for other 0-5 Ma lava flows of similar latitude. Precise 40Ar/39Ar dating consisting of 31 incremental heating experiments on 12 transitional flows yields weighted mean ages of 775.6±1.9 and 900.3±4.7 ka for the M-B and Kamikatsura transitional flows, respectively. This Matuyama-Brunhes age is ∼16 kyr younger than ages for M-B flows from the Canary Islands, Tahiti and Chile that were dated using exactly the same techniques and standards, suggesting that this polarity transition may have taken considerably longer to complete and been more complex than is generally believed for reversals. 相似文献
5.
V. A. Bolshakov 《Izvestiya Physics of the Solid Earth》2007,43(2):161-169
New magnetic and paleomagnetic data from the loess-soil Volodarka section (the Ob River) are presented. In particular, a secondary magnetization aligned with the contemporary magnetic field and persisting on heating to 500–600°C is shown to be present. It is shown that the transition zone of the Matuyama-Brunhes reversal is most likely located within the soil complex, consisting of three paleosol horizons separated by two horizons of loesslike loam that belong to the same interglaciation. 相似文献
6.
Steven C. Cande 《Earth and Planetary Science Letters》1978,40(2):275-286
Marine magnetic anomalies 33 and 34, corresponding to the first two reversals following the long normal polarity interval in the Cretaceous, are anomalously skewed by 30° to 40° throughout the North and South Atlantic. This phenomenon is most likely related to some aspect of the dipole paleomagnetic field. Specifically the magnetic field at the time of anomalies 33 and 34 appears to be characterized by the following: the dipole field gradually decreases in average intensity between reversals and/or there is an increase in the frequency or duration of undetected short polarity events toward the end of long periods (>106 years) of predominantly one polarity. Such long-period trends in the field are in conflict with the popular model for the generation of the earth's magnetic field that treats reversals as a Poisson process and assumes that the core has no memory greater than about 104 years. 相似文献
7.
Takesi Yukutake 《Physics of the Earth and Planetary Interiors》1979,20(2-4):83-95
In view of the classification of the geomagnetic field into its axisymmetric and non-axisymmetric parts, studies of geomagnetic secular variations on the historical time-scale are reviewed. The westward drift of the geomagnetic field, which is one of the most conspicuous features of its secular variation, is examined first. The non-axisymmetric field during the past several hundred years can be well approximated by the superposition of two constant-magnitude fields, a standing and a drifting field, whose lifetimes are supposed to be longer than 1000 years. It is pointed out that the sectorial term of the non-dipole standing field is small compared with the drifting one. The lack of the n = M = 2 term of the standing field is particularly remarkable.
On the other hand, the equatorial dipole field is likely to consist of two components which are both drifting. One drifts westwards with a normal velocity and the other eastwards with a small velocity.
Besides the pronounced westward drift in an east-west direction, the poleward movements of particular foci of the secular variation are noted. This may, however, be related to the rapid growth of the axisymmetric quadrupole field.
The time variation of the dipole field is briefly examined. As far as the data on the historical time scale are concerned, an antiparallel relationship seems to exist between the variations in the dipole and the quadrupole field. As the dipole moment decreases, the magnitude of the quadrupole moment increases. Finally, characteristic oscillation periods of the dipole field are examined. Although the data are few, a 60–70-year period, a 400–600-year and a 8000-year period emerge as the dominant periods. 相似文献
8.
Peter Olson 《Physics of the Earth and Planetary Interiors》1983,33(4):260-274
The behavior of the main magnetic field components during a polarity transition is investigated using the α2-dynamo model for magnetic field generation in a turbulent core. It is shown that rapid reversals of the dipole field occur when the helicity, a measure of correlation between turbulent velocity and vorticity, changes sign. Two classes of polarity transitions are possible. Within the first class, termed component reversals, the dipole field reverses but the toroidal field does not. Within the second class, termed full reversals, both dipole and toroidal fields reverse. Component reversals result from long term fluctuations in core helicity; full reversals result from short term fluctuations. A set of time-evolution equations are derived which govern the dipole field behavior during an idealized transition. Solutions to these equations exhibit transitions in which the dipole remains axial while its intensity decays rapidly toward zero, and is regenerated with reversed polarity. Assuming an electrical conductivity of 3 × 105 mho m?1 for the fluid core, the time interval required to complete the reversal process can be as short as 7500 years. This time scale is consistent with paleomagnetic observations of the duration of reversals. A possible explanation of the cause of reversals is proposed, in which the core's net helicity fluctuates in response to fluctuations in the level of turbulence produced by two competing energy sources—thermal convection and segregation of the inner core. Symmetry considerations indicate that, in each hemisphere, helicity generated by heat loss at the core-mantle boundary may have the opposite sign of helicity generated by energy release at the inner core boundary. Random variations in rates of energy release can cause the net helicity and the α-effect to change sign occasionally, provoking a field reversal. In this model, energy release by inner core formation tends to destabilize stationary dynamo action, causing polarity reversals. 相似文献
9.
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
10.
V. V. Shcherbakova V. E. Pavlov V. P. Shcherbakov I. Neronov V. A. Zemtsov 《Izvestiya Physics of the Solid Earth》2006,42(3):233-243
The territory of Karelia (Baltic Shield) is virtually not represented in the global paleomagnetic database for the Lower Riphean time interval (1650—1350 Ma). As regards the paleointensity H an, the huge interval 1–2 Ga in length is represented in the global paleointensity database by only eight determinations concentrated in the interval 1–1.35 Ga. The paper presents results of paleomagnetic studies of volcanic and subvolcanic rocks composing the Early Riphean Salmi Formation, which outcrops in the valley of the lower Tulemaioki River in the northern coast area of Lake Ladoga. Results of the study indicate that, in the Early Riphean time, the East European craton was located in the tropical region of the Southern Hemisphere between 15° S and 40° S. The inferred value of H an is close to the lower boundary of the interval (1.36–11.56) × 1022 A m2, encompassing previously published intensity values of the paleofield 1–1.35 Ga; this supports the hypothesis on the existence of long intervals of a lower field in the period in question [Maquoin et al., 2003]. 相似文献
11.
A. Tilgner 《地球物理与天体物理流体动力学》2013,107(3):225-234
In geodynamo simulations which simulate the generation of an axial dipolar magnetic field, the generation mechanism appears to be adequately described as an α2-dynamo with an anisotropic α-effect. The anisotropy in the α-effect favors an equatorial dipole field, however, which calls into question the interpretation in terms of an α2-dynamo. It is shown in this article with kinematic dynamo calculations and exemplary velocity fields with an anisotropic α-effect that both types of dipolar magnetic field can be generated. Two examples of working dynamos in a sphere with flows with zero α-effect are also provided. 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.
The mean tangential stresses at a corrugated interface between a solid, electrically insulating mantle and a liquid core of magnetic diffusivity λ are calculated for uniform rotation of both mantle and core at an angular velocity Ω in the presence of a corotating magnetic field B. The core and mantle are assumed to extend indefinitely in the horizontal plane. The interface has the form z = η(x, y), where z is the upward vertical distance and x, y are the zonal and latitudinal distances respectively. The function η(x, y) has a planetary horizontal length scale (i.e. of the order of the radius of the Earth) and small amplitude and vertical gradient. The liquid core flows with uniform mean zonal velocity U0 relative to the mantle. Ω and B possess vertical and horizontal components.The vertical (poloidal) component Bp is uniform and has a value of 5 G while the horizontal (toroidal) field BT = Bpαz, where α is a constant. When |α| ? 1, the mean horizontal stresses are found to have the same order of magnitude (10?2 N m?2) as those inferred from variations in the decade fluctuations in the length of the day, although the exact numerical values depend on the orientation of Ω as well as on the wavenumbers in the zonal and latitudinal directions.The influence of the steepness (as measured by α) of the toroidal field on the stresses is investigated to examine whether the constraint that the mean horizontal stresses at the core-mantle interface be of the order of 10?2 N m?2 might provide a selection mechanism for the behaviour of the toroidal field in the upper reaches of the outer core of the Earth. The results indicate that the restriction imposed on α is related to the value assigned to the toroidal field deep into the core. For example, if |α| ? 1 then the tangential stresses are of the right order of magnitude only if the toroidal field is comparable with the poloidal field deep in the core. 相似文献
14.
A key non-linear mechanism in a strong-field geodynamo is that a finite amplitude magnetic field drives a flow through the Lorentz force in the momentum equation and this flow feeds back on the field-generation process in the magnetic induction equation, equilibrating the field. We make use of a simpler non-linear?α?2-dynamo to investigate this mechanism in a rapidly rotating fluid spherical shell. Neglecting inertia, we use a pseudo-spectral time-stepping procedure to solve the induction equation and the momentum equation with no-slip velocity boundary conditions for a finitely conducting inner core and an insulating mantle. We present calculations for Ekman numbers (E) in the range 2.5× 10?3 to 5.0× 10?5, for?α?=α 0cos?θ?sin?π?(r?ri ) (which vanishes on both inner and outer boundaries). Solutions are steady except at lower E and higher values of?α?0. Then they are periodic with a reversing field and a characteristic rapid increase then equally rapid decrease in magnetic energy. We have investigated the mechanism for this and shown the influence of Taylor's constraint. We comment on the application of our findings to numerical hydrodynamic dynamos. 相似文献
15.
We are using a three-dimensional convection-driven numerical dynamo model without hyperdiffusivity to study the characteristic structure and time variability of the magnetic field in dependence of the Rayleigh number (Ra) for values up to 40 times supercritical. We also compare a variety of ways to drive the convection and basically find two dynamo regimes. At low Ra, the magnetic field at the surface of the model is dominated by the non-reversing axial dipole component. At high Ra, the dipole part becomes small in comparison to higher multipole components. At transitional values of Ra, the dynamo vacillates between the dipole-dominated and the multipolar regime, which includes excursions and reversals of the dipole axis. We discuss, in particular, one model of chemically driven convection, where for a suitable value of Ra, the mean dipole moment and the temporal evolution of the magnetic field resemble the known properties of the Earth’s field from paleomagnetic data. 相似文献
16.
S. V. Starchenko 《Izvestiya Physics of the Solid Earth》2017,53(6):908-921
A hydromagnetic dynamo is only possible at a sufficiently powerful convection. In the Earth’s core, it is probably the nonthermal convection very much in excess of its critical level with the molecular transporr coefficients. However, in the case of medium- or large-scale fields, the critical energy level caused by the turbulent tranport coefficients is likely to be slightly below the actual level. This probably explains both the 22-year success of this type of simplified geodynamo models and the energy scaling laws for hydromagnetic fields, which generalize these models. Also the review of energy-dependent analytical and observational estimates of vortex fields, hydromagnetic scale sizes, and velocities in the core is presented. These typical parameters are partly in a new way linked to the observed and more ancient magnetic variations. New, albeit, simplified and self-evident, substantiation is given to the paleomagnetic hypothesis about the predominance of the axial dipole under a certain time averaging. In (Pozzo et al., 2012) and more recent works, it is shown that the adiabatic heat flow and electrical conductivity in the Earth’s core are severalfold higher than the generally accepted estimates. Here, the dynamo supporting Braginsky’s convection (Braginsky, 1963) (under the crystallization of the heavy fraction of a liquid onto the solid core) started less than 1 Ga ago, whereas the more ancient geodynamo was supported by the compositional convection of another type. The known mechanisms implementing this convection, which differ by the scenarios of magnetic evolution, are reviewed. This may help identify the sought mechanism through the most ancient paleomagnetic estimates of the field’s intensity and through the numerical models. The probable mechanisms of generation and their absence for the primordial and recent magnetic field of the studied terrestrial planets are discussed. 相似文献
17.
High-precision 40Ar/39Ar ages for a series of proximal tuffs from the Toba super-volcano in Indonesia, and the Bishop Tuff and Lava Creek Tuff B in North America have been obtained. Core from Ocean Drilling Project Site 758 in the eastern equatorial Indian Ocean contains discrete tephra layers that we have geochemically correlated to the Young Toba Tuff (73.7 ± 0.3 ka), Middle Toba Tuff (502 ± 0.7 ka) and two eruptions (OTTA and OTTB) related to the Old Toba Tuff (792.4 ± 0.5 and 785.6 ± 0.7 ka, respectively) (40Ar/39Ar data reported as full external precision, 1 sigma). Within ODP 758 Termination IX is coincident with OTTB and hence this age tightly constrains the transition from Marine Isotope Stage 19–20 for the Indian Ocean. The core also preserves the location of the Australasian tektites, and the Matuyama-Brunhes boundary with Bayesian age-depth models used to determine the ages of these events, c. 786 and c. 784 ka, respectively. In North America, the Bishop Tuff (766.6 ± 0.4 ka) and Lava Creek Tuff B (627.0 ± 1.5 ka) have quantifiable stratigraphic relationships to the Matuyama-Brunhes boundary. Linear age-depth extrapolation, allowing for uncertainties associated with potential hiatuses in five different terrestrial sections, defines a geomagnetic reversal age of 789 ± 6 ka. Considering our data with respect to the previously published age data for the Matuyama-Brunhes boundary of Sagnotti et al. (2014), we suggest at the level of temporal resolution currently attainable using radioisotopic dating the last reversal of Earths geomagnetic field was isochronous. An overall Matuyama-Brunhes reversal age of 783.4 ± 0.6 ka is calculated, which allowing for inherent uncertainties in the astronomical dating approach, is indistinguishable from the LR04 stack age (780 ± 5 ka) for the geomagnetic boundary. Our high-precision age is 10 ± 2 ka older than the Matuyama-Brunhes boundary age of 773 ± 1 ka, as reported previously by Channell et al. (2010) for Atlantic Ocean records. As ODP 758 features in the LR04 marine stack, the high-precision 40Ar/39Ar ages determined here, as well as the Matuyama-Brunhes boundary age, can be used as temporally accurate and precise anchors for the Pleistocene time scale. 相似文献
18.
Linear α2Ω-dynamo waves are investigated in a thin turbulent, differentially rotating convective stellar shell. A simplified one-dimensional model is considered and an asymptotic solution constructed based on the small aspect ratio of the shell. In a previous paper Griffiths et al. (Griffiths, G.L., Bassom, A.P., Soward, A.M. and Kuzanyan, K.M., Nonlinear α2Ω-dynamo waves in stellar shells, Geophys. Astrophys. Fluid Dynam., 2001, 94, 85–133) considered the modulation of dynamo waves, linked to a latitudinal-dependent local α-effect and radial gradient of the zonal shear flow. These effects are measured at latitude θ by the magnetic Reynolds numbers R α f(θ) and R Ω g(θ). The modulated Parker wave, which propagates towards the equator, is localised at some mid-latitude θp under a Gaussian envelope. In this article, we include the influence of a latitudinal-dependent zonal flow possessing angular velocity Ω*(θ) and consider the possibility of non-axisymmetric dynamo waves with azimuthal wave number m. We find that the critical dynamo number D c?=?R α R Ω is minimised by axisymmetric modes in the αΩ-limit (Rα→0). On the other hand, when Rα?≠?0 there may exist a band of wave numbers 0?m?m ? for which the non-axisymmetric modes have a smaller D c than in the axisymmetric case. Here m ? is regarded as a continuous function of R α with the property m?→0 as R α→0 and the band is only non-empty when m??>1, which happens for sufficiently large R α. The preference for non-axisymmetric modes is possible because the wind-up of the non-axisymmetric structures can be compensated by phase mixing inherent to the α2Ω-dynamo. For parameter values resembling solar conditions, the Parker wave of maximum dynamo activity at latitude θp not only propagates equatorwards but also westwards relative to the local angular velocity Ω*(θ p ). Since the critical dynamo number D c?=?R α R Ω is O (1) for small R α, the condition m ??>?1 for non-axisymmetric mode preference imposes an upper limit on the size of |dΩ*/dθ|. 相似文献
19.
Michael E. Evans 《Studia Geophysica et Geodaetica》2012,56(3):725-734
New results from the La Lieude Formation now complete the magneto-stratigraphic coverage of the Permian redbeds preserved in the Lodève Basin of southern France. The majority of the samples yield reversed polarity, with a mean of declination D = 187.4°, inclination I = ?0.5° (Fisherian statistics k = 44.2, ??95 = 5.6°, n = 16). Together with previously published results, these data indicate that the entire basin lies within the Permo-Carboniferous Reversed Superchron (PCRS). But the stratigraphically highest sample exhibits normal polarity, suggesting that the transition marking the end of the PCRS may be close. The unusual behaviour of the geodynamo that generates superchrons prompts one to ask if there are concomitant influences on the morphology of the field. The intersecting palaeomeridian method offers a means of pursuing this question. An updated analysis suggests that there are currently no compelling reasons for adding significant higher-order terms to the geocentric axial dipole (GAD) model for the Late Permian. 相似文献
20.
Mabel Mena Avto Goguitchaichvili Miguel Cervantes Solano Juan Francisco Vilas 《Studia Geophysica et Geodaetica》2011,55(2):279-309
The Early Cretaceous may be considered a key period for understanding the evolution of the Earth’s magnetic field. Some still
unsolved problems are related to the mode of paleosecular variation (PSV) of the Earth’s magnetic field before and during
the Cretaceous Normal Superchron. We report here a detailed rock-magnetic, paleomagnetic and paleointensity investigation
from 28 lava flows (331 standard paleomagnetic cores) collected in the Argentinean part of the Parana Flood Basalts (Formation
Posadas) in order to contribute to the study of PSV during the early Cretaceous and to obtain precise Cretaceous paleomagnetic
pole positions for stable South America. The average paleofield direction is precisely determined from 26 sites, which show
small within-site dispersion and high directional stability. Five sites show evidences for the self-reversal of thermoremanent
magnetization. 23 sites yielded normal polarity magnetization and only 3 are reversely magnetized. Moving windows averages
were used to analyze the sequential variation of virtual geomagnetic pole’s (VGP) axial positions. Interestingly, the axial
average VGP path traces an almost complete cycle around the geographical pole and passes near the location of all previously
published Paraná Magmatic Province poles. Both paleomagnetic poles and average VGP paths are significantly different from
the pole position suggested by fixed hotspot reconstructions, which may be due to true polar wander or the hotspot motion
itself. Only 15 samples from 5 individual basaltic lava flows, yielded acceptable paleointensity estimates. The site mean
paleointensities range from 25.2 ± 2.2 to 44.0 ± 2.2 μT. The virtual dipole moments (VDMs) range from 4.8 to 9.9 × 1022 Am2. This correspond to a mean value of 7.7 ± 2.1 × 1022 Am2 which is 96% of the present day geomagnetic field strength. These intensities agree with the relatively high values already
reported for Early Cretaceous, which are consistent with some inferences from computer simulations previously published. 相似文献