RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 8, ES1002, doi:10.2205/2005ES000185, 2006
[3] Hospers [1954] was the first to prove that the virtual pole, averaged over the last several hundred years, coincides with a high accuracy with the geographic pole. This result, as well as those obtained by many other researchers [Irving, 1964; Opdyke and Henry, 1969, to name but a few], served as a basis for advancing a GAD hypothesis which was later tested repeatedly.
[4] Many researchers [McElhinny, 1973; Merrill and McElhinny, 1977, 1983; Quidelleur et al., 1994], the first of them being Wilson [1970], proved that in addition to its dipole component the magnetic field of the Earth, averaged over the last several million years, might include also some non-dipole members of the second order, whose total contribution, however, was not higher than 5% of the dipole component of the field. McElhinny et al. [1996] investigated in detail which of the second-order members of the harmonic expansion of the geomagnetic field could be recorded confidently using the paleomagnetic data availably for the last 5 Ma. Their analysis proved, first, that there were no confident indications that the time averaged field (TAF) included any unzoned (sectorial, tesseral) components and, secondly, that only some geocentric axial quadrupolar member might be established more or less reliably. This result was confirmed independently by Quidelleur and Courtillot [1996].
[5] It is important to remind in this connection that Khramov et al. [1982] and Yanovskii [1978] assumed the potential asymmetry of the paleomagnetic field from the Carboniferous to the Triassic, which had been associated, in their opinion, with the displacement of the dipole, oriented along the axis of the Earth rotation, toward the western segment of the Pacific Ocean. A comprehensive discussion of this point is offered below.
[6] Khramov et al. [1982] suggested the possibility of some displacement of the dipole relative to the Earth's center (which is equivalent to the presence of some unzoned members in the spherical harmonic decomposition of the paleomagnetic field (TAF)), proceeding from the papers of Adam et al. [1975] and Benkova et al. [1973]. In their papers these authors used the nonspherical harmonic representation of the averaged field, assuming obviously that the model they used, although being highly idealized, had a greater physical content than any spherical model. However, as mentioned by Merrill et al. [1996], none of the models, stipulating nonspherical decomposition, is satisfactory for describing the physical geometry of the internal sources of the geomagnetic field. Moreover, the modern dynamometric theory infers that the "real" sources of the field must be much more complex and numerous, compared to any physical models based on nonspherical expansion. For this reason, proceeding from the convenience of mathematical description, most of the present-day researchers prefer to describe the field in terms of spherical harmonic expansion. In this case the above-mentioned displacement of the dipole center means that some unzoned members were involved in spherical harmonic expansion. Apart from the authors mentioned above, the existence of unzoned members was proved by Creer et al. [1973] and Geordi [1974], who inferred that the values of unzoned constituents might be comparable with those of the zoned expansion members. However, Wells [1973] proved rigorously that only zonal members were really significant, some unzoned constituents being produced by the irregular distribution of the analyzed data in space. Later, proceeding form the analysis of the larger data base, McElhinny et al. [1996] proved that the explanation of the observed data does not require the use of any unzonal coefficients.
[7] All of the above considerations are pertinent to the time interval corresponding to the Quaternary and, partly, to the Neogene, when the movements of the lithospheric plates can be neglected during the analysis of paleomagnetic data. It is obvious that the assumption of the significant contribution of unzoned components to the paleomagnetic field of the older periods of time become even less proved in connection with the uncertainties of paleogeographic reconstructions and the space and time heterogeneity of the data distribution.
[8] Some models based on the analysis of the data available for the Pliocene, Pleistocene, and Holocene periods suggest the presence of an octupolar zonal member in addition to the dipole and quadrupole ones. The axial octupole of these models is always lower than 3% (between 1% and 1.6% [Carlut and Courtillot, 1998; Johnson and Constable, 1997]; and 2.9% in the model of Kelly and Gubbins [1997]. McElhinny et al. [1996] estimated the value of the octupolar member to be between 1% and 3%, noting that the accuracy of the data available does not allow them to rank these results as statistically significant.
[9] Gubbins and Kelly [1993], Johnson and Constable [1995, 1997], and Kelly and Gubbins [1997] interpreted the results of their complete spherical harmonic analyses of the geomagnetic field, averaged for the last 5 Ma, as the existence of low, yet statistically significant unzoned members. This conclusion was discussed in detail by Carlut and Courtillot [1998] and also by McElhinny and McFadden [2000], who proved that because of the low values of the inferred non-dipole members the very fact of their discovery depends on the potential minor inaccuracies of the paleomagnetic record and also on the use of the data that did not meet the modern requirements to laboratory processing.
[10] To sum up, at the present time we can be more or less sure
that the geomagnetic field of the last 5 Ma can be described
fairly well by the field of an axial geocentric dipole with some low
contribution of an axial geocentric quadrupole.
Merrill et al. [1996]
estimated this contribution as the
g02/g01 ratio equal to
0.038
0.012. The presence of a quadrupolar member may cause the
error of computing the paleomagnetic pole as high as 3-4o compared
to a purely dipole model. Considering that this value is lower than the
typical error of locating the paleomagnetic pole, found using a 95%
confidence circle, we can state that the GAD model describes the
geometry of the geomagnetic field for the last 5 Ma
[Merrill and McFadden, 2003].
[11] The data available for the geomagnetic field intensity during the last 10 Ma also show a good agreement with the model of a geocentric axial dipole [Tanaka et al., 1995].
[12] The analysis of the planetary geometry of the geomagnetic
field for the older epochs is aggravated by the fact that one has to be
sure concerning the fact that large movements of lithospheric plates
might or might not take place. In the cases where these movements
did occur (the view shared presently by the overwhelming majority of
geologists and geophysicists), we must first reconstruct the plate
tectonic pattern for the time of interest, using some independent data
(for instance, marine anomalies and bathymetry), and then study the
distribution of the paleomagnetic trends in the "old system of the
coordinates". In the case of the Cretaceous and younger epochs this
analysis suggests it to be unlikely that the non-dipole members had
ever been higher than a few percents of the geocentric axial dipole
[Coupland and Van der Voo, 1980;
Livermore et al., 1983, 1984].
Recently,
Besse and Courtillot [2002, 2003]
analyzed in detail the paleomagnetic data for the time of 0-200 Ma,
available in one of the latest versions of the Global Paleomagnetic
Database (GPMDB).
Using the modern kinematic models
[Müller et al., 1993;
Nürnberg and Müller, 1991;
Royer and Sandwell, 1989;
Royer et al, 1992],
all data were recalculated for one (African) plate and then, using
the time-average paleomagnetic field over the past 25 million years
[Wilson, 1971],
they calculated the paleomagnetic poles for each time window of 20 Ma.
These poles were found to be confined to the hemisphere
opposite, in terms of the reference point, to some hemisphere at an
angular distance usually not higher than
2o from the geographic pole.
Moreover, the geographic pole always resides inside a 95-percent
confidence interval corresponding to each of the calculated
paleomagnetic poles. It is only when the whole time interval (200 Ma)
is taken into consideration the deviation of the average paleomagnetic
pole from the geographical one to the opposite hemisphere (relative to
the reference point) becomes statistically significant. This can be
taken as the real indication of some "far-side" effect which can be
produced by the fact that the geomagnetic field contained a
quadrupole component with the value of 3
2% of the dipole. This
value has no practical significance for any paleotectonic reconstructions
based on paleomagnetic data. In this sense the results obtained by
Besse and Courtillot [2002, 2003]
validate the GAD hypothesis for the time interval of 0-200 Ma.
[13] In the case of older periods of time the uncertainty of plate tectonic reconstructions grows rapidly calling for the use of other methods for estimating the geometry of the Earth magnetic field.
[14] In 1976 M. E. Evans offered a new method for testing the GAD hypothesis in Precambrian and Phanerozoic rocks [Evans, 1976], based on the comparison of the real distribution of paleomagnetic inclinations, identified for a fairly long period of time, with the theoretical ones, calculated proceeding from the assumption of the dipole character of the field and the uniform distribution of "paleomagnetic measurements" over the surface of the Earth. The statistical agreement of the observed and calculated data was treated as the evidence proving the dipole character of the magnetic field; otherwise the hypothesis was discarded. It should be noted, however, that the correct application of this method calls for the use of a great number of reliable paleomagnetic data, this requirement being unsatisfied in the case of Late Proterozoic or Early Paleozoic data.
[15] The Evans method used to process Precambrian and Early Paleozoic data [Kent and Smethurst, 1998; Piper and Grant, 1989] showed the anomalous distribution of paleoinclinations, which may suggest the substantial contribution of non-dipole sources to the geomagnetic field. Admitting the fact that the observed pattern of the paleoinclination distribution may reflect the irregular (low-latitude) distribution of the continents in the time period discussed, which might have been caused by the fact of their being parts of a supercontinent, Kent and Smethurst [1998] offered a view that the contribution of the non-dipole components during the Proterozoic had been significantly higher than that during the subsequent periods of the geological history, and that the intensity of the zonal octupolar field at that time might be as high as 25% of the dipole one.
[16] However, McFadden [2004] and Meert et al. [2003] proved that the basic hypothesis on the uniform distribution of the paleomagnetic data over the Earth surface, on which the M. E. Evans method had been based, was not reliable, and hence the results of the analyses performed by J. Piper and S. Grant, as well as by D. Kent and M. Smethurst, should be dealt with as preliminary ones.
[17] Meanwhile, the authors of some recent papers [Si and Van der Voo, 2001; Torsvik and Van der Voo, 2002; Van der Voo and Torsvik, 2001, to name but a few], reported the results of their calculations which offer a serious challenge to the central axial dipole hypothesis.
[18] Using the original method, Van der Voo and Torsvik [2001] analyzed the European and North American paleomagnetic data base, including the data collected by Torsvik et al. [2001] for the time interval of 300-40 Ma. The results of this analysis can be treated as the indication of the fact that during the period of 120-40 Ma and 300-200 Ma the total geomagnetic field included some notable zonal octupolar component, the contribution of which might be as high as 10% of the dipole component. No obvious indications were found for the presence of a quadrupolar component in this time interval. The time interval of 200-120 Ma did not show any significant deviations from the dipole model.
[19] The assumed existence of an octupolar component with g03/g01 roughly equal to 0.1 allows one to solve some problems, such as the well known contradiction between the central Asian and Euroasian paleomagnetic data for the Cretaceous and Paleogene, the direct use of which calls for the significant reduction of the crust between the Central Asian continental blocks and North Eurasia, which is absolutely inadmissible in geological terms. It should be noted, however, that this problem seems to have been solved without using the hypothesis of the substantially non-dipole character of the geomagnetic field. Bazhenov and Mikolaichuk [2003] proved that the Tien Shan Paleogene basalts studied by them show primary magnetization, the inclination of which agrees fairly well with the curve of the apparent migration of the North Eurasian pole. This result proves the fact that inclination was underestimated in the previously studied Paleogene sedimentary rocks (primarily continental red beds) of Middle Asia, this precluding their use for paleotectonic reconstructions.
[20] If the time-averaged geomagnetic field (TAF) could be represented for the time of 300-200 Ma as a sum of the dipole and octupole fields, this would remove the substantial contradictions arising between the geological and paleomagnetic data during the reconstruction of Pangea. In order to achieve the better agreement between the paleomagnetic poles of Laurussia and Gondwana, which are brought together in the Pangea-A model, ranked in this paper as the most substantiated model, Torsvik and Van der Voo [2002] believe that the contribution of the octupolar source varied in time.
[21] It is important to mention that the assumption of the notable contributions of the zonal components to TAF complicates (though insignificantly) the necessary calculations, yet do not preclude the possibility of using paleomagnetic data in paleogeographic and paleotectonic reconstructions.
[22] The hypothesis advanced by R. Van der Voo and T. H. Torsvik was discussed actively during the last 2-3 years. In March 2003, at the conference held in honor of N. D. Opdyke, this problem was discussed by McElhinny [2003] who mentioned that the results obtained by R. Van der Voo and T. H. Torsvik could not be taken as a proof for the existence in the past of some substantial non-dipole component and could be explained reasonably in terms of the GAD hypothesis. Courtillot and Besse [2004] devoted a special paper to the problem raised by the authors mentioned above. Having analyzed a broader data base, they proved that during the 200-year period of time discussed the contribution of any octupolar source had not been greater than 3%, the error being greater than this value, which makes the latter to be statistically insignificant. At the same time they emphasized that the results of their analysis showed a weak (3%) but trustworthy quadripolar signal.
[23] To sum up, the numerous studies carried out by the present time show, with a high probability, that the geological history had been dominated by a dipole filed with some zonal (axially symmetric) sources operating in some individual periods of time.
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Figure 1 |
[25] In principle, in addition to the methods mentioned above, paleomagnetic data can be used for testing the dipole nature of the geomagnetic field also by way of comparing the paleomagnetic trends obtained for large undeformed crustal blocks. In particular, these blocks include epi-Hercynean platforms the constituents of which were not usually displaced relative to one another, at least since the time of their formation. As to the epi-Hercynean platforms, the largest and best known is the North Eurasian one. Khramov et al. [1982] analyzed the Late Permian data available for this platform and found that the distribution pattern of the paleomagnetic trends were in good agreement with a central dipole field with its pole located in the northwestern part of the Pacific Ocean. A similar work was done using the results of the Mesozoic paleomagnetic determinations available for Africa (described in detail in the book by McElhinny et al. [1996]). The results of this work also confirmed the consistency of a dipole hypothesis for the time interval concerned.
[26] A large volume of high-quality data, meeting the modern requirements, was accumulated during the last decade for the Permian-Triassic trap rocks of the Siberian Craton. During the study reported in this paper, an attempt was made to test the GAD hypothesis for the Paleozoic-Mesozoic boundary by way of comparing the respective Siberian paleomagnetic poles with the European poles of the same age. Also the estimation of the possible non-dipole component contribution to the averaged magnetic field of that time was carried out.
Citation: 2006), New paleomagnetic data for the Permian-Triassic Trap rocks of Siberia and the problem of a non-dipole geomagnetic field at the Paleozoic-Mesozoic boundary, Russ. J. Earth Sci., 8, ES1002, doi:10.2205/2005ES000185.
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