Dipole Offset--The Time-Average Palaeomagnetic Field Over the Past 25 Million Years

Wilson, R. L.

doi:10.1111/j.1365-246x.1971.tb03616.x

Cited by 158 publications

(104 citation statements)

References 9 publications

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“…This is shown in Figure 8, for the latitudinal band between 0 and 20°for which there were 616 data. The longitudes of the VGPs are plotted relative to their site longitude [Wilson, 1971]. The approximately circularly symmetrical form of the VGPs in this plot shows that the poles have no tendency to streak in any particular direction.…”

Section: Resultsmentioning

confidence: 99%

“…In particular, the position of the mean pole for each latitudinal band of Table 1 has been plotted in Figure 10 and given in Table 2, in site longitude coordinates, introduced by Wilson [1971] as a convenient way of looking at this phenomenon. Nine of the poles are far sided, eight are right handed and six are both, including the three poles that are furthest away from the geographic pole.…”

Section: Resultsmentioning

confidence: 99%

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Latitudinal signature of Earth's magnetic field variation over the last 5 million years

Harrison

2009

Geochem Geophys Geosyst

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[1] In order to investigate the latitudinal effect of the geomagnetic field variation, a new data set consisting of virtual geomagnetic poles (VGPs) from all latitudes has been produced. Since the updated data set was limited to data with VGPs within 45°of the geographic poles, data from lava flows with low-latitude VGPs were added. More rigorous criteria were used to winnow the data. The data were divided into groups from different observation latitudes. In each group it was shown that the distribution of VGP latitudes could be described by a predominance of poles (average 82%) following a Fisher distribution with the rest following a distribution that would produce a uniform number of poles as a function of latitude. A distribution composed of two Fisher distributions also fit the data very well. For the case using a Fisher distribution plus a uniform distribution, the Fisher distribution changed such that the angular standard deviation (ASD) of VGPs from a set of observations taken at the equator is about 10°and the ASD at 60°observation latitude is about 19°. These results are similar to some results seeking to determine the ASD of VGPs as a function of observation latitude using other methods, which have been recently published, but there are also discrepancies. The results allow us to model inclination distributions as a function of observation latitude for comparison with data in which only the inclination is known, such as data from drill holes. It is shown that in order not to have doubt about the polarity of the inclination data, a drill hole has to be located at an absolute latitude greater than 27°for there to be less than a 5% error. This has major importance for the location of the ''Mission to the MOHO'' of the Integrated Ocean Drilling Program. The results also confirm that the sources of the nondipole field located in the outer core have to be more than twice as strong at high latitudes than at low latitudes so as to produce the observed increase in VGP scatter with observation latitude. The model of the secular variation proposed here is in no way a theory about how the secular variation happens, but it does allow those who wish to develop such a theory to have a model distribution with which to check their predictions.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Latitudinal signature of Earth's magnetic field variation over the last 5 million years

Harrison

2009

Geochem Geophys Geosyst

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“…The far-sided effect, first identified by Wilson [1970], has been attributed to the presence of a persistent axial quadrupole superimposed on the main axial dipole [Wilson, 1971] (Figure 7b and Table 5) corresponds to a lower energy spectrum [Lowes, 1974] for each degree (l = 2 to 4) of the spherical harmonic representation (Table 5; because the value of the axial dipole coefficient is set to 30 gT, the energy spectrum is not calculated for degree I = 1). The differences between the two models are more easily analyzed using the difference of the Br component at the CMB (Figure 7c).…”

Section: Implications For Taf Modelsmentioning

confidence: 99%

Paleomagnetic directions and K/Ar dating of 0 to 1 Ma lava flows from La Guadeloupe Island (French West Indies): Implications for time‐averaged field models

Carlut¹,

Quidelleur²,

Courtillot³

et al. 2000

J. Geophys. Res.

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Abstract. Twenty-six lava flows spanning the last million years were sampled in La Guadeloupe, French West Indies. Because of the lack of continuous volcano-stratigraphic sections in La Guadeloupe, dating is necessary in order to describe the temporal evolution of the geomagnetic field in this time interval. New K/Ar ages ranging from 50 ka to I Ma have been obtained on andesires using the Cassignol-Gillot technique at the Universitd Paris Sud -Institut de Physique du Globe de Paris Orsay laboratory. Additional flows (with K/At ages obtained using the same dating technique) were also sampled for paleomagnetic investigations. More than 200 samples were analyzed using both alternating field AF and thermal stepwise demagnetization techniques. Duplicate samplings of two flows at three different sites demonstrate that within-How dispersion is negligible for the andesitic lava flows sampled in this study. Direct comparison with an earlier paleomagnetic study performed on the island indicates that, for the three investigated flows, modern demagnetization techniques yield much better defined paleomagnetic directions. The MatuyamaBrunhes transition was recorded in a three-flow section and is dated at 781 + 18 ka, in good agreement with other recent radiometric age determinations. The mean paleomagnetic pole calculated from the 23 normal polarity flows is indistinguishable t¾om geographic north, which implies that no significant persistent axial quadrupole term can be identified at this site for the last million years. This result contradicts earlier results and has important implications for models of the time-averaged field (TAF). An Occam algorithm was used to construct a TAF model from the global volcanic database from the last 5 Ma. Substitution of the mean direction calculated from the earlier study for the Lesser Antilles by our new mean value reduces the quadrupole term by more than 30•. This effect, which was produced by changing data from a single site, demonstrates that older paleomagnetic sites may need to be reinvestigated. Furthermore, it also highlights the limitations of TAF models that can be inferred from paleomagnetic databases in their present state.

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“…The GAD hypothesis has served as an excellent first order model, especially for the use of palaeomagnetic data in terms of continental drift (IRVING, 1964;MCELHINNY, 1973). Detailed analyses of the data covering the past few million years have shown that there are second order effects which produce significant departures from the GAD hypothesis (WILSON, 1970(WILSON, , 1971(WILSON, , 1972WILSON and MCELHINNY, 1974;Cox, 1975). MERRILL and MCELHINNY (1977) have shown that the time-averaged field is best represented by a series of zonal harmonics in a spherical harmonic expansion.…”

Section: Introductionmentioning

confidence: 99%

Comparison between the Thelliers' and Shaw's palaeointensity methods using basalts less than 5 million years old.

Senanayake

McElhinny

McFadden

1982

J. geomagn. geoelec

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We present 45 new determinations of the intensity of the geomagnetic field from basaltic rocks, less than 5 Ma in age, at sites mainly in the southern hemisphere. For 43 samples both Thelliers' (modified by Coe) and Shaw's (modified by Kono) methods were used and in some cases repeat measurements were made on several specimens. The results using the two methods are in excellent agreement and show no systematic difference. The within-specimen and between-sample measurements at each site exhibit internal consistency that can be associated with standard errors in the palaeointensities of 10%.Very importantly the specimens for the determinations were individually selected using a new simple method based on low temperature susceptibility variation down to liquid nitrogen temperatures. Better than 90% success rate was achieved with such specimens in providing good palaeointensities using either method. We attribute the high success rate and the agreement between methods to the method of specimen selection.

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Dipole Offset--The Time-Average Palaeomagnetic Field Over the Past 25 Million Years

Cited by 158 publications

References 9 publications

Latitudinal signature of Earth's magnetic field variation over the last 5 million years

Latitudinal signature of Earth's magnetic field variation over the last 5 million years

Paleomagnetic directions and K/Ar dating of 0 to 1 Ma lava flows from La Guadeloupe Island (French West Indies): Implications for time‐averaged field models

Comparison between the Thelliers' and Shaw's palaeointensity methods using basalts less than 5 million years old.

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