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Fig. 6.2. Analysis of global paleomagnetie results for the past 5 Myr by averaging over 45° longitude sectors: (a) normal and (b) reverse. The paleomagnetie poles tend to plot too far away, which typically puts them over the geographic pole. The number of spot readings of the field is indicated for each average. Polar stereographic projection at 76°N. After McElhinny et al. (1996).

Fig. 6.2. Analysis of global paleomagnetie results for the past 5 Myr by averaging over 45° longitude sectors: (a) normal and (b) reverse. The paleomagnetie poles tend to plot too far away, which typically puts them over the geographic pole. The number of spot readings of the field is indicated for each average. Polar stereographic projection at 76°N. After McElhinny et al. (1996).

paleomagnetie poles from Europe and Asia for the past few million years tended to plot too far away from the observation site along the great circle joining the site to the geographic pole. This has been referred to as the far-sided effect. Successive analyses by Wilson (1970, 1971), McElhinny (1973a), Wilson and McElhinny (1974), Merrill and McElhinny (1977, 1983), Quidelleur et al. (1994), Quidelleur and Courtillot (1996) and McElhinny et al. (1996) confirmed that this occurs on a global scale. Whereas in tectonic studies the selection criteria given in Table 6.1 would require DC > 3 (Table 6.2), for data in the time range 0-5 Ma it has been considered that data with DC > 2 are acceptable. The far-sided effect is illustrated in Fig. 6.2 from the analysis of McElhinny et al. (1996) in which global data for the past 5 Myr have been averaged in 45° longitude sectors. The position of the mean pole is related to the sector in each case, but the average of all eight sector poles still gives the geographic pole within a degree. Note that the reverse data tend to plot further over the pole from the sampling region than do the normal data.

Wilson (1971) introduced the concept of the common-site longitude pole position as a convenient way to analyze the overall far-sided effect. The method is to place all observers at zero longitude by replacing the pole longitude with the common-site longitude given by the difference between the pole and site longitudes. The data shown in Fig. 6.2 for the past 5 Myr have been analyzed in this way and the global mean pole positions are summarized in Table 6.3 either as an overall mean or as a common-site longitude mean. The mean common-site longitude poles for the normal and reverse global data are shown in Fig. 6.3.

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