The Earth's magnetic field is primarily dipolar

Details The Earth's magnetic field is primarily dipolar The Earth's magnetic field is primarily dipolar

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....7046b&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #7046

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The question of the geometry of the Earth's magnetic field has been, and should remain, a central concern for all paleomagnetists. The founding assumption that the field has always been dominantly dipolar has been under recent challenge; stable, long standing octupolar contributions of up to 10% of the main dipole have been proposed for several periods in the Phanerozoic (e.g. ref. 1). Uncertainties that limit interpretation of paleomagnetic data arise from physical, field and laboratory problems. We note mainly uncertainties in rock or magnetization age, inclination shallowing in sediments, possible remagnetization, lack of proper averaging of secular variation in lavas, improperly modeled tectonics or unnoticed deformations of large blocks or plates, failure of reference APWPs to be valid, or uncertainties in past plate motions based on oceanic kinematic parameters... There are so many instances in which these problems have been demonstrated to occur or are likely (at no major cost to geophysical hypotheses and theories) that they must have been all excluded with satisfactory likelihood before the major and 'expensive' hypothesis that the field could be very significantly non-dipolar over long geological periods must be entertained. We will discuss a number of data that pertain to this problem. (a) In a recent review of the global paleomagnetic data base (ref. 2), when all data were averaged in 20 Ma windows, we were unable to find conclusive evidence for significant long term departures from a dipolar geometry, except for a contribution from a quadrupolar component of some 3% pm 2% (grand average) of the axial dipole. This confirms a result which had been suggested since the early 70's and vindicated by all recent analyses of the best data sets from the last 5 Ma (with a value up to possibly ca. 7%; see for instance Elmaleh et al, this meeting). Detailed analyses of key time periods when enough data with widespread enough coverage are available are clearly desirable. (b) There were early suggestions that low Cenozoic and Mesozoic inclinations in Central Asia might reveal non dipole fields (ref. 3). Recent work (ref. 4) has shown that tectonic problems or inclination shallowing due to original syn-sedimentary effects were the likely cause of widespread Asian low inclinations. In central Asia, for many times and locations, no significant difference in paleolatitude is found between the Eurasian reference curve and Cretaceous to Present volcanics, whereas paleolatitudes derived from sediments are 10 to 25 degrees shallower. Thus, an analysis of the sedimentary data may be interpreted to indicate a significant octupole component whereas an analysis of the volcanic data from the same region will show no significant octupolar signature. There are however a few cases when the volcanics may not match the Eurasian reference (e.g. Hankard et al, this meeting). (c) Ongoing work on the global data base and some new data from the Permian should provide further constraints on the geometry of the field at that time and the extent to which non-dipolar terms may be required. In (of course only provisionnal) conclusion, we believe that there is as yet not sufficiently detailed and robust evidence to reject the basic hypothesis that the Earth's field has remained on (geological, i.e. Ma durations) average close to an axial dipole, with a quadrupolar component in general no larger than 5%, throughout the Phanerozoic, and as yet ill-constrained higher order (octupolar?) components probably not in excess of a few percent. Ref.: (1) Si and Van der Voo, Terra Nova, 13, 471-478, 2001; (2) J. Besse and V. Courtillot, JGR, 107, doi:10.1029/2000JB000050, 2002; (3) M. Westphal, EPSL, 117, 15-28, 1993; (4) S. Gilder et al., JGR, 106, 30,505-30,521, 2001 and EPSL in press, 2003; J.P. Cogné et al, JGR, 104, 17,715-17,734, 1999.

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