A palaeomagnetic test of the axial dipole assumption and implications for continental distribution through geological time

Details A palaeomagnetic test of the axial dipole assumption and implications for continental distribution through geological time A palaeomagnetic test of the axial dipole assumption and implications for continental distribution through geological time

May 1989

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989pepi...55...37p&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 55, Issue 1-2, p. 37-53.

Physics

12

Scientific paper

The distribution of the magnetic inclination I in the global palaeomagnetic data is examined over broad windows of geological time extending from the Cenozoic back to the Archaean. Distributions are compared with the theoretical predictions from the uniform global sampling of dipolar and higher order geomagnetic field sources. The distributions are found to accord more closely with a dipolar source than with a multipolar source over most of geological time, reinforcing a conclusion by Evans based on 10% of the present data base. The exception is the interval 600-300 Ma, broadly coincident with Palaeozoic times, when the data critically fail to accord with any simple multipole assumption. We interpret this to be a symptom of the remagnetisation effect: the sedimentary-derived (and to a much lesser extent the igneous- and metamorphic-derived) record from these times has been extensively remagnetised in late Palaeozoic times as the result of a unique incidence of diagenetic, tectonic and thermal processes. Otherwise, we conclude that the palaeomagnetic data, interpreted on the basis of the axial dipole assumption, can be used reliably to evaluate tectonic models for older (> 600 Ma) and younger (< 250 Ma) parts of geological time.
Models for the global distribution of the continental crust for those cases that have postulated episodic or long-term accretion of the crust into quasi-rigid supercontinents are also evaluated. The Proterozoic supercontinent yields the most acceptable solution to the I distribution observed over the long interval of Precambrian times (3000-600 Ma); it also explains the variable ratios of high-latitude palaeomagnetic poles from the ancient shield segments of continental crust. Owing to the failure of the Palaeozoic data set to accord with the dipole assumption, the I distributions cannot be used to test the relationship between the supercontinent of Pangaea and the geoid symmetry during the earlier part of its history, but after 250 Ma it was most closely constrained to the form of the geoid positive. Over geological time as a whole the continental crust has shown a marked preference for the I distribution defined by the present geoid positive, rather than the distribution defined by the geoid negative. We therefore conclude that the deep mantle effects responsible for the geoid have, on average, had a surface response which has tended to move the continental crust towards this sector of the globe.

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