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Paleomagnetism of the Ntonya Ring Structure, Malawi

1968; American Geophysical Union; Volume: 73; Issue: 2 Linguagem: Inglês

10.1029/jb073i002p00725

2156-2202

J. C. Briden,

Geological and Geochemical Analysis

Journal of Geophysical Research (1896-1977)Volume 73, Issue 2 p. 725-733 Paleomagnetism of the Ntonya Ring Structure, Malawi James C. Briden, James C. BridenSearch for more papers by this author James C. Briden, James C. BridenSearch for more papers by this author First published: 15 January 1968 https://doi.org/10.1029/JB073i002p00725Citations: 42AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat Abstract The natural remanent magnetization of specimens from eleven sites in syenitic and granitic rocks from the Ntonya ring structure is dominated by a large, randomly oriented IRM. Alternating-field demagnetization resulted in the isolation of a much smaller and stable component in specimens from seven sites. The mean direction of stable NRM was 311.3°E, +42.8° (downward), which corresponds on the geocentric dipole model to a paleomagnetic pole at 15.1°N, 27.8°W. At the other four sites either the stable component was too small relative to the IRM to be isolated from it by af demagnetization or it was not detected at all. The dispersion of stable NRM directions is considerably less than the dispersion associated with the present geomagnetic field. Because of its high stability against alternating-field and thermal demagnetization the systematically directed NRM is considered to be thermoremanence, acquired when the rocks cooled through the Curie point of the principle magnetic minerals (magnetite). Isotopic age determinations suggest that this stable NRM originated between 520 and about 600 m.y. ago, the latter being the most probable age. This study provides the first detailed result from late Precambrian or Lower Paleozoic rocks in Africa. References Beuf, S., B. Biju-Duval, J. Stevaux, G. Kulbicki, Ampleur des glaciations 'Siluriennes' au Sahara: leurs influences et leurs consequences sur la sedimentation, Rev. Inst. Franc. Petrole Ann. Combust. Liquides, 21, 363, 1966. Bloomfield, K., Infracrustal ring-complexes of southern Malawi, Geol. Surv. Malawi Mem., 4, 1965. Briden, J. C., Ancient secondary magnetizations in rocks, J. Geophys. Res., 70, 5205, 1965. Briden, J. C., Recurrent continental drift of Gondwanaland, Nature, 215, 1339, 1967. Brock, A., Paleomagnetic result from the Hook intrusives, Zambia, Nature, 216, 359, 1967. Cahen, L., N. J. Snelling, The Geochronology of Equatorial Africa, North Holland, Amsterdam, 1966. Chamalaun, F. H., Origin of the secondary magnetization of the old red sandstones of the Anglo-Welsh cuvette, J. Geophys, Res., 69, 4327, 1964. Clifford, T. N., The Damaran Episode in the Upper Proterozoic-Lower Paleozoic structural history of southern Africa, Geol. Soc. Am. Spec. Paper, 1967. Creer, K. M., The dispersion of the geomagnetic field due to secular variation and its determination for remote times from paleomagnetic data, J. Geophys. Res., 67, 3461, 1962. Fisher, R. A., Dispersion on a sphere, Proc. Roy. Soc. London, A, 217, 295, 1953. Graham, K. W. T., A. L. Hales, Preliminary paleomagnetic measurements on Silurian sediments from South Africa, Geophys. J., 5, 318, 1961. Gough, D. I., A spinner magnetometer, J. Geophys. Res., 69, 2455, 1964. Gough, D. I., N. D. Opdyke, The paleomagnetism of the Lupata alkaline volcanics, Geophys. J., 7, 457, 1963. Gough, D. I., N. D. Opdyke, M. W. McElhinny, The significance of paleomagnetic results from Africa, J. Geophys. Res., 69, 2509, 1964. Helsley, C. E., Paleomagnetic results from the Middle Cambrian of northwest Africa (abstract), Trans. Am. Geophys. Union, 46, 67, 1965. Irving, E., Paleomagnetism and Its Application to Geological and Geophysical Problems, John Wiley & Sons, New York, 1964. McElhinny, M. W., An improved method for demagnetizing rocks in alternating magnetic fields, Geophys. J., 10, 369, 1966. McElhinny, M. W., T. J. Neal, Portable field sampling equipment, Methods in Paleomagnetism, Elsevier, Amsterdam, 1966. McElhinny, M. W., N. D. Opdyke, The paleomagnetism of some Carboniferous glacial varves from Central Africa, J. Geophys. Res., 731, 1968. McElhinny, M. W., J. C. Briden, D. L. Jones, A. Brock, Geological and geophysical implications of paleomagnetic results from Africa, Rev. Geophys., 6, 1968. Opdyke, N. D., The paleomagnetism of the Permian redbeds of southwest Tanganyika, J. Geophys. Res., 69, 2477, 1964. Snelling, N. J., E. I. Hamilton, A. R. Drysdall, C. J. Stillman, A review of age determinations from Northern Rhodesia, Econ. Geol, 59, 961, 1964. Watson, G. S., A test for randomness of directions, Monthly Notices Roy. Astron. Soc., Geophys. Suppl., 7, 160, 1956. Citing Literature Volume73, Issue215 January 1968Pages 725-733 ReferencesRelatedInformation