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In the absence of an independent polarity constraint, the near-zero inclination of the Umkondo magnetization direction, resulting from the equatorial position of Kalahari ∼1100 Ma, permits either polarity assignment and makes it possible to rotate Kalahari by 180° without shifting its latitudinal position.
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For the Midcontinent rift, we used the average of the published poles for the Logan sills, Coldwell Complex, Powder Mill Group, and North Shore Volcanic Group (16, 21), which have yielded U-Pb crystallization ages of 1107.9 ± 1.8 to 1108 ± 1 Ma (14, 36, 37). The position of this average pole is 49.1°N, 209.2°E, with a radius for the 95% circle of confidence of 7.7°. Aligning this pole and the mean Umkondo pole places the two cratons ∼3000 km apart. The separation could be decreased by ∼1700 km if the extreme limits of the error circles for the poles were used.
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note
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We thank G. Brandl for advice on sample sites in South Africa; T. Machacha (Director) and T. Najaule, Botswana Geological Survey, for permission to sample borehole cores from the Xade Complex; B. Vink and M. Wendorff for assistance in Botswana; F. Venter for permission to sample in Kruger National Park, South Africa, and J. Venter for assistance in the park; and A. Maloof for discussions of paleomagnetic data. Supported by NSF grant nos. EAR-9909269 (R.E.H.) and EAR-9909854 (I.W.D.D. and W.A.G.), and EAR-9526702 (S.A.B.), and by the Tectonics Special Research Centre, University of Western Australia (I.W.D.D.). This paper is dedicated to the memory of H. Munyanyiwa, who first pointed out to us the potential significance of the Umkondo sills in eastern Zimbabwe.
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