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Regional magnetic overprinting of Witwatersrand Supergroup sediments, South Africa.

Abstract:

Shales from the late Archean Wiwatersrand Supergroup of South Africa yield stable, well-grouped remanent magnetizations after thermal and alternating field demagnetization. Virtual geomagnetic poles from 17 sites located throughout the central and W parts of the basin indicate that rocks of the basin were remagnetized after folding. A paleomagnetic fold test shows that the Fisherian precision parameter, K, decreases from 22 to 3 after restoring the beds to paleohorizontal. Illite crystallinity indices show that the remagnetized samples were subjected to at least middle greenschist facies temperatures, whereas magnetic blocking temperature estimates indicate upper greenschist facies temperatures. A K/Ar age obtained from clays of 1945+ or -40Ma (2sigma ) is assigned to this metamorphic event. The new paleomagnetic pole obtained from the Witwatersrand Basin sites (19oN, 46oE, K = 22, ALPHA 95 = 8o) is consistent with other 1950-2050Ma poles from Southern Africa, confirming independently the K/Ar age. The distinctive overprint direction seen in shale specimens may be useful as a directional marker for orienting deep bore cores.-Authors