The longest conductivity anomaly in the world explained: sulphides in
fold hinges causing very high electrical anisotropy
Alan G. Jones, T.J. Katsube, and Pamela Schwann
After almost three decades of study, from its initial discovery in the
1960s to laboratory analyses of rock samples last year, we can now
identify the most probable cause of the North American Central Plains (NACP)
conductivity anomaly for much of its 1,500-km strike extent. Tectonic
processes operating during Paleoproterozoic Trans-Hudson orogenesis,
with closure of the 5,000-km-wide Manikewan ocean, included
subduction and compression of sediments deposited during a hiatus in
volcanism as the first of the advancing arcs approached the Archean
continental margin to the west (Wyoming and Rae/Hearne cratons).
These sediments were folded, and syngenetic sulphides within them
migrated to concentrate along fold hinges, preferentially along strike,
leading to high anisotropy in electrical conductivity (over 2-3 orders
of magnitude). Mapping of the anomaly in similar tectonic
environments, from the southern Dakotas to northern Manitoba,
suggests that these processes were active along the whole western and
northern margin of the orogen.
However, other processes, possibly invoking graphitic emplacement in a
foredeep, more likely account for the southern terminus
of the anomaly from the Black Hills to southeastern Wyoming.
Journal of Geomagnetism and Geoelectricity, 49, 1619-1629, 1997.
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Alan G Jones / 10 June 2004 /