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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.

The Longest Conductivity Anomaly in the World Explained: Sulphides in Fold Hinges Causing Very High Electrical Anisotropy