A summary of electromagnetic studies on the Abitibi-Grenville transect

Electromagnetic surveys on the Abitibi-Grenville Lithoprobe transect have e lucidated a number of conductivity signatures that can be genetically linke d to Precambrian tectonic processes. Some major fault zones are moderately conductive, possibly signalling graphite deposition from a mantle CO2 flux along crust-penetrating fault systems. However, conductive (graphitic) meta sedimentary rocks characteristic of foreland basins are apparently absent f rom the transect area. A weak inverse correlation between metamorphic grade and electrical conductivity was observed by following rock units across th e Grenville Front into high-grade equivalents within the parautochthonous b elt. A uniformly conductive mid-crustal layer extends across the Grenville Front, apparently without change in character. The existence of this ubiqui tous mid-crustal conductor has been interpreted to mean that electrical con ductivity is controlled by the present-day pressure, temperature, and fluid saturation of the lower crust, independent of ancient structure, mineralog y, or metamorphic grade. Lower crustal (upper mantle?) electrical anisotrop y is pervasive across the transect area. An apparent spatial correlation of conductivity anisotropy with Archean tectonic deformation patterns has bee n interpreted to indicate that the lithosphere has remained intact since th e Neoarchean.