High‐resolution electromagnetic images of conducting zones in an upthrust crustal block

A high‐resolution, controlled‐source, electromagnetic survey in the Kapuskasing region, over exposed deep crustal material upthrust along ramp‐and‐flat geometry faults, reveals a shallow weakly‐conductive layer (5,000–10,000 Ω·m) within a resistive host (50,000–100,000 Ω·m). This layer, at ≈1 km depth, correlates spatially with a zone of enhanced reflectivity, and coincident breaks in both conductivity and reflectivity suggest that a high‐angle fault zone offsets the layer. The cause(s) of this enhanced conductivity is conjectural until this unit is drilled, but the two probable candidates are: (1) electrolytic conduction in saline‐filled porous rocks, or (2) ionic conduction along connected grain‐boundary films of graphite. Or both conduction mechanisms may be operating with the graphite interconnectivity provided by the brines.