Publication 56

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

Alan G. Jones, Richard C. Bailey, and Marianne Mareschal


A high-resolution electromagnetic survey over exposed deep crustal material upthrust along ramp-and-flat geometry faults reveals a shallow weakly-conductive layer within a more resistive host. This layer, at a depth of around 1~km, correlates spatially and in fine details with a region of enhanced reflectivity proposed as a suitable continental drilling target. Coincident breaks in conductivity and reflectivity are suggestive of high-angle faults or shear zones offsetting the layer. Although the reflective/conductive layer has a shallow northward dip, the EM data show that the conductive region does not approach the surface to the south, implying that there is little to be gained by relocating the drillhole. The cause of the observed moderately-enhanced conductivity is unknown. The associated enhanced reflectivity suggests rock heterogeneity, which may facilitate electrolytic conduction in a few percent of saline-filled porosity. Alternatively, physical rock property studies indicate the presence of conductive, grain-boundary graphite which, if interconnected, would provide a pathway for ionic conduction. Or perhaps both conduction mechanisms are operating with the interconnectivity of the graphite provided by the saline fluids.


Geophysical Research Letters, 21, 1807-1810, 1994.

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Alan G Jones / 10 June 2004 /