Electromagnetic sounding and crustal electrical conductivity in the region of the Wopmay Orogen, Northwest Territories, Canada

Abstract

Temporal variations of the three components of the geomagnetic field were recorded at eight sites along a 240 km profile across the Early Proterozoic Wopmay Orogen. After an empirical separation of these data into normal and anomalous parts, horizontal-to-vertical-field transfer functions in the period range 40–1200 s display evidence for a minor anomaly spatially located near the allochthonous shelf margin at the eastern edge of the Hepburn Batholith. The observations can be partially simulated by a two-dimensional 20 ? m body (30 km wide, 2 km thick) embedded in the surface of a very resistive layered Earth model derived from inversion of magnetotelluric sounding data at a central station. The body correlates spatially with metamorphosed graphitic pelites of the Odjick Formation (Epworth Group), a unit of deep-water facies interpreted as continental slope–rise deposits. Laboratory measurements on samples of the pelite yielded resistivity values of the order of 104 ?∙m, so the enhanced conductivity of the body is more likely caused by water filling cracks associated with the pelites' well-developed cleavage and schistosity, rather than by the graphite. A scalar audiomagnetotelluric survey across the Wopmay fault zone, a prominent structure that bisects the orogen, gave results very much distorted by three-dimensional effects. The electric-polarization apparent resistivities of these data indicate a shallow conductor 2 km east of the fault scarp, 1–2 km wide. Models of the feature suggest that its vertical extent is at least 1–2 km.