The conductivity of the crust and mantle beneath the Kapuskasing Uplift: electrical anisotropy in the upper mantle

Abstract

SUMMARY Kapuskasing Uplift has been interpreted as an oblique cross-section of up to 25km of crust and thus provides the opportunity to examine the properties of exposed mid- to lower-crustal material. A magnetotelluric (MT) survey mapped a remarkably uniform upper crust and provided no evidence for upper crustal conductive zones that could be related to the often observed increase in mid and lower crustal conductivity. The only shallow conductive anomaly is related to the Ivanhoe Lake Cataclastic Zone but its electrical signature does not appear to extend more than one kilometer in depth. A regional apparent resistivity curve was determined from MT data and confirmed by subsequent controlled source electromagnetic surveys. Regional curves are essential for proper structural interpretation but are often difficult to determine because of electric field distortions. The data show a decrease in resistivity at depths below 15 km which is typical of continental crust in many areas of the world. There is a clear difference in orthogonal apparent resistivity and phase curves at periods greater than 10s which is most pronounced in a N65E and N25W coordinate frame. Two different models are proposed to explain the data. The first consists of a conducting 2-D slab of approximately 50 km width in the lower crust and striking N65E. The strike direction is consistent with a number of regional structural trends but there is no other direct supporting evidence for it. The second model invokes either micro or macro electrical anisotropy in the upper mantle. There is considerable evidence for seismic anisotropy in the region. The electrical anisotropy could be the result of preferential conduction along the c-axis of olivine crystals, hydrogen dissolved in the olivine lattice with preferential conduction along the a-axis, or possible alignment of dykes, joints and faults in the upper mantle. If the anisotropy is related to strain induced orientations of crystals or fabric, it will provide evidence for the motions of the mantle associated with plate tectonics.