THE CONDUCTIVITY OF THE CRUST AND MANTLE BENEATH THE KAPUSKASING UPLIFT - ELECTRICAL ANISOTROPY IN THE UPPER MANTLE

The Kapuskasing Uplift has been interpreted as an oblique cross-sectio n of up to 25 km of crust and thus provides the opportunity to examine the properties of exposed mid- to lower-crustal material. A magnetote lluric (MT) survey mapped a remarkably uniform upper crust and provide d 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 Ca taclastic Zone but its electrical signature does not appear to extend more than one kilometer in depth. A regional apparent resistivity curv e was determined from MT data and confirmed by subsequent controlled s ource electromagnetic surveys. Regional curves are essential for prope r structural interpretation but are often difficult to determine becau se of electric field distortions. The data show a decrease in resistiv ity at depths below 15 km which is typical of continental crust in man y areas of the world. There is a clear difference in orthogonal appare nt resistivity and phase curves at periods greater than 10 s which is most pronounced in a N65-degrees-E and N25-degrees-W coordinate frame. Two different models are proposed to explain the data. The first cons ists of a conducting 2-D slab of approximately 50 km width in the lowe r crust and striking N65-degrees-E. The strike direction is consistent with a number of regional structural trends but there is no other dir ect supporting evidence for it. The second model invokes either micro or macro electrical anisotropy in the upper mantle. There is considera ble evidence for seismic anisotropy in the region. The electrical anis otropy could be the result of preferential conduction along the c-axis of olivine crystals, hydrogen dissolved in the olivine lattice with p referential conduction along the a-axis, or possible alignment of dyke s, 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 tectonic s.