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

Abstract

S U M M A R YThe 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 N65"E and N25"W 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 N65"E. 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.

Anisotropy Versus Heterogeneity in Continental Solid Earth Electromagnetic Studies: Fundamental Response Characteristics and Implications for Physicochemical State

“…Studies in the Canadian Precambrian Shield terranes have been some of the most prominent with regards to deep electrical anisotropy, especially those of the Abitibi-Grenville provinces in the East (Jones, 1992;Kellett et al, 1992;Kurtz et al, 1993;Mareschal et al, 1994Mareschal et al, , 1995Boerner et al, 2000). There, a pervasive resistivity anisotropy in the lowermost crust or uppermost mantle exists over a transect length of >300 km, with the more conducting direction $N55°E parallel to observed Archean tectonic patterns.…”

A synthesis of electromagnetic studies in the Lithoprobe Alberta Basement Transect: constraints on Paleoproterozoic indentation tectonics

“…Estimating the depth of the anisotropy can be difficult without introducing subjective constraints (Rasmussen 1988;Kellett et al 1992;Kurtz et al 1993). In the MT data, it is (Fig.…”

Electric and Magnetic Field Galvanic Distortion Decomposition of BC87 Data.

“…Relevant studies include Chakridi et al (1992), Jones and Dumas (1993), Eisel and Bahr (1993), , Kurtz et al (1993), Ogawa et al (1994), Marquis et al (1995), Boerner et al (1995), and Gupta and Jones (1995). Physically, the 3D galvanic distortion is caused by the presence of electric charges along discontinuities or gradients in electrical conductivity associated with small-scale (relative to the background inductive scale length) surface structures.…”