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Electrical conductivity structure of the lower crust and upper mantle in western Canada Caner, Bernard

Abstract

Geomagnetic induction techniques have been used to study the structure of the crust and upper mantle in western Canada. Geomagnetic depth-sounding (GDS) has been used primarily for mapping, and magnetotellurics (MT) for quantitative interpretation. Self-consistent models of electrical conductivity structure have been derived from the combined MT/GDS data. The conductivity structure models have been considered in conjunction with other relevant geophysical information: heat-flow, seismology and aeromagnetic surveys. No definite petrological models can be derived because of the order-of-magnitude uncertainties in the relations between electrical conductivity, temperature and composition. However, if we exclude geochemically improbable solutions, the following two distinct results can be extracted: a) In southwestern Canada (boundaries not clearly defined, but at least as far east as Lethbridge), the uppermost mantle is moderately conducting (resistivity 30-50 ohm-meters). This indicates a temperature of at least 750°C at depth 35 km., and provides independent confirmation (without assumptions of crustal structure) of the heat-flow derived estimates of Roy et al (1968b). b) In a sharply delineated region starting from about 0-30 km west of the Rocky Mountain Trench, the lower crust (from a depth of about 10-15 km) is conductive. The most likely interpretation is a hydrated lower crust, as proposed by Hyndman and Hyndman, 1968. Hydration alone is sufficient to explain the observed conductivities, i.e. higher temperatures are not necessarily required for this model. However, given the information from (a) above, some partial melting of hydrated granitic materials should occur in this zone; this is in good agreement with the geological evidence of granitic intrusives in this region.

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