Publication 79
Regional electrical conductivity structure of the southern Canadian Cordillera
and its physical interpretation
Juanjo Ledo and Alan G. Jones
Abstract
The regional crustal structure of the southern and central Canadian Cordillera of western
Canada is interpreted from the inversion of magnetotelluric data along five profiles crossing
the physiographic morphogeological belts, with emphasis on the Intermontane and Omineca belts.
Decomposition of the tensor impedance data demonstrates that large-scale regional structure
can be reasonably approximated along each profile as two-dimensional, with dominant geoelectric
strike of either -30O or +15O, depending on profile location. These profile-specific strike
directions are consistent with a local clockwise rotation of crustal structures in the southern
Intermontane and Omineca belts suggested by others based on paleomagnetic data and palinspastic
reconstructions. Comparing the resistivity models derived from two-dimensional inversions of the
distortion-corrected data along each profile allows us to construct an orogen-scale three-dimensional
resistivity model for southern British Columbia, which shows generally a resistive upper crust
overlying a conductive lower crust. The conductivity of the lower crust beneath the Intermontane
Belt is independent of latitude, and is similar for all profiles. In contrast, a two orders of
magnitude variation in lower crustal conductivity is observed along strike in the Omineca Belt,
with higher conductivities to the south in the region of Eocene extension, and lower conductivities
to the north in the unextended part of the belt. Such spatial association has noteworthy implications
for the cause of lower crustal conductivity in active, or recently-active, young regions. Our
preferred joint interpretation of the observed lower crustal resistivities and other geophysical
data is in terms of fluids, with brines dominating for the most part but partial melt possible at
the base of the crust. The along-strike variation in the Omineca Belt we attribute mostly to variation
in fluid content and interconnectivity, with the lowermost crust of the southern Omineca Belt being
partially molten. This physical state difference is a consequence of degree of extension, and implies
that mantle-derived fluids are important for lower crustal crustal conductivity.
Source
J. Geophysical Research, 106 , 30,755-30,770, 2001.
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Alan G Jones / 10 June 2004 /
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