Electrical resistivity structure of the Flathead Basin in southeastern British Columbia, Canada

Abstract

In June 1985, wide-band magnetotelluric data were acquired at 12 equally spaced sites along a 30 km profile crossing the Flathead (Kishenehn) Basin in southeastern British Columbia, Canada. These data have been modelled by both one-dimensional inverse techniques and two-dimensional forward trial-and-error fitting. The results indicate the presence in the area of the following three major zones of low electrical resistivity (10–500 Ω∙m):1. Sediments of the 10 km wide Flathead sedimentary basin, extending to a depth of about 2 km, dominate the responses in the middle of the profile.2. In the eastern part of the profile, in the area of the Lewis Range, a thin [Formula: see text] zone of low resistivity (35 Ω∙m) is imaged at a depth of some 3 km extending eastward from the edge of the basin. We associate this zone with the less dense thrusted Mesozoic clastic rocks lying directly below the Proterozoic rocks of the Lewis thrust sheet.3. Beneath the Flathead Basin is a third zone, of higher resistivity (500 Ω∙m), which extends to deep within the crust. This zone may originate from mantle upflow, as recently proposed to explain the existence of Cordilleran conductors in other localities. Additionally, to model the long-period geomagnetic transfer function responses, we are required to postulate the existence of a zone of low resistivity in the middle to lower crust, 50 km west of the survey line, corresponding to the location of the Rocky Mountain Trench.