Near-surface and crustal-scale images of the Nechako basin, British Columbia, Canada, from magnetotelluric investigations¹This article is one of a series of papers published in this Special Issue on the theme of New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia.²Geological Survey of Canada, GSC Contribution No.: 20100129.

Abstract

Two-dimensional (2-D) models of audio and broadband magnetotelluric (MT) data collected throughout the southern Nechako basin in south–central British Columbia, Canada, provide electrical resistivity images of the Cretaceous to Oligocene volcanic and sedimentary packages and underlying crustal-scale features. Analysis of distortion effects and structural dimensionality indicate that the MT responses are primarily one-dimensional (1-D) at periods less than 0.1 s. Departures from a 1-D response occur with maximum phase differences occurring between 0.1 and 10 s. The upper crustal resistivity models reveal a low resistivity layer at near surface depths, interpreted as Chilcotin basalts, that blankets portions of the region to depths less than 50 m, but locally thicken up to 200 m. Cretaceous sedimentary units (those showing the highest potential for hydrocarbons) are characterized by moderately low resistivities that are laterally variable. More uniform, lower resistivities appear to be associated with the Eocene volcaniclastic groups, suggesting that the MT method can distinguish between these units and may be useful in targeting areas that are more prospective for hydrocarbon exploration. In general midcrustal resistivity values are high, consistent with values of typical volcanic terranes; however, a low-resistivity zone is identified at 8–10 km depth that correlates closely with the location of seismic reflectors as well as recent microseismic activity. This low-resistivity zone is interpreted as a midcrustal reservoir of magma, the top of which marks the upper limit of fluids migrating from lower crust depths. Additionally, several crustal-scale faults are imaged.

Mapping crustal structure of the Nechako–Chilcotin plateau using teleseismic receiver function analysis,

“…They interpret the base of the Nechako Basin sedimentary and volcaniclastic units as a rapid increase in resistivity values (fig. 5 of Spratt and Craven 2011). Their estimate of 6-8 km for the base of this unit is in very good agreement with our estimated thickness of 7.4 and 6.1 km, respectively (combined sediment and volcaniclastic), at UBRB and FPLB (Table 1).…”

“…The magnetotelluric (MT) study of Spratt and Craven (2011) covers the central portion of the Nechako region, and their line A passes very close to our seismic stations UBRB and FPLB (Kim 2010). They interpret the base of the Nechako Basin sedimentary and volcaniclastic units as a rapid increase in resistivity values (fig.…”

The Rock Physical Property Database of British Columbia, and the distinct petrophysical signature of the Chilcotin basalts,

“…However, knowledge of the thickness and actual areal distribution of this cover is limited because the region is dominated by glacial cover. If the rock physical properties fingerprint of the Chilcotin basalts can be well defined, then inversion of geophysical data can be used to define its areal extent and thickness (e.g., Hayward and Calvert 2009;Spratt and Craven 2011).…”