A Model of Crustal Conductive Structure In the Canadian Cordillera

Abstract

S U M M A R YResults from magnetotelluric soundings in t h e frequency range 0.016-160 H z are used t o model conductive structure in the upper and middle crust along a section across strike of the Canadian Cordillera, a region of continental accretion associated with three successive subductions over the last 200 Ma. 2-D finite-difference modelling is applied t o secure a first-order fit to observed phase and apparent resistivity pseudosections in two polarizations, with t h e electric field along geological strike (E-polarization) and across strike (B-polarization). T h e fit to observed fields, and its limitations, are shown in detail by comparing calculated and observed pseudosections. T h e misfits are instructive and a r e t o b e expected where 2-D modelling is fitted t o 3-D structure in the Earth. T h e model nevertheless indicates two major resistive and three major conductive blocks in t h e upper and middle crust. T h e larger resistive block probably represents granitoid plutons of the Coast Plutonic Belt, which are shown t o continue beneath the adjoining Intermontane Belt. T h e three conductive blocks a r e believed t o result from saline water in fractures. T h e block under the Coast Plutonic Belt may draw its water from the currently active subduction offshore, whereas t h e block in line with the Fraser Fault may draw its water from the atmosphere a n d the block under the Omineca Belt from a global-scale upcurrent in the mantle.

The electrical resistivity of Canada’s lithosphere and correlation with other parameters: contributions from Lithoprobe and other programmes

“…Thickest elastic thickness, >100 km, is found beneath Hudson Bay and to the west of Hudson Bay. Thinnest elastic thickness on the North American Craton, <40 km, is found beneath the Williston Basin in southern Saskatchewan correlative (2) Shaocheng, J., Rondenay, S., Mareschal, M., and Seneschal, G., 1996 1985Majorowicz and Gough (1991; Gough and Majorowicz (1992); Majorowicz et al (1993aMajorowicz et al ( , 1993b; Majorowicz and Gough (1994) (2014) with the trace of the Trans-Hudson Orogen. For the Cordillera, elastic thickness is in the range 20-40 km (Fluck et al 2003).…”

“…The Canadian MT database comprises high-quality broadband MT data acquired by modern systems as part of Lithoprobe activities plus other major programmes run over the last 30 years, such as the Federal Geothermal Energy Programme (GEP) in the early 1980s, the Federal Exploration TECHnology initiatives EXTECH-III Flores et al (1985); Ingham et al (1987); Jones et al (1988); Gupta and Jones (1990); Majorowicz and Gough (1991); Gough and Majorowicz (1992); Jones et al (1992aJones et al ( ), (1992b; Eisel and Bahr (1993); Jones (1993a); Jones and Dumas (1993); Jones et al (1993b); Lilley (1993); Majorowicz et al (1993aMajorowicz et al ( ), (1993b; Majorowicz and Gough (1994); Cook and Jones (1995); deGroot-Hedlin (1995); Gupta and Jones (1995); Marquis et al (1995); Chave and Jones (1997); Ritter and Ritter (1997); Soyer and Unsworth (2006); Rippe et al (2013) Ph.D. thesis: Marquis (1992) Jones and Savage (1986); Jones (1988aJones ( ), (1988b; Jones and Craven (1990); Agarwal and Weaver (1993); Ellis et al (1993); Jones (1993b); Jones et al (1993a); Takasugi et al (1993); Uchida (1993); Wu et al (1993); Zhao et al (1993); Tournerie et al (1995); ; …”

Cordilleran Intermontane thermotectonic history and implications for neotectonic structure and petroleum systems, British Columbia, Canada

“…Observed increased reflectivity and high electrical conductivity in the middle crust can be caused by fluids trapped in ductile shear zones (Majorowicz and Gough 1994). The depth to the brittleductile transition in the Canadian Cordillera is indicated by a mid-crustal conductor that occurs at depths where temperature is 450-500°C (Majorowicz and Gough 1991;Majorowicz et al 1993).…”