3D Electrical Resistivity of the Crust and Upper Mantle of the Southern Canadian Cordillera from Inversion of Magnetotelluric Data: Linking Geothermal Potential to Regional Scale Crustal Structure

Abstract:

The southern Canadian Cordillera is located in the backarc region of the Cascadia subduction zone. This backarc is relatively hot, with high heat flow and shallow convection in the asthenosphere. To the east, a sharp transition occurs with the thick, cold lithosphere of the North American craton. Regions of the southern Canadian Cordillera, especially in the Omineca belt, host some of Canada’s most promising geothermal resources. These geothermal manifestations are often associated with active faults. However, the factors controlling the distribution of these resources are poorly understood, as is their relationship to crustal and upper mantle structure.

Magnetotelluric (MT) exploration is a geophysical method widely used in geothermal exploration since it measures resistivity, a property sensitive to temperature and the presence of fluids. MT is extensively used on reservoir scale studies in the upper few kilometres, but it can also image deeper and investigate the zones where fluids originate. Previous MT studies in the southern Canadian Cordillera defined an extensive layer of low resistivity in the mid-crust. This has been inferred to be a combination of partial melt and aqueous fluids. However, all prior MT studies used a 2D approach that is limited in terms of reliably imaging the subsurface resistivity structure. To develop a fully 3D model for this region, additional MT data were collected from 2002-2018. A 3D resistivity model of the region 48-54^oN and 112-122^oW, obtained from 336 MT stations with a typical spacing of 22 km, shows a number of significant features including:

(1) Zones of low resistivity throughout the southern Omineca and southwestern Foreland belts that are shallowest near the Southern Rocky Mountain Trench, some of which extend to depths of > 50 km.

(2) Low-resistivity zones that connect the low-resistivity crustal layer and the surface along a number of faults.

(3) A transition from low-resistivity upper mantle beneath the Cordillera to high-resistivity upper mantle in the craton beneath the Foreland belt. A sharp Cordillera-craton boundary that includes sub-vertical and west-dipping regions.

This 3D model gives an overview of the resistivity structure and shows that crustal fluid distribution can be mapped. Implications for future geothermal research and development will be discussed.