Models of conductive structure under the Canadian Cordillera

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Summary Extensive 2-D forward modelling has been used to study conductive structures beneath the Cordillera, the Rocky Mountains and the Rocky Mountain Trench of western Canada. Both large-scale geology and magnetovariation anomalies indicate generally 2-D structure. Eleven stations close to a profile transverse to strike were chosen from two magnetovariation arrays. The response parameter modelled at these stations is the real part of Z/H for a linearly polarized uniform horizontal primary field, along the profile and transverse to the structures, calculated from single-station transfer functions at periods of 600 and 1500 s. Induction arrows indicate that the quadrature phase response is related mainly to currents induced in sedimentary rocks in another conductor. Very large real Z/H, at stations in the Main Ranges, and small values 50 km SW of these, in the Rocky Mountain Trench, can be modelled by currents induced in highly conductive rocks, rising to within a few kilometres of the surface, beneath the Main Ranges. This conductive ridge is well constrained in position and depth. It is joined to the Canadian Cordilleran Regional conductive layer, and is essentially a thickening of the CCR conductor along its edge. The best purely inductive model places the top of the CCR conductor within 5 km of the surface, but it can be modelled at greater depth if the entire conductive structure carries a small amount of channelled current in addition to the induced currents. Available magnetotelluric soundings indicate a midcrustal depth to the CCR conductor SW of Revelstoke, but depths of 5 km or less under the Rocky Mountains and Trench, in good agreement with our models fitted to long-period magnetovariation fields. The nature of the CCR conductor has been considered elsewhere: its high conductivity may be due to silicate partial melt in interconnected pores at upper mantle depths, with saline hot water in interconnected pores and fractures at crustal depths.