Very-Long-Period Investigation of the Mantle Beneath the Canadian Shield: A Lakebottom Magnetotelluric Observatory

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A major feature of the plate tectonic model is the concept of a thermal boundary layer (TBL) at the earth's surface. This TBL serves as the upper boundary of the convective systems which drive the plate motions, and is dominated by a conductive thermal regime. Heat flow, subsidence, and other data have been used to argue for no significant differences between the deep structures under old ocean basins and old continents, both of which are at or very near a thermal equilibrium state. This suggests that the continental and oceanic lithospheres have the same thickness of about 150 km at equilibrium. Differences must exist in the upper lithosphere and crust to explain the buoyancy and mechanical behavior of continental material which helps govern its role in plate tectonics. A number of competing models for the deep structure of the continents have been proposed which will be tested by estimating the deep electrical conductivity structure beneath the Canadian Shield. The conductivity is a thermally activated process, and should be extremely sensitive to the radically different deep sub-shield mantle conditions associated with differing models of the continental lithosphere. The PIs will operate a very long period (VLP) magnetotelluric observatory in the bottom of a deep freshwater lake. This innovative deployment scheme overcomes many of the difficulties previously encountered in obtaining VLP MT data.