The continental middle to lower crust (CLC) remains one of the most enigmatic parts of the Earth about which comparatively little is known. Remote-sensing geophysical and geochemical data illustrate our gaps in complete knowledge of the state and composition of the CLC. Generally, it is thought that the CLC, when compared to the upper crust, is somewhat more uniform in its properties. However, studies undertaken in the last decade have revealed that the CLC can be as heterogeneous as the upper crust, but that some regions of surprising homogeneity exist. In this chapter, I will describe attempts to image one particular physical parameter of the CLC, namely its electrical conductivity sigma. Electromagnetic (EM) sounding methods, as with most geophysical methods, are sensitive to the structure that exists today, and as such are dependent on the current state and composition of the CLC, rather than its state at formation or as a consequence of tectonic or metamorphic activity as are petrological studies on exhumed samples from the CLC. All EM methods give responses that are volumetric averages of the Earth's conductivities sensed by the diffusive EM fields, and as such are in the same class of geophysical techniques as seismic surface wave studies, potential field methods, and geothermal investigations. These are distinct from seismic reflection and refraction methods which deal with non-diffusive waves. However, with EM methods the governing field equations (Section 2) and wide range of the physical parameter being sensed (electrical conductivity, Section 1.1) ensure a far greater resolving power to anomalies than with the other diffusive geophysical techniques. Also, EM and seismic methods are the only geophysical techniques for which probing of the deep crust is assured; with all others there are inherent screening effects and depth ambiguities. Imaging the electrical conductivity structure beneath the surface has a number of uses, classified mainly by the depth of investigation: 1: Identifying zones of mineralization, which are of economic importance. 2: Detecting fluids in the deep crust, such as those above downgoing slabs, e.g., the Juan de Fuca plate (Kurtz et al., 1986a, 1990; EMSLAB, 1989). 3: Resolving one physical property of crustal zones. As expressed by Dohr et al. (1989)\\ ``magnetotellurics works like a broad paint brush, colouring the layers bounded by the seismic reflections''. 4: Determining the structure of the mantle, in particular the depth to the ``electrical asthenosphere'', which is a zone of enhanced conductivity in the upper mantle below the lithosphere (e.g., Jones, 1982). This depth often correlates with the depth to a seismic low velocity zone. Of these uses, (2) and (3) are relevent to the electrical conductivity of the CLC. Haak and Hutton (1986) have reviewed this topic, and I intend this chapter to complement their work by adding new results and interpretations, only repeating certain of their points for completeness. After a discussion of the physical parameter that EM methods are sensing, I will review EM methods appropriate for determining the conductivity of the CLC and their problems and limitations. A summary of recent (1980s) EM results pertinent to the topic follows with specific details from two geographic areas: the Kapuskasing structure in northern Ontario and the Valhalla complex in southeastern British Columbia. Finally, I examine the proposed causes of the observed enhanced electrical conductivity of the CLC focussing on the two currently most popular: saline fluids and grain-boundary films of carbon.
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