Imaging and observing the electrical Moho

Defining the depth to the base of the crust is of profound importance for understanding and developing theories of the secular variation of our planet, including crustal formation and evolution. Since its discovery 100. year ago, the seismic Moho is, almost ubiquitously, used as the crust-mantle boundary reference. Laboratory studies on crustal and mantle rocks suggest that there should be a concomitant step-lie change in electrical conductivity, an electric Moho (eMoho), at the crust-mantle boundary. We examine resolution properties of electrical and electromagnetic methods for imaging and observing an eMoho for three generic models, one to represent the Archean, one for the Proterozoic, and the third for the Phanerozoic. We show that in all three cases, given the existence in most localities of a conducting lower crust compared to the upper crust and upper lithospheric mantle, the problem is difficult and at the edge, and often beyond, confident resolution. For some highly unusual localities however, the lower crust is more resistive than the underlying upper mantle, and in those cases it is possible to resolve an eMoho, but very high quality data are required. Examples of eMoho observations around the globe are discussed, focussing on the results from a site on the southwestern part of the Slave Craton, northwestern Canada.