Electrical conductivity, which depends on ionic fluids, metal or carbon content and temperature, provides complementary information to properties determined from seismic or potential field data. Our ability to image conductivity structure has recently undergone a revolution. These projects involve high resolution magnetotelluric (MT) profiling in the Trans-Hudson Orogen (THO) and the southeast Appalachians (SEA). The common element in the two experiments is illumination of processes in fossil zones of continental collision. The THO experiment will coincide with an 850 channel COCORP seismic reflection line across the Williston Basin and will be closely coordinated with the Canadian Lithoprobe transect of the THO. Its targets include the largest known conductivity anomaly in N. America, sutures between the orogen and adjacent Archean provinces and possibly anomalous mantle under the orogen. It is expected to provide a definitive demonstration of the resolving power of MT data in nearly ideal conditions. The SEA experiment will take advantage of the fact that the ocean edge can amplify MT response to certain structures. It is expected to settle the controversy about the location of the Alleghanian suture, map the sedimentary package under the thrust associated with this collision event, provide important insight into the preservation of retrograde metamorphic conditions in deep continental crust and estimate the volatile content in (and the presence of partial melt beneath) very old oceanic lithosphere.
