A combined continental-ocean magnetotelluric (MT) transect of the southern Appalachians orogenic belt has been completed that spans nearly 600 km on land and extends over 1000 km offshore beneath the Atlantic Ocean. The study is revealing the long-term changes in the lithosphere that occur as active subduction regimes degenerate to fossil regimes, and tests the degree to which the latter are modified by subsequent extension. The electrical resistivity structure of active subduction has been examined by several researchers in recent years (Wannamaker and Hohmann, 1991; Jones, 1993), but here is the first thorough examination of a currently passive, former convergent, margin.
It is in orogenic belts such as the Appalachians that most present-day continental crust is defined, through metamorphic reworking of older material, addition of new material by magmatism, or the imbrication of protocontinental fragments in the course of subduction. Within orogenic belts, localised "suture" zones mark the transitions between continental fragments. Continental deformation in these zones can extend through the entire lithosphere, and its role in controlling the loci of future orogenies is a question fundamental to understanding the evolution of the Earth. The lithological properties of greatest importance to continental deformation are the composition and metamorphic state of the material, the presence of fluids, the temperature, and pre-existing zones of weakness.
Electrical resistivity (or its inverse, conductivity) is strongly affected by such properties. Indeed, major deep conductivity anomalies have been recognised in reconnaisance surveys of orogenic belts throughout the world (e.g., Jones, 1993). Most of these anomalies were located using magnetometer array data whose interpretation unfortunately is often ambiguous. A far more detailed image for the southern Appalachians is being sought with modern quality MT data and analysis. Our instruments were deployed primarily in the spring and summer of 1994, and procide data on electrical resistivity from depths of about 1 km to over 200 km.