TECTONIC INTERPRETAT ION OF REGIONAL CONDUCTIVITY ANOMALIES

Abstract. This review paper selects key results from electromagnetic induction s udies of a variety of distinctive tectonic phenomena in the top 200 km of the Earth. Its main theme is that electromagnetic data are ssential for an understanding of tectonism involving partial melting, recycling of large volumes of fluids (CO2 and H20) and underthrusting of metasedimentary rocks. The wide variety of tectonic regimes in which these processes are known to be important is reflected in the choice of case studies. A discussion of conductivity models for young oceanic lithosphere and asthenosphere is followed by results from induction studies across the S.E. Australian passive margin, the North American active margin, the Ryukyu Island-Arc and the Oregon Cascades continental rc. The importance of partial melting and free fluid movement i apparent in these regions. Terrain accretion and/or continent-continent collisions recorded at palaeosuture zones in Ireland, Germany and Scandinavia have left distinctive conductivity structures. These are often associated with grain-boundary graphite ither in weakly-metamorphosed black shales in underthrust edimentary basins or precipitated from CO2-rich fluids. They are discussed in the context of the evolution of mature continental crust. All of the case studies are based on experiments published since 1989 in which the electromagnetic results have been central to an integrated geophysical nd geological interpretation. 1