Electrical discontinuities in the continental lithosphere imaged with magnetotellurics

Magnetotelluric (MT) models reveal the continental lithosphere to be extremely electrically complex. In this chapter, I have combined experimental conductivity data on typical lithospheric minerals with estimates of standard lithospheric compositions to produce forward models of the predicted electrical structures of hypothetical, "typical" lithospheres. These models show consistently high (> 105 ohm · m) resistivities in the upper crust, compositionally dependent lower crustal resistivities between 10² and 10⁴ ohm · m, and lithospheric mantle resistivities that decrease with depth and lie between 10⁵ and 10² ohm · m. Most models predict a resistivity discontinuity at the crust-mantle boundary but often this may not be resolved by MT data. A review of MT models from the continental lithosphere shows that many reveal electrical discontinuities that are not predicted by these models of standard lithospheres. In the upper crust, fluids and conductive minerals, such as graphite or sulfides, are likely causes of conductive anomalies. In the lower crust and upper mantle, amphibole may be an important additional cause of conductors. With increasing temperature, high hydrogen contents can also produce conductive anomalies in the mantle. MT models reveal many features in the continental lithosphere that are not predicted by seismic or geochemical models, including large-volume conductors in tectonically stable lithospheric mantle, lateral conductivity discontinuities extending for tens of kilometers, and narrow conductors that penetrate the crust and mantle. Serious efforts to understand these features in the context of their conductivity causes and tectonic settings will significantly advance our understanding of the composition and evolution of the continental lithosphere.