Interpretation of electrical conductivity in the lower crust

Values of electrical conductivity of possible lower crustal materials to interpretate lower crustal magnetotelluric soundings were studied. Two groups of physical conditions that can enhance electrical conduction in the crust at depths of 15 to 30 km are discussed: (1) a hot, dry regime obtained in regions of high heat flow or at intermediate depths; and (2) a warm, wet regime suitable to explain high lower crustal conduction in regions for which high temperatures are not expected. Statistically significant differences between rock types are found: mafic rocks are better conductors than granites by about half an order of magnitude and within the mafic group, aphanitic rocks have higher conductivity than phaneritic ones. Plots of log conductivity vs temperature are presented for each rock type to show mean log conductivity values together with standard deviations so that most probable temperature ranges can be inferred from conductivity. The use of this information is to infer temperature for a high conductivity layer at 15 to 25 km depth in the Zuni Area of New Mexico is demonstrated.