THE ORIGIN OF ELECTRICALLY CONDUCTIVE LOWER CONTINENTAL-CRUST - SALINE WATER OR GRAPHITE

Magnetotelluric measurements show the lower continental crust to be electrically conductive globally, typically 100-1000 times more conductive than dry rocks measured in the laboratory and 10-100 times more conductive than the middle to upper crust. The relatively uniform low resistivity is a fundamental regional characteristic of the lower continental crust upon which local anomalies are superimposed. Conduction mechanisms must exist that are not active in the upper crust. This paper documents geophysical observations that provide important constraints on the mechanisms. They include the following: (1) the resistivity and the depth to the conductive layer show little lateral change across many large-scale geological terrane and structural boundaries; (2) the lower crust on average is more conductive in Phanerozoic areas than in Precambrian areas (10-30 Omega m vs. 100-300 Omega m, for a layer of about 10 km thickness); (3) the depth to the top of the conductive layer is generally shallower in young areas than in Precambrian regions (10-20 km vs. 20-35 km); the depth appears to be associated with heat flow and commonly corresponds to a present temperature of 350-400 degrees C; (4) in some areas there appear to be correlations between the resistivity of the lower crust and seismic reflectivity, seismic velocity, and seismic attenuation. The most probable candidates for the conduction mechanism are small amounts of interconnected saline pore fluids and interconnected thin films of graphite. An explanation involving about 1% interconnected saline pore fluids is consistent with these constraints, but there are difficulties reconciling the pore interconnection necessary to give low resistivity with the low permeability required to keep the lower-crustal fluid from escaping upward, and with the inference from metamorphic geology that the lower crust consists of dry granulite rocks. The explanation involving thin graphite films less readily explains some of the geophysical constraints such as the difference between Phanerozoic and Precambrian regions and the depth control by temperature. However, this mechanism allows a lower crust with a dry granulite mineralogy as otherwise inferred especially in shield areas.