Electrical conductivity of a water-saturated quartz-mica-garnet-schist, collected from a surface outcrop near the Denali Fault Zone in the Yukon-Tanana terrane of east central Alaska, increases slightly with pressure to about 200 MPa. This behavior is unlike that exhibited by other Yukon-Tanana samples or by most rocks from other locations. Detailed petrographic examination of the sample revealed the presence of a stringer of carbonaceous material generally less than 10 μm thick enclosed in and intergrown with one of the muscovite layers and extending for about 2 cm along the foliation.

The stringer is probably responsible for the anomalous conductivity change with pressure, making the sample the first for which anomalous electrical conductivity behavior can be attributed to carbon associated with a specific feature. The carbonaceous stringer together with its host muscovite layer are deformed and broken around a rotated garnet porphyroclast. The deformation was accommodated by plastic deformation of quartz and therefore occurred in the ductile regime under conditions at least equivalent to greenschist facies metamorphism. We interpret the textural relations to indicate that the carbonaceous material was formed by fluid deposition in a fracture formed within the muscovite layer, possibly during the main phase of metamorphism and deformation, and that the mica and carbon stringer were then deformed by the noncoaxial deformation responsible for rotation of the garnet porphyroclasts. Together the facts that the deformation resulting in garnet rotation was ductile and that the garnet rotation disrupted the stringer demonstrate that the carbonaceous stringer was present at depth (i.e., >10 km). Brittle deformation on the microscopic scale is observed to have broken the connectivity of the carbon stringer, explaining in part why the rock does not exhibit anomalously high conductivity at 0.1 MPa (1 atm) pressure. The brittle deformation is interpreted to have been caused by unloading due to uplift. The observations indicate that carbonaceous material may exert a primary control on crustal electrical conductivity because it may be present as interconnected arrays in grain boundaries or microfractures or in megascopic, throughgoing fractures. ¿ American Geophysical Union 1995