Deep crustal electrical conductivity in the Adirondacks

Abstract

Controlled and natural source electromagnetic induction experiments in the Adirondack Precambrian shield region of northern New York State indicate that the electrical conductivity of the crust is consistent with laboratory studies of moist igneous rock and in serious discord with such measurements on dry rock. A highly conductive layer is found in the lower crust at a depth of ∼22 km. Four three‐axis flux gate magnetometers were deployed at distances of 20 to 62 km from a central source location in Newcomb, New York. A single turn horizontal loop of wire 4.5 km in diameter carrying 65 rms amperes of square wave current generated an oscillating magnetic dipole source field at frequencies of .05 and .1 Hz. The magnetic field data were computer processed to determine the amplitude and phase of the vertical and horizontal magnetic field components relative to the source. These data, together with previous data in the frequency range 0.4 to 400 Hz, indicate a conductivity of 1×10−4mhos/m extending to 10 km, underlain by a conductivity of 1×10−3mhos/m to 22 km. At 22‐km depth the conductivity increases to between ∼4×10−2 and ∼8×10−2 mhos/m. Geomagnetic fluctuations in the period range 10 to 200 s have been analyzed using the gradient method of Kuckes and Schmucker. These data indicate that the conductive feature in the lower crust is of finite extent with a conductance of ∼400 mhos. Comparison of the Adirondack results with the results of controlled and natural source experiments in South Africa and North America indicate similar crustal conductivities among these stable shield regions.