Deep crustal electrical conductivity; Evidence for water in the lower crust

Abstract

Controlled source magnetic induction experiments in the Adirondack Precambrian shield region of northern New York State indicate that the electrical conductivity in both the upper and lower crust are in conformity with laboratory studies of moist, igneous rock and in serious discord with such measurements on dehydrated rock. A two order of magnitude increase in the conductivity found at 20 km depth suggests a structural or phase change there. A single turn loop of wire 1.5 km in diameter was used to generate an oscillating magnetic dipole source field over the frequency range 0.4 to 390 Hertz. The amplitude and phase of the resulting fields were measured as a function of distance to 90 km from the source. Analysis of these measurements indicates approximate horizontal stratification of the electrical conductivity. The uppermost layer, which is no more than a few hundred meters thick, has a conductivity thickness product of 0.4 (ohms)−1. It is underlain by a conductivity of about 8 × 10−5 (ohm meters)−1 to a depth of about 15 km. The conductivity between 15 and 20 km is approximately 10−3 (ohm meters)−1; the conductivity is greater than 10−2 (ohm meters) −1 below 20 km.

Limits on lithospheric stress imposed by laboratory experiments

“…Without repeating all the arguments, the evidence for a water-filled, interconnected pore space seemed compelling at least to about 5 km. Recent deep resistivity measurements [Nekut et al, 1977] suggest that the same conclusion holds to 20 km, and a recent study of crustal permeability argues for high conductivities to 8 km [Brace, 1980]. High fluid conductivity necessitates interconnecting fractures, since porosity in crystalline terrains must be a percent or less.…”

Time‐dependent changes in failure stress following thrust earthquakes

“…Laboratory experiments show that most rocks experience significant hydraulic weakening when exposed to water. There is ample evidence to suggest that crustal theologies in seismogenic zones are wet [e.g., Brace, 1972;Nekut et al, 1977;Brace, 1980]. (3) Dry feldspar aggregates are considered.…”

Hydroselsmicity: A Hypothesis for The Role of Water in the Generation of Intraplate Seismicity

“…Nekut et al (1977) suggested that in the crystalline rocks of the Adirondacks a water-filled pore space to a depth of 20 km is required to explain the relatively high electrical conductivity of the crust. Permeabilities inferred from earthquake mi-gration and other large-scale crustal phenomena ranges (Brace, 1980;, for crystalline rocks, from 10-12 to 10-1°CM2 which are about the same as the more permeable intervals in boreholes.…”