High electrical conductivity in a model lower crust with unconnected, conductive, seismically reflective layers

Geophys. J . Inf. (1992) 108, 895-905 High electrical conductivity in a model lower crust with unconnected, conductive, seismically reflective layers A. M. Merzer'" and Simon L. Klemperer2t ' State of Israel, Armament Development Authority, Electromagnetics Division, POB 2250(87), Haifa 3 1021, Israel British Institutions' Reflection Profiling Syndicate, Bullard Laboratories, Cambridge University, Madingley Road, Cambridge CB3 OEZ,U K Accepted 1991 October 1. Received 1991 October 1; in original form 1990 May 21 Key words: continental crust, electrical conductivity, lower crust, seismic reflection profiling. 1 INTRODUCTION Electromagnetic and magnetotelluric experiments have shown that continental lower crust almost always has electrical conductivity orders of magnitude higher than predicted by laboratory measurements on dry rocks at lowercrustal temperatures and pressures (Shankland & Ander 1983: Haak & Hutton 1986). Average conductivity values are 20 to 30 Bm for Phanerozoic lower crust, but lo4 Bm for dry rocks (Hyndman & Shearer 1989). Possible causes of high crustal conductivity, away from areas of elevated geothermal gradient and crustal magma * Part of this work was carried out while on sabbatical leave at the Material Mechanics Laboratory, Mechanical Engineering Department, Technion-Israel Institute of Technology, Haifa, Israel. t Now at: Department of Geophysics, Mitchell Building, Stanford University, Stanford, C A 94305-2215, USA. chambers, are the existence in the crust of saline water or of minerals such as metals, metal sulphides and oxides, ferrous-ferric silicates and graphite (e.g. Parkhomenko 1982). These minerals are common accessories but are rarely dominant, so that they are implausible explanations for the worldwide occurrence of lower-crustal conductive layers. The one possible exception is graphite, which has recently been reported to be present as a thin grainboundary film in some plutonic rocks (Frost et al. 1989). However there is no good reason for any of these minerals to be concentrated into the lower crust (Haak & Hutton 1986), which is normally much more conductive than the upper crust. Though specific mineral assemblages may be an adequate explanation of certain crustal conductors, the most plausible explanation for the common occurrence of lower-crustal conductive zones is the presence of a few tenths of a per cent to a few per cent of saline fluids throughout the lower crust. 895 SUMMARY In this paper we derive the electrical conductivity for a model lower crust containing unconnected, highly conductive lamellae within a highly resistive matrix. Lateral overlap, with small vertical separation, of lamellae of the dimensions imaged by seismic reflection profiling (a few hundred metres thick and a few kilometres across) could increase lower-crustal conductivity from the low values predicted by laboratory measurements on dry rocks to the high values observed in field experiments. The model does not depend on the cause of high conductivity within the lamellae. However, lamellation of the lower crust may provide a way of lithologically trapping saline water in permeable, conductive lamellae within an impermeable, non-conductive matrix, and so resolve the apparent contradiction between the low crustal permeabilities required for maintenance of high pore pressure over geological time periods and the high degree of pore interconnection required for the high observed conductivity. The permeable lamellae and impermeable matrix would be of very different lithologies, as implied by the high amplitudes of the lower-crustal reflections. For a typical example the model gives resistivities that compare favourably with the modified Archie's Law. The model can also give anisotropic resistivity effects, which are quantitatively compatible with results from field experiments. 896 A . M . Merzer and S . L. Klemperer conductivity in the middle crust. A middle-crustal high-porosity layer might contain fluids trapped there by an impermeable layer due to mineral precipitation from cooling fluids (Jones 1987) or due to the brittle-ductile transition and associated change from equilibrium to non-equilibrium (fracture-dominated) fluid-rock geometries (Bailey 1990; Warner 1991). Hyndman & Shearer (1989) suggest as alternative resolutions of the paradox the possibilities that lower-crustal grain sizes are much smaller than typically observed for high-grade rocks, thus reducing permeability; that fluid conductivity in the lower crust is much higher than the conductivity of sea-water at elevated temperatures, thus reducing the required porosity; or that an as yet unknown physical bonding mechanism holds the fluid in the rock without reducing its conductivity. In this paper we suggest that the paradox can be resolved by considering the macroscopic geometries in which the saline fluids-or other conductors-may be contained; and we calculate the conductivity of a model crust, in which saline fluid is confined to isolated, permeable, porous regions within a resistive matrix. Theoretical support for the model is presented, and order-of-magnitude calculations are made. Finally geological aspects of the model are considered including anisotropy effects. 2 SEISMIC REFLECTION IMAGES OF THE LOWER CRUST Not only is the lower continental crust normally more conductive than the upper crust, but it is also commonly highly reflective (Fig. 1) (e.g. Leven et a/. 1990; Matthews & Smith 1987). Correlations between lower-crustal zones of SWAT 3 2.0 KM 10 0 Figure 1. Typical BIRPS (British Institutions' Reflection Profiling Syndicate) seismic section to show the character of lower-crustal reflections (fig. 1 of Reston 1987). Section extends vertically from c. 5 s two-way traveltime (c. 15 km depth) to c. 11 s (c. 35 km). The Moho reflection, base of the lower crust, is the deepest bright reflector at 9.8 s on the left of the reflection section, 9.2 s on the right-hand side. If saline fluids are present in the lower crust, crustal conductivity must depend strongly on their detailed distribution within the much more resistive rock matrix. In the lower crust at temperatures above 350" to 400 "C textural equilibrium between fluids and host rock is rapidly achieved on a geological time-scale (Watson & Brenan 1987). The fluid distribution under this circumstance is controlled by the local porosity and the local dihedral angle. For small porosity, if the dihedral angle is greater than 60°, fluid is distributed in isolated pores at grain corners, and the rock has very low permeability. If the dihedral angle is less than a", fluid coats all grain edges and forms an interconnected pattern through the whole rock, and the rock is highly permeable. A problem for interpreting lower-crustal conductivity as due to saline water has been that if the fluid forms a connected network, and so is conductive [resistivity of typical saline water at lower-crustal temperatures is about 0.02Q (...truncated)