HIGH ELECTRICAL-CONDUCTIVITY IN A MODEL LOWER CRUST WITH UNCONNECTED, CONDUCTIVE, SEISMICALLY REFLECTIVE LAYERS

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.