Water in the lower continental crust: modelling magnetotelluric and seismic reflection results

SUMMARY Magnetotelluric and multichannel seismic reflection measurements indicate that the Phanerozoic lower continental crust is commonly electrically conductive and reflective, in contrast to a more resistive and transparent middle to upper crust. A few per cent free saline water can provide an explanation for both results along with the apparent requirement that neither the conductive nor the reflective properties are retained when lower crustal rocks are brought to the upper crust. Common 10 km thick and 20-30 Qm resistivity layers can be explained with 0.5-3 per cent pore water, if there are equilibrium pore geometries and the salinity is close to that of sea-water as suggested by lower crust fluid inclusions. Seismic velocities and impedances must be affected if such porosity exists. Seismic reflectors with reflection coefficients of 5-10 per cent can be explained by layers or lamellae with porosity contrasts of 1-4 per cent and reasonable effective pore aspect ratios of 0.1-0.03. A minimum temperature of 350°C is estimated from a correlation between heat flow and depth to the top of conductive and reflective layers. The upward limit in the crust may occur at an impermeable boundary formed by hydration reactions at the top of greenschist facies conditions or by precipitation of silica. It also may be associated with the minimum temperature for ductile behaviour and equilibrium grain boundary pore configurations. The maximum temperature is about 700 "C according to the evidence indicating that there is no free water in granulite facies conditions. Areas that have been subject to such high temperature conditions without the subsequent addition of water, i.e. the lower crust of shields, are generally non-reflective and electrically resistive.