Imaging of deep fluids in Archaean crust

Abstract

DESPITE the importance of aqueous fluids in tectonic, metamorphic and magmatic activity in the Earth's crust, few techniques are available for directly observing their distribution at depth, even in stable continental crust. Such observations as have been made (electromagnetic deep sounding, which by inference^1,2 images saline fluids, and super-deep drilling³) suggest that fluids are present in varying amounts throughout the crust, in direct conflict⁴ with the normal petrological inference of a very dry deep crust. Here we report the results of an unusually high-resolution electromagnetic sounding method to image the deep electrical-conductivity structure below a 30-km profile in Ontario which crosses Archaean lower-crustal material of high metamorphic grade exposed by a Proterozoic thrust fault, the Ivanhoe Lake cataclastic zone. The electrical-resistivity image obtained shows, embedded in very resistive crystalline crust, two nearly continuous, sub-horizontal zones of enhanced electrical conductivity at depths of 2–3 and 5–6 km. Below this, the commonly observed^2,5-7 mid-crustal rise in conductivity is seen at depths of 15–20 km. The image does not correlate well with the structure inferred from surface geology, suggesting that the transport and distribution of fluids was not strongly controlled by lithological boundaries or the thrust fault itself.