Geological implications of a permeability-depth curve for the continental crust

Abstract

The decrease in permeability ( k ) of the continental crust with depth ( z ), as constrained by geothermal data and calculated fluid flux during metamorphism, is given by log k = −14 − 3.2 log z, where k is in meters squared and z is in kilometers. At moderate to great crustal depths (>∼5 km), this curve is defined mainly by data from prograde metamorphic systems, and is thus applicable to orogenic belts where the crust is being thickened and/or heated; lower permeabilities may occur in stable cratonic regions. This k-z relation implies that typical metamorphic fluid flux values of ∼10−11 m/s are consistent with fluid pressures significantly above hydrostatic values. The k-z curve also predicts that metamorphic CO2 flux from large orogens may be sufficient to cause significant climatic effects, if retrograde carbonation reactions are minimal, and suggests a significant capacity for diffuse degassing of Earth (1015–1016 g/yr) in tectonically active regions.