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

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.