Permeability of the continental crust: dynamic variations inferred from seismicity and metamorphism

The variation of permeability with depth can be probed indirectly by various means, including hydrologic models that use geothermal data as constraints and the progress of metamorphic reactions driven by fluid flow. Geothermal and metamorphic data combine to indicate that mean permeability (k) of tectonically active continental crust decreases with depth (z) according to log k ! )14–3.2 log z, where k is in m and z in km. Other independently derived, crustal-scale k–z relations are generally similar to this power-law curve. Yet there is also substantial evidence for local-to-regional-scale, transient, permeability-generation events that entail permeabilities much higher than these mean k–z relations would suggest. Compilation of such data yields a fit to these elevated, transient values of log k ! )11.5–3.2 log z, suggesting a functional form similar to that of tectonically active crust, but shifted to higher permeability at a given depth. In addition, it seems possible that, in the absence of active prograde metamorphism, permeability in the deeper crust will decay toward values below the mean k–z curves. Several lines of evidence suggest geologically rapid (years to 10 years) decay of high-permeability transients toward background values. Crustal-scale k–z curves may reflect a dynamic competition between permeability creation by processes such as fluid sourcing and rock failure, and permeability destruction by processes such as compaction, hydrothermal alteration, and retrograde metamorphism.