PERMEABILITY OF CONTINENTAL OROGENIC BELTS CONSTRAINED BY INTERNAL AND EXTERNAL FORCING

The mean large-scale permeability of tectonically active, crystalline continental crust appears to be sufficient to accommodate both external (meteoric recharge) and internal (metamorphic and magmatic) fluxes under near-hydrostatic fluid pressures. We perform a series of numerical simulations that entail two-dimensional vertical sections with an arbitrary (e.g. constant) initial permeability, a surficial slope (5.5^o) and meteoric recharge flux (1x10^-6 kg m^-2 s^-1) typical of the western United States, and a global-mean metamorphic fluid flux (1.4x10^-8 kg m^-2 s^-1). Permeability is allowed to evolve dynamically, increasing instantaneously when fluid pressure exceeds a depth-dependent threshold value and otherwise decaying with time. The prescribed meteoric fluid flux stays relatively close to the upper boundary, where it affects shallow permeabilities. The basal metamorphic fluid flux rises and converges toward the lower-elevation outlet, and affects permeabilities at depth. Because both the surface flux and the basal flux converge toward the lower-elevation outlet, there is a large, wedge-shaped region of the domain with little flow, where permeabilities decay towards a prescribed lower limit (10^-19 m²). Permeabilities at shallow depths (<1 km) increase to values sufficient to accomodate the meteoric recharge. In zones of active fluid circulation, maximum simulated permeabilities from roughly 1-6 km depth agree with the best-fit curve from 169 well tests in the Black Forest (log k = -15.4-1.38 log z, Stober and Bucher, Hydrogeol. J., 2007). Maximum simulated permeabilities below 6 km depth agree with a crustal-scale permeability-depth relation estimated from geothermal and metamorphic data (log k = -14-3.2 log z, Manning and Ingebritsen, Rev. Geoph., 1999). Our simulations help to explain the global coherence of geothermal-metamorphic permeability data, and imply that permeability will decay rapidly with depth where internal (metamorphic and magmatic) forcing is absent, as is the case in inactive continental crust.