ROLE OF THE BRITTLE-DUCTILE TRANSITION IN LARGE-SCALE FLUID FLOW IN ACTIVE CONTINENTAL CRUST

Geothermal and metamorphic-fluid-flux data show that permeability (k, m²) decreases with depth (z, km) in tectonically or magmatically active continental crust according to logk=-14 - 3.2logz. This relationship describes permeability averaged over large time and length scales. The dependence of k on depth is greatest above the approximate position of the brittle-ductile transition (BDT, 10-15 km); below this depth, the data are equally consistent with a constant logk of -18.3. These results show that independent geological observations support a simple model for the hydrology of active continental crust consisting of two hydrologic regimes, bounded by the BDT. Below the BDT, permeability is nearly constant, low, and shows little variance among localities. Model metamorphic devolatilization rates are sufficient to sustain lithostatic pressure gradients and fluids must be mainly internally derived. Low fluid flux results in conductive heat transport but advective solute transport. Above the BDT, permeability and its variance increase strongly with decreasing depth. The transition corresponds to logk » -17, which is an effective upper limit on permeability at which elevated pore-fluid pressures can be sustained (Neuzil, 1995, AJS, 295, 742). Thus hydrostatic pressure gradients dominate above the BDT (except in thick sequences of low-k sediments) and fluids are primarily meteoric in origin. Generally, higher fluid fluxes may permit advective transport of heat as well as solutes. That the boundary between the two crustal hydrologic regimes corresponds to the BDT reinforces the idea that rock deformation controls crustal hydrology: the shallow, brittle crust is more permeable because elevated rock strength supports a high-k pore network, whereas the deep, ductile crust displays a nearly constant k because plastic deformation dynamically limits permeability. However, the boundary between the two regimes is not a discontinuity, but rather a large change in slope. The continuity in the k-z relationship across the BDT implies efficient communication between the two reservoirs.