HYDRODYNAMICS OF REGIONAL METAMORPHISM DOE TO CONTINENTAL COLLISION

Two-dimensional numerical models are used to examine variations of flu id flow and fluid pressure induced by topography, thermal buoyancy, an d production of metamorphic fluids during continental collision. After burial, the thermal and denudation history of the crust and permeabil ity are first-order controls on the pattern of fluid flow and its evol ution with time. Topography generally controls flow patterns at shallo w levels (over thermal buoyancy), and for permeabilities less than abo ut 1 mu D, metamorphic devolatilization centrals flow patterns at dept h. As a result, fluids are driven during metamorphism from the hinterl and to the foreland throughout the crust. Devolatilization fluxes are on the order of 10(6) kg m(-2); higher fluxes can be produced by focus ing of flow in areas of high permeability. Once cooling of the crust e nsues, metamorphic fluid production ceases, and fluid pressures, even in the models with permeabilities of less than 1 nD, drop toward hydro static values. Although most other hydrologic forcings not included in the models, such as expansion of pores due to unloading, will also ac t to decrease fluid pressures, ductile flow of rods may close poses an d prevent a drop in fluid pressure above the brittle-ductile boundary. Because of the low permeability that results, hydrofracturing is part icularly likely to happen at depths below the brittle-ductile boundary near the peak of metamorphism as cooling progresses downward through the crust. Widespread flow of fluids toward higher temperatures at dep th, as has recently been proposed, is unlikely because it requires a d ecrease in fluid pressures toward the hinterland and thus low topograp hy, low rates of metamorphic devolatilization, and high permeability.