Abstract

Two-dimensional numerical models are used to examine variations of fluid flow and fluid pressure induced by topography, thermal buoyancy, and production of metamorphic fluids during continental collision. After burial, the thermal and denudation history of the crust and permeability are first-order controls on the pattern of fluid flow and its evolution with time. Topography generally controls flow patterns at shallow levels (over thermal buoyancy), and for permeabilities less than about 1 mu D, metamorphic devolatilization controls flow patterns at depth. As a result, fluids are driven during metamorphism from the hinterland to the foreland throughout the crust. Devolatilization fluxes are on the order of 10 6 kg m (super -2) ; higher fluxes can be produced by focusing of flow in areas of high permeability. Once cooling of the crust ensues, metamorphic fluid production ceases, and fluid pressures, even in the models with permeabilities of less than 1 nD, drop toward hydrostatic values. Although most other hydrologic forcings not included in the models, such as expansion of pores due to unloading, will also act to decrease fluid pressures, ductile flow of rocks may close pores and prevent a drop in fluid pressure above the brittle-ductile boundary. Because of the low permeability that results, hydrofracturing is particularly 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 depth, as has recently been proposed, is unlikely because it requires a decrease in fluid pressures toward the hinterland and thus low topography, low rates of metamorphic devolatilization, and high permeability.

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