Trapping of aqueous fluids in the deep crust

Free aqueous fluids in the deep (ductile) crust have residence times there which are very short in geological terms. If not consumed by retrograde metamorphic reactions, they empty very rapidly upwards. This can be shown by applying the theory of magma percolation in a ductile host to the problem of aqueous fluid percolation. Below the brittle‐ductile transition thermal activation of ductility is strong enough to facilitate the rapid evolution of porosity and consequently allow rapid fluid transport. In the cooler crust above the brittle‐ductile transition, however, this mechanism vanishes. As a consequence, fluid overpressures resulting from the accumulation of fluid in the transition zone must induce hydraulic fracturing rather than pore inflation. In regions of horizontally compressive regional stress, the commonest situation in stable continental crust, these fractures will be horizontal. Aqueous fluids rising through or liberated from the lower crust will therefore probably accumulate (at least initially) at the brittle‐ductile transition in horizontally extensive reservoirs with high horizontal permeability but low vertical permeability.