REACTION VEINS AND THE PERMEABILITY OF THE UPPER CRUST

The permeability of continental upper crust varies with time due to deformation related changes of aperture and fracture geometry and as a result of chemical reaction of flowing fluids with the solids exposed along the fractures. Particularly dissolution and precipitation of minerals contribute to the variation of the permeability with time. The time dependence of κ is difficult to measure directly and it has not been observed in well tests. At depths below the deepest wells and down to the brittle ductile transition zone evidence of permeability variation with time can be found in surface exposures of rocks originally from this depth. Exposed hydrothermal reaction veins are very common in continental crustal rocks and document fossil permeability and its variation with time. A model description of permeability variation with time in the deeper parts of the brittle continental crust suggests a permeability history with three distinct episodes: 1) initial abrupt permeability increase related to fracture generation, 2) slow continued permeability increase caused by dissolution of solids along the fractures by reactive fluid, 3) slow decrease of permeability due to precipitation of solid reaction products from the fluid supersaturated with these solids. The last period ends when the central fracture has been sealed and becomes impervious. The permeability of a representative volume of the crust is the sum of the contributions from all fractures at a given instant in time.