The Hydrologic Characteristics of an Orogenic Wedge During Burial of Pelites

Abstract:

The burial and exhumation of oceanic sediments in orogenic wedges is one of the most important processes that occur during continental collisions. During progressive burial, pelites lose porosity and permeability, and eventually pass into the lithostatic pressure regime where the porosity and permeability are thought to be very small. However, prograde metamorphic reactions produce fluids that must ascent though the pore space or fractures. The hydrologic characteristics of schists undergoing burial were modeled using the program SUTRAMET, which is a modified version of the program SUTRA (Voss & Provost, 2002) that allows modeling of metamorphic reactions at high pressures and temperatures and incorporates transient porosity and permeability changes due to overpressure and changing volume of the metamorphic assemblage.

The initial model crustal section includes a hydrostatic pressure regime to the depth of 5 km with φ ≥ 0.007 and k ≥ 10^-17 m². Within the lithostatic pressure domain, schists have φ ≤ 0.003 and k ≤ 10^-18 m². Below the schists that are undergoing burial (3 or 6 mm/y) is an impermeable substrate with φ = 0. P and T are maintained at the top and the bottom boundaries and P is also maintained at the top of the lithostatic domain. T-dependent thermal diffusivity and heat capacity were applied. The initial T gradient is ~30°/km. The only heat sink is endothermic metamorphic reactions and the only heat source is a small amount of radioactivity. The initial schist was assumed to have 17.9 wt.% qtz, 13.2% ab, 28.7% ksp, 9.0% ms, 22.8% chl, and 8.5% law. By the time it reaches the bottom of the crustal section, metamorphic reactions produce the anhydrous assemblage 14.5% qtz, 7.8% an, 13.7% ab, 36% ksp, 0.2% sil, and 27.5% grt.

Results show that fluid pressure remains slightly above lithostatic within the initially lithostatic pressure regime as metamorphic fluids are released, even at the slow burial rate of 3 mm/y, because the fluids pass to the hydrostatic regime very slowly. Fracturing and enhancement of permeability due to shrinking of the mineral assemblage moderate the amount of excess pressure within the crustal column. The results also show that very large amounts of water are released at discrete horizons within the crust due to discontinuous metamorphic reactions as schists pass through the horizons during burial.