MECHANICAL ROLES OF TRANSIENT METAMORPHIC FLUID PRODUCTION IN EMBRITTLEMENT OF DUCTILE SHEAR ZONES

Fluids released during prograde metamorphism may rise and weaken brittle fault zones but models indicate that fluid production is too short-lived and fluxes are too low to solve the weak fault paradox except temporarily. In contrast, short-lived fluid production events probably have important effects in shear zones that carry rocks across the ductile-brittle transition (DBT) of the crust. Combined structural and fluid inclusion studies of two major normal shear zones in the Alps (Brenner and Simplon Lines; Axen et al., 2001, J. Geophys. Res.) indicate that prograde fluid evolution was essential to their ductile-to-brittle evolution at abnormally high-temperature conditions. In particular, a prograde fluid-production event aided thinning of the ductile shear zones from ~2 km to ~50 m as minor dilatant brittle structures formed in conditions of 450°-575°C and 400-750 MPa. Formation of these structures apparently stopped mylonitization abruptly throughout most of the shear zones’ thicknesses and aided fluid influx into the remaining ~50 m-thick ductile shear zones. These remaining zones must have been weaker than the subjacent nondeforming parts, but strain rate very likely increased in them, probably causing increase of absolute and differential stress levels as well. Once the fluid-production event ended, the remaining ~50 m apparently quit deforming mylonitically and a brittle fault evolved, probably above ~450°C and ~400 MPa (~15 km). Thus, transient evolution of fluid was important for embrittlement of most of the shear zone, and the end of fluid production probably caused the final change to frictional slip. The mechanical, fluid, strain rate, and differential stress characteristics of the DBTs in the areas where shear-zone deformation was focused were probably significantly different from the DBTs to either the east or west.