Laboratory-determined Rheology of Rocks at Hydrothermal Conditions and the Transition from Brittle to Ductile Deformation

Abstract:

While damaging earthquakes commonly nucleate in the cold, brittle crust, increasing pressure and temperature result in a transition zone to more ductile behavior in the lower crust to upper mantle. Under these conditions, brittle rheology must be replaced by a mode of deformation favoring creep and slow slip. Heterogeneity in mineralogy, stress, fluid pressure and fluid composition will result in a broad zone of mixed deformation properties that include slow slip and tremor, localized sources of higher frequency radiated energy and repeating sources. Observed sensitivity of low frequency earthquakes to small amplitude tidal stresses suggests near-lithostatic pore pressures within the transition zone. We examine experimental determinations of rheological properties of rocks and minerals at high pressure and temperature and controlled pore pressure. Some rocks, composed primarily of framework silicates, show a transition from brittle to ductile behavior near 350°C, consistent with inferred temperatures of the transition zone in subduction zones and in plate-bounding strike slip fault systems. Other minerals, such as serpentine, have a complex rheology that shifts from brittle to ductile deformation with temperature, pressure and strain rate. An additional weakening mechanism is observed when, for example, ultramafic rocks are brought in contact with quartz-bearing rocks under hydrothermal conditions. In this case, mineral assemblages are no longer in equilibrium and reactions at load-bearing grain contacts lead to reductions in strength that are strongly strain rate sensitive. These various weakening processes are examined in the context of fault stability and the transition from brittle to ductile deformation.