Does Slow Slip and Tremor Require High Fluid Pressure? Thoughts From Observations in Retrograde Faults

Subduction zone tremor and slow slip appear spatially correlated with elevated pore fluid pressures related to prograde dehydration reactions in water-saturated subducting rocks. However, tremor and slow slip have also been observed in other tectonic settings, notably on the San Andreas and Alpine faults, where displacement of previously dehydrated rocks occurs along trajectories of decreasing temperature and pressure. Here, fluids are not produced by local dehydration reactions, and external fluids introduced to the fault may be consumed by retrograde hydration reactions. Thus, we suggest that tremor and slow slip in retrograde shear zones imply weakening and strain localization without high fluid pressures, and test this hypothesis against observations in an exhumed, retrograde, continental shear zone. The Kuckaus Mylonite Zone (KMZ), Namibia, accommodated crustal scale strike-slip displacement of granitoid gneisses and migmatites at 450 - 480°C and 270 - 420 MPa, conditions representative of the continuation of continental fault zones below the base of the crustal seismogenic zone. High strain zones within the KMZ comprise fine grained retrograde mineral assemblages requiring rehydration of the original high temperature mineral assemblages in the host rock. Lack of local fluid sources implies that rehydration required influx of an external fluid. We see only minor evidence for hydrothermal vein formation related to this fluid, but note metasomatic alteration and local evidence for dissolution-precipitation. We propose that the main contribution of the fault zone fluid was to allow growth of retrograde, weak, mineral phases, and facilitate grain size reduction through nucleation of new grains, activating grain-size sensitive creep at low driving stresses. Tremor and slow slip observed on active faults under similar conditions may also not require high fluid pressures, but rather be explained by reaction weakening under hydrostatic fluid pressure conditions.