Fault-gouge dating in the Southern Alps, New Zealand

We interpret the fault gouge ages to date growth of newly formed illite during gouge formation at temperatures of ~ 300-350 °C towards the base of the seismogenic zone. Simple backcalculation using current uplift/exhumation and convergence rates, and dip angles of 45-60° at the Alpine Fault support that interpretation. We infer that the fault gouge ages record faulting and gouge formation as the rocks passed very rapidly through the brittle-ductile transition zone on their way to the surface. Rb-Sr and 40Ar/39Ar ages on muscovite from Alpine Schist mylonite date muscovite growth at ~ 11 Ma together with a younger phase of cooling/shearing at ~ 1.5-2 Ma. Our ages from the Alpine Schist indicate extremely rapid cooling exceeding 200 °C/Ma. The fault gouge age from the Two Thumbs Fault is significantly too old to have formed as part of the late Neogene/Quaternary Southern Alps evolution.