LAUSR

Abstract We report two 40Ar/39Ar illite ages from fault gouge directly above the current trace of the Alpine Fault in New Zealand at Gaunt Creek (1.36 ± 0.27 Ma) and Harold Creek (1.18 ± 0.47 Ma), and one 40Ar/39Ar illite age from fault gouge from the Two Thumbs Fault on the east side of the Southern Alps. Metamorphic muscovite clasts inherited into the Alpine Fault gouge yielded 40Ar/39Ar ages of 2.04 ± 0.3 Ma at Gaunt Creek and 11.46 ± 0.47 Ma at Harold Creek. We also report Rb-Sr muscovite-based multimineral ages of Alpine Schist mylonite adjacent to the dated fault gouge at Harold Creek (13.1 ± 4.3 Ma) and Gaunt Creek (8.9 ± 3.2 Ma). 40Ar/39Ar muscovite ages from the Gaunt Creek mylonite yielded plateau ages of 1.47 ± 0.08 Ma and 1.57 ± 0.15 Ma. Finally, we report zircon fission track (0.79 ± 0.11 and 0.81 ± 0.17 Ma) and zircon (U-Th)/He ages (0.35 ± 0.03 and 0.4 ± 0.06 Ma) from Harold Creek. 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.