Middle to Late Miocene Age for the End of Amphibolite-Facies Mylonitization of the Alpine Schist, New Zealand: Implications for Onset of Transpression Across the Alpine Fault

Abstract : We report five new Rb-Sr muscovite-based isochron ages, which are the first to constrain the timing of amphibolite-facies mylonitization of the Alpine Schist in the Southern Alps of New Zealand. The ages range from 13.1 ± 4.3 to 8.9 ± 3.2 Ma (2σ uncertainties) for mylonite directly above the Alpine Fault. The weighted mean age of 10.74 ± 0.57 Ma is within uncertainty of a published ⁴⁰Ar/³⁹Ar illite/mica upper-intercept age of 11.5 ± 0.5 Ma measured at the same locality. The end of amphibolite-facies mylonitization occurred at metamorphic conditions of ~560-570 °C and ~0.9-1.1 GPa as derived from pseudosection analysis in the NCTiKFMASH system. We interpret Miocene metamorphism to reflect transpressional crustal thickening and formation of a thick crustal root supporting early Southern Alps topography at or prior to 10.74 ± 0.57 Ma. Additional ~2-1 Ma Rb-Sr biotite, ⁴⁰Ar/³⁹Ar muscovite, and ⁴⁰Ar/³⁹Ar biotite ages reflect isotopic closure during rapid cooling along the Alpine Fault in the Pleistocene. The Miocene mylonitization ages and the Pleistocene cooling ages define a distinct two-stage cooling and exhumation history for the Alpine Schist with initial cooling of ~10 °C/Myr and exhumation rates of 2-4 km/Myr. Final cooling since ~2 Ma was >100 °C/Myr at exhumation rates of ~5-6 km/Myr. We interpret the two-phase cooling history by movement of the mylonite through a strongly nonlinear thermal structure. An older 60.5 ± 0.7 Ma metamorphic event is also preserved as a Rb-Sr crystallization age of a predeformational muscovite-plagioclase assemblage in a sheared pegmatite.