Provenance changes and movement on the Alpine Fault indicated by heavy minerals from Cretaceous-Cenozoic sediments in south Westland

Abstract

Abstract Late Cretaceous sediments of south Westland contain only ultra-resistant heavy minerals suggesting that there has been substantial intrastratal solution, or that strong chemical weathering was active in the provenance area. The lower part of the Paleocene Tokakoriri Formation is strongly volcanic, but in the upper part the dominant influence was granite or hornfels and (probably associated) older sedimentary rocks such as Greenland Group. Miocene to Recent sediments record a sequence of pronounced changes of provenance. The upper Miocene Tititira Formation shows the early Tertiary source continuing, with increased influence from another granite or altered older sediments. The base of the early Pliocene Halfway Formation is marked by an abrupt flood of hornblende, presumably from Fiordland, though the Tititira source also continues. The influence of hornblende declines towards the top of the formation, and is further reduced (though still prominent) in the overlying “Teer unit”. The Pleistocene Cascade Conglomerate is flooded by olivine from Red Mountain ultramafics. Recent sediments with less olivine and more pyroxene suggest greater influence from more serpentinised ultramafics north of Red Mountain. The sequence of source areas is consistent with the hypothesis that most displacement on the Alpine Fault has taken place since the Miocene.