Structures within greenschist facies Alpine Schist, central Southern Alps, New Zealand

Abstract

Abstract Ductile structures in the greenschist facies rocks of the Alpine Schist that lie in the headwaters of the Whataroa, Callery, and Balfour valleys formed during Mesozoic deformation, similar to those in the Otago Schist to the south. A general westward increase in metamorphic grade is locally disrupted and repeated by several late metamorphic faults. The first appearance of biotite occurs at tectonic boundaries in the Callery and Balfour sections. Metagreywackes are juxtaposed against a greenschist‐metapelite‐metachert sequence by a synmetamorphic ductile high strain zone in the lower Balfour valley. A prominent quartz rodding lineation in the high strain zone defines a stretching direction that plunges about 30° southwest. Structures in the high strain zone are locally cut by quartz (±biotite) veins, which have been weakly deformed. Fluid inclusions in quartz veins indicate that the veins formed about 8–10 km deep and that uplift was initially nearly isothermal. Pervasive foliation that formed during Mesozoic deformation was deformed into upright, open, kilometre‐scale folds, with some foliation‐parallel boudinage, associated with the Miocene inception of the Alpine Fault. The resultant north to northeast trending folds plunge gently southwards. Deformation associated with Alpine Fault movement was only minor in the Alpine Schist greenschist facies and resulted in passive differential uplift of deeper parts of the schist pile in the west.