New Zealand Journal of Geology and Geophysics abstracts

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

D. CRAW
M. S. RATTENBURY*
R. D. JOHNSTONE

Department of Geology
University of Otago
P.O. Box 56
Dunedin, New Zealand

*Present address: Institute of Geological & Nuclear Sciences, P.O. Box 30 368, Lower Hutt, New Zealand.

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 30deg. 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 inthe west.

Keywords  folding; structure; metamorphism; Alpine Schist; Torlesse Terrane; Alpine Fault; Southern Alps

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