Structure of the alpine schists of South Westland, Southern Alps, New Zealand

Abstract

The structure of the metamorphic rocks forming the Southern Alps in South Westland, New Zealand, is described. Metamorphic grade increases north-westwards from greywacke and argillite east of the Main Divide through Chlorite Zone schists to high-grade schists and gneisses of the Biotite, Garnet, and Oligoclase Zones. The metamorphic zonal sequence is interrupted and repeated by post-metamorphic strike faults and folds, and terminated on the north-west by the Alpine Fault.After geosynclinal sedimentation in the late Paleozoic and early Mesozoic, the thick geosynclinal sediments were deformed and metamorphosed in the middle Jurassic to early Cretaceous Rangitata Orogeny. Three movement phases separated by relatively static periods of metamorphic crystallisation are distinguished. In the first movement phase (F ,), movements were dominantly translational, forming large recumbent folds with axial surfaces dipping south and south-west. The F I movements were accompanied by Chlorite Zone metamorphism and development of schistosity and lineation. In the first static phase (M,), garnet and biotite porphyroblasts grew in the schists. In the second movement phase (F2 ) , the F, recumbent folds were isoclinally folded about steep axial planes, and a second schistosity and lineation produced by transposition of the first. The F, folds and lineations plunge south and south-west in the same direction as the regional dip produced by the F , folding. In the second static phase (M,), garnet, biotite, and amphibole grew in the schists, and the isograd pattern was established, and transgressed the F, and F, fold axes on a regional scale. In the postmetamorphic (F3 ) deformation, the orogenic belt was uplifted by folding and faulting, the Alpine Fault began to move, and some folding about steep axes took place under a wrench regime. Lamprophyre dike swarms, probably Cretaceous in age, are the only post-tectonic intrusives known. They have probably been displaced 110 miles from similar dikes in North Westland by dextral strike-slip movement on the Alpine Fault in the late Cainozoic Kaikoura Orogeny.

Schist structure east of the Alpine Fault bends, southeast Nelson, New Zealand

“…Certainly some minerals (e.g., biotite; P43684) have been recrystallised during phase 2, and rotated (pinwheel) garnets occur within the F 2 zone (e.g., P43685). A similar problem, of schists developing metamorphic textures during more than one event, was first noted by Grindley (1963) and later by Findlay (1979) in the Mount Cook region.…”

Contortions in the schists of the Cromwell district, Central Otago, New Zealand

“…comm.). Phase 3 folding and metamorphism seems better developed in the Cromwell district than in these other areas, although Cooper (1974) and Grindley (1963) record post-Phase 2 static recrystallisation.…”

Potassium-argon age of some New Zealand lamprophyre dikes near the Alpine Fault

“…The zone of southern dikes is bounded to the north by the Alpine Fault near Paringa River (Mason, 1961;Grindley, 1963) and extends as a broad belt south-westwards for 105 ian to the Matukituki River (Fig. Ib).…”

“…Belts of mylonite mapped on opposite sides of the Alpine Fault may once have been continuous and since displaced 230 km (Grindley, 1963). The mylonites are thought to have been formed in the Early Cretaceous (Grindley, 1963) or Late Jurassic to Early Cretaceous (Reed, 1964). They are cut by the dikes (Young, 1968).…”