Multiphase deformation and its relationship to metamorphic crystallisation at Haast River, South Westland, New Zealand

Abstract

The temporal and spatial relationships between multiphase folding, faulting, dike swarm intrusion, and metamorphic crystallisation are discussed for a suite of meta-morphic rocks from the Haast River area (S87, S98), South Westland. Metamorphic reconstitution is sufficiently far advanced in the map area that most structures developed contemporaneous with sedimentation in the New Zealand Geosyncline are now obliterated. The oldest recognisable mesoscopic structures, representing the initiation of Rangitata Orogenic movements in this area, are tight mesoscopic, isoclinal F₁ folds. Macroscopic folds of the same generation are considered to be recumbent isoclinal folds or nappes. In localised areas F₁ folds are refolded by a second synmetamorphic folding phase, F₂. The hinge zone of a macroscopic, isoclinal, F₂ antiformal fold has been located and mapped out in the eastern sector of the map area. Although metamorphic temperatures were maintained at high values during F₁ and especially F₂ deformation, the metamorphic maximum post-dated these folding phases. Late-metamorphic, broad, open folding gave rise to the Mount Victor Fold Belt, Haast Antiform, and Thomas Synform structures which folded the metamorphic isograds. The F₃ folding was followed by faulting along an east-west direction with localised kink-style, F₄ folding developed along fault planes. The major fault of this phase, the Burke Fault, has a maximum vertical movement of 2700 m, and has a reverse dip-slip character. The eastern sector of the area was then intruded by an alkalic-lamprophyric dike swarm with magma channelled along early east-west faults and along an ac joint system developed during F₂ folding. A later north-east trending fault set offset the earlier east-west faults and has a normal dip-slip character. Kink or chevron folds (F₅) are also locally developed along fault planes.