Geology of Inchbonnie, North Westland

Angus, Paul V. M.

The geology of the Haast Schist in Inchbonnie area, Westland, is studied. The Haast Schist is dominated by quartzofeldspathic lithologies, although pelitic, metavolcanic and metachert horizons occur. Metamorphic grade includes The Greenschist and Amphibolite Facies which increase from the chlorite zone through biotite to the garnet zone as the Alpine Fault is approached.
Pre-and synmetamorphic deformation (D1) is characterised by a regional schistosity S1, sub-parallel to compositional banding. S1 is the result of F1 deformation, a period of tight isoclinal folding. Departure from true parallelism of S0 and S1 planes produces L1 , a steeply plunging lineation. Broad open F2 synrnetamorphic folds occur along slightly plunging axes. S2 surfaces are delineated by growth of white mica and opaques and incipient development of quartz-albite segregations. Post-metamorphic deformation (D3) is dominated by strike-slip movement on the Alpine fault, producing conjugate joints and faults orientated at 097° and 147°. The 097° set is dominated by the Taramakau branch of the Hope Eault.
Pounamu Ultramafic bodies occur within Greenschist and Amphibolite Facies rocks, and are totally enclosed within quartzofeldspathic lithologies. These consist of a serpentinite-magnesite or talc-magnesite core, surrounded by talc-magnesite, talc, tremolite, chlorite, muscovite, biotite and albite monomineralic and bimineralic zones. These define a 600m wide zone (Pounamu Ultramafic Belt, (PUB), consisting of two bands interpreted as limbs of-an F2 macroscopic fold which closes in the Newton Range. The PUB is subsequently dextrally displaced 5 km by the Hope Fault along with the associated chlorite-actinolite greenschists and the garnet isograd.
A study of mineral microstructures of fault rocks associated with the Alpine Fault indicates that (l) strain is consistently greater on the east side of the Alpine Fault, and (2) a typical shear stress of 70 MPa close to the Alpine Fault zone occurred during mylonitisation.
Twelve marine terrace remnants within the Harper Pass area were compared with Huon Peninsula, New Guinea. By correlating the 400 m terrace with the 56 ka New Guinea Terrace, an uplift rate (on the eastern side of the Alpine Fault) of 3.7 mm/yr prior to 135 ka, and 6.7 mm/yr from 135 ka to present, was obtained. The change is attributed to an increase in convergence between the Australian and Pacific Plates.
Rounded quartz pebbles sampled from five of the marine terrace remnants in the Inchbonnie area were studied, using the Scanning Electron Microscope. Quartz pebbles from the lower terraces (830-892) have surface textures characteristic of a beach environment, and surface textures subsequently became dominated by chemical features as the terrace sequence progressed. This is attributed to silica dissolution in the surface weathering zone.
The relationship of the Alpine Fault and Hope Fault (Member of the Marlborough Fault System) is considered in a plate tectonic theme, and it is considered that Hope Fault dextrally offsets the Alpine Fault in at least two localities.

128 p. : ill. (some col.), maps ; 30 cm.

1984Angus

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