Fraser complex and alpine fault tectonics, central Westland, New Zealand

Rattenbury, Mark Sinclair

Fraser Complex, replacing Fraser Formation, is a variably mylonitised suite of diverse igneous and high-grade metamorphic rocks in central Westland, New Zealand. Fraser Complex is bounded to the east by the Alpine Fault against Alpine schist of the Older Torlesse Terrane, and bounded to the west by the Fraser Fault and the Bald Hill Range Thrust against Karamea Terrane Greenland Group, granitoids and Cenozoic sediments. Interlayered amphibolite facies gneisses from Fraser Complex are collectively known as Mt Misery Metamorphic Suite, comprising the metapelitic garnet+biotite+sillimanite±kyanite±K-feldspar Hokitika Gneiss, the metabasic hornblende±epidote±biotite±garnet Doctor Creek Gneiss, and the migmatitic biotite+K-feldspar±garnet Hokitika Gneiss. The variability of garnet-biotite and garnet-hornblende geotherrnometry is attributed to retrogressive effects.
Intrusive into Mt Misery Metamorphic Suite are Fraser Peak Granite and Doughboy Tonalite, S-type granitoids which are chemically distinct from Karamea and Separation Point Batholith granitoids, and have probably derived from melting of Hokitika Gneiss. Camptonite lamprophyre, trachyte and basalt dykes intrude Mt Misery Metamorphic Suite, Fraser Peak Granite and Dough boy Tonalite. Most of the dykes have a similar mineralogy and geochemistry to Hohonu Range dykes and may be early Cretaceous also.
Enveloping Mt Misery Metamorphic Suite and igneous lithologies are numerous mylonite zones. Mylonitisation does not increase in intensity towards either the Alpine Fault or the Fraser Fault. Mylonite foliation generally dips steeply to the southeast, striking subparallel to the Alpine Fault trend. Mylonitic lineations generally plunge gently NE or SW, and the variability is attributed largely to sheath folding. Microstructural shear senses are predominantly dextral, although deformation is inhomogeneous. Mylonite protolithology can usually be sourced to Mt Misery Metamorphic Suite, Fraser Peak Granite or dyke lithologies. Albite-chlorite-actinolite-titanite, with some biotite recrystallisation, and deformation/recrystallisation of quartz and feldspar indicates mid greenschist facies conditions during mylonitisation. Mylonitisation postdates all dyke intrusions with one known exception, a basaltic dyke which truncates mylonite foliation and gives a minimum age of mylonitisation of late Miocene. The basaltic Smith Gorge dyke intrudes Doctor Creek Gneiss, not mylonite. Intrusion of lamprophyre, trachyte and most basalt dykes gives a maximum age of mylonitisation, of early-mid Cretaceous, rather than a minimum age as previously believed.
High-grade gneisses occur throughout Westland and share similar metamorphic grade and mineralogy with Fraser Complex. Regional correlation of the gneisses as basement to Cambro-Ordovician Greenland Group is challenged by a 158 Ma U-Pb zircon age interpreted to date highgrade metamorphism of Fraser Complex. K-Ar dating of Fraser Complex gneisses and mylonites has indicated excess argon contributions to hornblende ages of 228-298 Ma, and possibly to whole rock ages of 45-91 Ma and to biotite ages of 44-61 Ma. The K-Ar biotite ages are incompatible with mid greenschist facies conditions during dextral strike-slip mylonitisation associated with Alpine Fault movement in the mid-late Tertiary.
Uplift of Fraser Complex occurred along the Fraser Fault around 9 Ma, and ceased before deposition of Pliocene silts and conglomerates. The Fraser Fault is a steeply dipping zone of cataclasite, and is redefined to exclude locally crosscutting thrust faults. Initiation of movement and rapid uplift along the Bald Hill Range Thrust is recorded in Shadow Formation, a late Quaternary, periglacial marine silt and conglomerate sequence. Thrust surfaces underlying schist nappes up to 2 km northwest of the most recent Alpine Fault trace, with the Bald Hill Range Thrust are believed to be older, now inactive surface traces of the Alpine Fault which have migrated with time. Newly-discovered recent traces of the Alpine Fault show thrusting of Alpine schist over recent gravels. Two successively-underthrust gravels from Kaka Creek yielded radiocarbon ages of 3350 and 2600 years B.P., and give a minimum convergence rate across the Alpine Fault of 6.7 ± 1.5 mm/year. These ages extend an apparent 500 year return interval of large earthquakes in a relatively aseismic portion of the Alpine Fault.

xiii, 178 p Photos and Illus; 30cm.

1988Rattenbury

Abstract Only

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