GEOMETRY, KINEMATICS AND SIGNIFICANCE OF LATE TERTIARY DEXTRAL TRANSPRESSIONAL STRUCTURES IN NORTHERNMOST FIORDLAND, NEW ZEALAND

High-pressure (14-16 kbars) granulite and amphibolite facies orthogneisses exposed in the Fiordland belt of southwestern New Zealand record a polyphase history of Early Cretaceous to Late Tertiary tectonism. A dominant subvertical, NNE-striking foliation (S1) defined by amphibole, garnet, clinopyroxene and plagioclase assemblages formed at lower crustal depths (>45 kms) during Early Cretaceous convergence along the margin of Gondwana. At the northwestern boundary of the granulite belt, a NNE-striking subvertical shear zone (the Anita shear zone) tuncates S1 and separates high pressure rocks from Paleozoic cover rocks to the west. Both the Anita shear zone and S1 are reactivated by regional strike-slip faults that include, from west to east, the Alpine fault, the Pembroke fault and the Harrison-Kaipo (HK) fault. This latter fault zone forms the eastern boundary of the granulite belt. Deformed pegmatites that display a greenschist facies protomylonitic foliation (S2) and truncate S1 form part of the fabric of the fault zones. Preliminary K-Ar isotopic work on phengites from these pegmatites yielded 12-20 Ma ages. At the southern end of the HK fault zone, en echelon dextral faults strike to the NE and offset lithologic contacts in a step-wise geometry. At its northern end, this fault zone curves to the north, and intersects and merges with the NE-striking Pembroke fault. Faults at this intersection accommodated reverse displacements. Elsewhere, the dominant NE-striking fault zones record bulk dextral strike-slip kinematics. Our data strongly suggest that northernmost Fiordland was affected by a regional-scale Late Tertiary dextral transpressional restraining bend formed by curved faults that separate the Fiordland granulites from weakly unmetamorphosed rocks to the north, west, and east. Our data also suggest that final Late Tertiary exhumation of the granulites in northern Fiordland was controlled by dextral transpressional tectonics and erosion. This mechanism contrasts with areas containing Early Cretaceous granulites located in south-central Fiordland where exhumation processes were influenced by mid-Cretaceous ductile normal faulting.