No abstract

Geological observations require that episodic slip on the Alpine fault averages to a long-term displacement rate of 2-3 cm/yr. Patterns of seismicity and geodetic strain suggest the fault is locked above a depth of 6-12 km and will probably fail during an earthquake. High pore-fluid pressures in the deeper fault zone are inferred from low seismic P-wave velocity and high electrical conductivity in central South Island, and may limit the seismogenic zone east of the Alpine fault to depths as shallow as 6 km. A simplified dynamic rupture model suggests an episode of aseismic slip at depth may not inhibit later propagation of a fully developed earthquake rupture. Although it is difficult to resolve surface displacement during an ancient earthquake from displacements that occurred in the months and years that immediately surround the event, sufficient data exist to evaluate the extent of the last three Alpine fault ruptures: the 1717 AD event is inferred to have ruptured a 300-500 km length of fault; the 1620 AD event ruptured 200-300 km; and the 1430 AD event ruptured 350-600 km. The geologically estimated moment magnitudes are 7.9 ± 0.3, 7.6 ± 0.3, and 7.9 ± 0.4, respectively. We conclude that large earthquakes (M w >7) on the Alpine fault will almost certainly occur in future, and it is realistic to expect some great earthquakes (M w ≥8).

This paper provides a comprehensive description of all major plutonic rock units in Fiordland between Lakes Poteriteri and Te Anau, and the heads of Doubtful and George Sounds. Plutonic rocks comprise c. 80% of the basement in the area described, the remainder being metase dim entary and metavolcaniclastic rocks. The plutonic rocks, of which c. 50% are granitoids, were emplaced in three phases-at c. 492 Ma, between c. 365 and 318 Ma, and between 168 and 116 Ma. Correlatives of the Devonian Karamea Suite emplaced between c. 375 and 367 Ma, and the Triassic to Early Jurassic part of the Darran Suite emplaced between c. 230 and 168 Ma, are not present in the area described here.

Lamprophyre dikes in north-west Otago and north Westland yield whole rock potassium-argon ages mainly lying between 136 and 107 m.y. These ages correspond roughly to the ages inferred by others for the wall rock, suggesting that the ages give the latest major cooling event and that the dikes were possibly intruded earlier.In the dikes, the mica and amphibole ages are younger than their host whole rock age, while the plagioclase ages are older. The mineral ages are best explained as due to loss of argon from micas and amphiboles, and absorption by plagioclase and possibly other tectosilicates, the whole rock remaining approximately a closed system. The redistribution of argon is thought to have occurred at slightly lower temperatures than that causing outgassing of the whole rock. The whole rock dates support an earlier suggestion that the dikes were intruded as a single belt that has since been separated into two parts by a 120 km displacement along the Alpine Fault. The Alpine Fault displacement, which is dextral, was probably in two major periods: one in the Early Cretaceous, and the other in the Late Cenozoic.

This paper provides a first description of all major plutonic rock units between Resolution Island and Lake Poteriteri in southwest Fiordland. Plutonic rocks, of which c. 95% are granitoids, comprise c. 60% of the basement in southwest Fiordland. Approximately 50% of the plutonic rocks were emplaced between c. 355 and 348 Ma, 5% at c. 164 Ma, 25%between c. 140 and 125 Ma, and 20% between c. 125 and 110 Ma. These episodes of plutonism occurred in response to terrane amalgamation, continental thickening, and subduction along the convergent margin of Gondwana. Correlatives of Devonian plutonic rocks which occur in Nelson are absent from the area described here.A wide variety of plutonic rocks were emplaced at c. 355-348 Ma. These include relatively small plutons of K-and Rb-rich gabbro-diorite and members of at least three distinct suites of granitoids. Plutons of two-mica ± garnet granodiorite, granite, and minor tonalite share affinities with the S-type Ridge Suite and are the most widespread c. 355-348 Ma old granitoids in southern Fiordland. Plutons rich in Ca, Fe and Zr, depleted in K and Na, and containing quartz diorite, tonalite, and minor granodiorite with the unusual assemblage red-brown biotite, garnet ± hornblende ± clinopyroxene also occur widely in southern Fiordland. These plutons are similar to peraluminous A-type granitoids, indicating A as well as I and S-type plutonism occurred in the Western Province at this time. The Newton River and Mt Evans Plutons have no correlatives amongst c. 355-348 Ma granitoids in southern Fiordland, but their chemistry is similar to that of the older Karamea Suite.