Plate Tectonics and Regional Stratigraphic-Metamorphic Relations in the Southern Part of the New Zealand Geosyncline
The evolution of the New Zealand geosyncline is examined in terms of both the plate tectonics model and such geological constraints as regional metamorphic and biostratigraphic patterns and interrelations between paired sedimentary facies. A common western provenance is tentatively concluded for both major sedimentary facies belts of the geosyncline; the inner (Hokonui) belt acted as a volcanotectonic sediment trap during times of increased volcanic activity, while during periods of volcanic quiescence, quartzo-feldspathic debris from the adjacent “continental” foreland was transported across the inner belt into the outer (Torlesse) basin. In general the Torlesse rocks appear to decrease in age but increase in metamorphic grade toward the inferred western (continental) source, and it is suggested that the older (Carboniferous and Permian) rocks were never overlain by substantial thicknesses of younger strata. Development of an early extensional basin, by strike-slip displacements, in Carboniferous to Jurassic times is consistent with both this concept and with the apparently anomalous regional biostratigraphic zonation of the Torlesse rocks. Differences in geosynclinal evolution between New Zealand and California appear to be accountable in terms of the greater age of the New Zealand geosyncline and the early period of extensional growth. The Late Jurassic-Cretaceous Rangitata orogeny was an important event occurring relatively late in the depositional history of the New Zealand geosyncline, and is interpreted as representing the initiation of the present system of Pacific basin sea-floor spreading. A corresponding early extensional event is not recognized in the Jurassic-Cretaceous Coast Range geosyncline of California, the evolution of which appears to be entirely related to subduction and arc activity related to an East Pacific rise spreading axis. Differences in detailed deformational style and metamorphism between the two areas may result from differences in spreading rates and from the steeper dip of the Jurassic-Cretaceous subduction zone in New Zealand.