A thermochronological study of southern Fiordland, New Zealand

Abstract

The Fiordland Block, in the southwestern part of the South Island of New Zealand, shows evidence of a complex tectonic evolution dominated by tectonic and intrusive events in the mid-Paleozoic and mid-Cretaceous and is thought to represent a fragment of the Paleozoic margin of Gondwana. Fiordland geology is dominated by the extensive granulite facies Cretaceous Western Fiordland Orthogneiss, which is surrounded by Paleozoic metasediments and intrusive rocks. This study concentrates on an area in southern Fiordland, where rocks of three crustal levels are exposed, separated by steep SW-NE trending faults (for example the Dusky Fault): the granulite facies (-9-12 kbar) Western Fiordland Orthogneiss, the amphibolite facies Cretaceous Supper Cove Orthogneiss which intruded into kyanite-grade ( -6-8 kbar) Paleozoic metasediments, and the andalusite to sillimanite-grade ( -3-4 kbar) Southwestern Fiordland Block. By determining and comparing the structural, metamorphic and thermochronological histories of the different crustal levels, a better understanding of the tectonic evolution of Fiordland, and of the exhumation of Fiordland in particular, has been gained. The Paleozoic history of Fiordland is characterised by andalusite to sillimanitegrade metamorphism and strong deformation associated with the emplacement of voluminous granitic intrusions. New U-Pb SHRIMP zircon dating of granitic intrusions in southern Fiordland yielded ages of ca 370 to 380 Ma, which are consistent with previously determined ages from other parts of Fiordland and from the once contiguous area of Westland-Nelson, in the northwestern part of the South Island of New Zealand. A ca 340 Ma U-Pb zircon age of a posttectonic pegmatite in the Southwestern Fiordland Block and a ca 335 Ma U-Pb zircon age from a weakly deformed metagabbro in the sillimanite-grade Central Fiordland Belt (to the east of the Southwestern Fiordland Block) constrain the younger age limit of the dominant tectonic event. The localised preservation of 220-270 Ma 40Ar-39 Ar hornblende and mica ages indicate that the area cooled to below -300°C by at least the Permian and give a minimum cooling rate of 3°-5°C/Ma for cooling following mid-Paleozoic metamorphic and deformation events. However, because most of the Paleozoic rocks are strongly affected by a Cretaceous thermal event, little is known about the tectonic and cooling history between the mid-Paleozoic and the mid-Cretaceous. The majority of the 40 Ar-39 Ar mica ages from the Paleozoic metasedimentary and intrusive sequence range between ca 100 and 120 Ma and indicate a strong thermal overprint in the Cretaceous. In the Southwestern Fiordland Block, the thermal event is associated with only very little deformation and with a localised, static, kyanite-grade overprint. No evidence for the presence of Cretaceous intrusions has been found in the Southwestern Fiordland Block. Immediately north of the Dusky Fault, which separates the Southwestern Fiordland Block from deeper crustal levels, kyanite-grade Paleozoic rocks are intruded by the voluminous 121 Ma Supper Cove Orthogneiss and are dominated by Cretaceous deformation and metamorphism (at 6-8 kbar and 600-650°C). The Supper Cove Orthogneiss is similar in geochemical composition to the Western Fiordland Orthogneiss, but was metamorphosed at lower pressures and could, therefore, represent higher levels of the Western Fiordland Orthogneiss complex. The Western Fiordland Orthogneiss, the Supper Cove Orthogneiss and the intruded Paleozoic cover experienced rapid cooling to temperatures around 300°C as result of rapid exhumation. 40Ar-39Ar mica ages of 107 Main the Western Fiordland Orthogneiss and 40 Ar-39 Ar hornblende and mica ages of 100 Ma and 90 Ma, respectively, in the Supper Cove Orthogneiss and kyanite-grade Paleozoic cover, indicate that the deeper level Western Fiordland Orthogneiss cooled more rapidly than the higher level Supper Cove Orthogneiss. This could indicate that the Western Fiordland Orthogneiss was exhumed from beneath the Supper Cove Orthogneiss along an extensional detachment zone, thereby bringing the two orthogneisses at a similar crustal level. This is consistent with the observation that, following the rapid cooling, both areas remained at similar temperatures until final exhumation and cooling in the late Tertiary, as indicated by K-feldspar age spectra. A similar model has been proposed for the thermal event in the Southwestern Fiordland Block: middle to lower crustal rocks were exhumed from beneath the Southwestern Fiordland Block, bringing hot lower crustal rocks at shallower levels and increasing the heatflow. The relatively small temperature difference between the Southwestern Fiordland Block and the mid to lower crustal Cretaceous orthogneisses following mid Cretaceous rapid cooling confirms that lower crustal rocks were brought within 5 km of the Southwestern Fiordland Block. These observations may indicate that several detachment zones were active at the same time at various levels in the crust. The continental extension resulted in the break-up of the Gondwana margin. K-feldspar 40 Ar-39 Ar age spectra from the Southwestern Fiordland Block and Central Fiordland Belt indicate slow cooling at rates of 1 °-2°0Ma from about 100 Ma to at least 50 Ma, when temperatures dropped to below ca 200°C. This is consistent with 60- 80 Ma zircon fission track ages. A minor deformation event around 50 Ma is indicated by resetting of the argon isotopic system in K-feldspars from the Western Fiordland Orthogneiss and Supper Cove Orthogneiss and by increased cooling rates indicated by Kfeldspar 40Ar-39 Ar age spectra from the Southwestern Fiordland Block. This is thought to be related to Eocene plate reconfiguration in the southwest Pacific. The initiation of the final exhumation and cooling is well constrained by Kfeldspar 40Ar-39 Ar age spectra from the Western Fiordland Orthogneiss and Supper Cove Orthogneiss, which show a sudden increase in cooling rate from <1 °C/Ma to -13°0Ma around 15 Ma. Final exhumation was probably the result of the start of compression across the precursor of the Alpine Fault and possibly of subduction of the Australian plate under southern Fiordland. Consistent ca 7 Ma apatite fission track ages across southern Fiordland indicate that the brittle Dusky Fault and other equivalent faults were mainly active between 15 and 7 Ma, during which the Western Fiordland Orthogneiss and Supper Cove Orthogneiss were brought up to the same crustal level as the Southwestern Fiordland Block. Show more