New insight into the dynamic development of the Southern Alps, New Zealand, from detailed thermochronological investigation of the Mataketake Range pegmatites

Abstract

Abstract The Southern Alps are an actively developing mountain range resulting from continuing oblique continent-continent collision between the Australian and Pacific plates through the South Island of New Zealand. The thermal histories of five granite pegmatites from the Mataketake Range in the southwest of this orogen are revealed here by U-Pb dating of zircon, K-Ar, 40 Ar- 39 Ar and Rb-Sr dating of muscovite, biotite and alkali feldspar, and fission-track dating of apatite and zircon. The constraint of these thermal histories, and in particular the high thermal resolution gained from analysis of K-feldspar by the 40 Ar- 39 Ar dating method, offers a novel insight into the dynamic evolution of the Southern Alps. The pegmatites were probably formed by localized anatectic partial melting at temperatures of 620–680°C between 67 and 82 Ma. Other than initial cooling to the ambient temperature of the host schist following intrusion (estimated at c. 500°C), these bodies share an apparently uniform thermochronological record, and this thermal history is thus largely indicative of cooling during local exhumation. Between 10 and 5 Ma, the Mataketake Range area cooled from c. 350 to 260°C, at a rate of 18°C Ma −1 . This corresponds to unroofing at between 0.8 and 1 km Ma −1 during this interval, and is probably a reflection of the inferred early history of the Australian-Pacific plate boundary through the South Island of New Zealand as a dominantly strike-slip feature. A reduction in cooling rate at 5 Ma, immediately followed by an episode of extremely rapid cooling at c. 350°C Ma −1 marks the initiation of rapid exhumation and the development of the perturbed geothermal structure of the modern Southern Alps. This supports suggestions that the present tectonic regime of the orogen developed in a single episode of tectonic reorganization 5 Ma ago.