Correlating Tectonic Histories Using Combined Thermochronological Constraints: an Example from Westland, Southern New Zealand.
Thermochronological constraint derived for individual samples through 40Ar-39Ar analyses of K-feldspar and (U-Th)/He dating of apatite provides a new means of correlating the tectonic histories of structurally ambiguous granitoid rocks from the Austrlaia-Pacific plate boundary zone in Westland, southern New Zealand. Extensive Quaternary cover and plains of fluvio-glacial outwash material limit structural continuity of this region. Thermochronological constraint is particularly valuable in such disrupted regions, with distinctive chronologies of cooling providing a basis for correlating between discontinuously exposed outcrops. By using complementary constraint from multiple chronometers to derive continuous thermotectonic histories for individual samples, we are able to directly isolate signals of tectonic change through time at a range of points along the plate boundary and distinguish them from diachronous spatial variation in deformation of the wider orogenic system. The thermal histories defined in this study allow the distinction of two tectonic domains, the distributions of which offer insight into the early evolution of the Australia-Pacific plate boundary through the region. Fault- bounded slivers of high-grade gneiss dispersed throughout Westland record differential exhumation at transpressional centres within a dominantly strike-slip tectonic setting between c. 22 and 25 Ma. Correlation to an analogous structural system in Otago and Southland east of the Alpine Fault demonstrates the operation of a broad zone of distributed deformation early in the development of the plate boundary. Abandonment of these local transpressional centres during the Miocene reflects the progressive evolution of a more optimally oriented plate bounding structure. Development of the modern phase of oblique convergence and rapid orogenic growth in the South Island is recorded in these basement samples by the synchronous development of rapid cooling, presumably reflecting erosional exhumation of samples, at 8 Ma.