WITNESS TO OROGENY: CONSTRAINING DEVELOPMENT OF THE SOUTHERN ALPS OF NZ USING THERMOCHRONOLOGY IN THE NEIGHBOURING WEST COAST REGION

We present preliminary thermochronological results from a new study investigating the initiation and development of the Alpine Fault zone and Southern Alps of New Zealand. Previous attempts to constrain the evolution of this rapidly exhumed orogenic belt have generally focussed on upper crust of the Pacific plate, east of the Alpine Fault. However, the insights obtained from the Pacific plate are ambiguous due to extreme translation and exhumation of material in the Southern Alps resulting from 90 ± 20 km of Australia-Pacific plate convergence in the past 5 Ma. This ambiguity is compounded by the uniform low to medium grade schist lithologies making up the Southern Alps, which make poor targets for geochronological study.

We resolve these difficulties by focussing on granites and pegmatites exposed along the West Coast region of the South Island. The West Coast represents the footwall of the eastward dipping Alpine Fault zone, a crustal-scale structure forming the active boundary between the Australian and Pacific plates. Unlike the Pacific Plate crust east of the fault, the West Coast has apparently accommodated relatively little deformation and exhumation during the development of the Southern Alps. However the rocks of this region have been in close proximity to the Alpine Fault throughout the interval of recent orogenesis, and have been thermally affected by the rapid adjacent uplift and exhumation of the Pacific upper plate rocks.

Preliminary results have been obtained from localities between the Jacksons Bay and Lake Kaniere areas of the West Coast. Apatite (U-Th)/He and K-feldspar ⁴⁰Ar- ³⁹Ar data from individual samples provide thermochronological records spanning from less than 1 M.yr. to more than 100 M.yr. ago. These records offer new insight into how uplift and exhumation patterns at this tectonically active margin changed during the transition from strike-slip to obliquely convergent motion, and how exhumation rates have varied along the Australia-Pacific plate boundary during the 5 Ma lifespan of the orogen.