Birth of a Plate Boundary: Detailed thermochronological constraint of Cenozoic Plate Boundary Evolution in the South Island of New Zealand

Detailed thermochronological investigations of individual samples collected west of the Alpine Fault zone in New Zealand provide new insight into the spatial distribution of early Australian-Pacific (AUS-PAC) plate boundary evolution that is not preserved elsewhere in the modern orogenic system of the South Island. Internally consistent cooling histories, derived through combination of ⁴⁰Ar-³⁹Ar, fission track and (U-Th)/He analyses of a range of minerals from individual samples, span the Cenozoic. These records provide direct constraint on the timing and character of three distinct episodes in the evolution of this tectonic boundary. Of particular note, the early Miocene propagation of the coherent AUS-PAC boundary through the South Island is expressed as short-lived, episodes of geographically localized cooling, which we infer to be due to exhumation, beginning at 23-25 Ma. This is consistent with development of this boundary as an initially distributed zone of deformation, progressively localizing onto discrete fault structures developing primacy of strain accommodation over time. Further work is in progress to map out the spatial distribution of this activity in greater detail, and thereby constrain the progressive development of the plate boundary structure. Rapid cooling universally observed from 8 Ma onwards corroborates the association of the modern obliquely convergent character of the AUS-PAC boundary through the South Island with a marked change in Pacific Plate motion at that time. Reconciling this regionally synchronous onset with previous geographically wider-spread studies of the Southern Alps supports recent suggestions that along-strike variation in age reported for individual thermochronometers results largely from structural inhomogeneity within the orogen, rather than diachronous tectonic development of the system.