Fission-track analysis unravels the denudation history of the Bonar Range in the footwall of the Alpine Fault, South Island, New Zealand

We apply fission-track thermochronology to shed new light on the tectonic history of Zealandia during Late Cretaceous continental extension and the onset of Late Tertiary mountain building in the Southern Alps of New Zealand. The Southern Alps are one of the fastest erosionally exhuming mountain belts on Earth. Exhumation of the Bonar Range in Westland just to the northwest of the Alpine Fault is orders of magnitude slower. We report apatite and zircon fission-track ages from samples that were collected along an ENE-WSW profile across the central Bonar Range, parallel to the tectonic transport direction of a prominent ductile fabric in the basement gneiss. Zircon fission-track (ZFT) ages show a large spread from 121.9 +/- 12.1 Ma to 74.9 +/- 7.2 Ma (1 sigma errors). The youngest ZFT ages of 78 to 75 Ma occur at low elevations on either side of the Bonar Range and become older towards the top of the range, thereby showing a symmetric pattern parallel to the ENE-trending profile across the range. Age-elevation relationships suggest an exhumation rate of 50-100 m Ma(-1). We relate the ZFT ages to slow erosion of a tectonically inactive spot in the Late Cretaceous magmatic arc of Zealandia. Therefore, the first main significance of the paper is that it demonstrates that not all of 110-90 Ma Zealandia was necessarily participating in extreme core complex-related extension but that there were enclaves of lithosphere that underwent slow erosion. The apatite fission-track (AFT) ages range from 11.1 +/- 1.9 Ma to 5.3 +/- 1.0 Ma and age-elevation relationships suggest an exhumation rate of c. 200 m Ma(-1). We relate the AFT ages to the inception of transpressive motion across the Alpine Fault and modest exhumation in its footwall in Late Miocene times. If so, the second significant point of this paper is that transpressive motion across the Alpine Fault was already under way by c. 11 Ma.