Tectonic architecture of the mountain front‐foreland basin transition, South Island, New Zealand, assessed by fission track analysis

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Abstract
Oblique continental convergence across the Alpine fault since the mid‐Miocene has loaded the Australia plate with the leading edge of the Pacific plate and formed a foreland basin. The basin occurs mostly offshore beneath the continental shelf, but remnants of the marine basin fill overlie Ordovician basement in a narrow coastal strip 10–20 km wide in Westland, between the Tasman Sea and the Alpine fault. The results of fission track analyses of apatite and zircon separates from this basement, integrated with the regional structure and stratigraphy, indicate that the foreland basin formerly extended across the whole of Westland and that its succession increased in thickness eastward toward the Alpine fault, reaching a maximum of about 4 km. Continuing covergence across the main thrust (Alpine fault) has caused its footwall to thicken by reverse fauting in a transitional zone 10–20 km wide immediately west of the mountain front (Southern Alps). Consequently, the inner margin of the foreland basin has been inverted and the succession almost completely eroded. In the southern part of Westland, upper crustal shortening in the Australia plate amounted to about 2 km; associated uplift started about 5 m.y. ago and involved only one major fault‐bounded block. In northern Westland, upper crustal shortening amounted to about 12 km; two major fault‐bounded blocks were involved, and uplift was underway by 10 Ma. Assuming a preuplift geothermal gradient of 25°C/km, the long‐term rate of uplift ranged between 0.8 and 2.0 mm/yr in southern Westland and between 0.4 and 1.2 mm/yr in different parts of northern Westland.