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

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 faultbounded blocks were involved, and uplift was underway by 10 Ma. Assuming a preuplift geothermal gradient of 25øC/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.

Transpressional development of the Australia‐Pacific boundary through southern South Island, New Zealand: Constraints from Miocene‐Pliocene sediments, Waiho‐1 borehole, South Westland

“…apatite fission track ages from between the SWFZ and the Alpine Fault are generally older than late Cenozoic (Seward & Nathan 1990;Kamp et al 1992), indicating that most rocks exposed at the surface in this region have not been subjected to temperatures greater than c. 110°C during the late Cenozoic. This suggests much lower degrees of cooling and uplift northwest of the Alpine Fault than for the region immediately southeast of the Alpine Fault, where zircon fission track and K-Ar ages are late Cenozoic (Adams 1981;Tippett&Kamp 1993).…”

“…Two possible tectonic mechanisms are proposed that may explain the subsidence preceding middle Miocene deposition: (1) lithospheric stretching and thermal subsidence (cf McKenzie 1978) related to rifting in the Solander Trough and Emerald Basin (Sutherland 1995a); (2) flexure of the Australian plate and foreland basin development due to northwest-directed thrusting in the middle Miocene. The general thickening of the Westland Basin towards the southeast, combined with the middle Miocene start of terrigenous sedimentation and increase in depth of deposition, was interpreted by Kamp et al (1992) and Sircombe (1993) to be due to the formation of a foreland basin in middle Miocene time. Kamp et al (1992) suggested that, during the early stages of foreland basin development (middle -late Miocene), basinal sediments were bounded on their southeastern edge by the Alpine Fault.…”

Gneissic rocks of the Bonar Range, central Westland, New Zealand

“…2). The augen mylonite and sheared granite from the Bonar Range yielded c. 75-79 Ma ages, indicating they had cooled through c. 200-265°C and had undergone no subsequent ductile deformation since the Late Cretaceous (White & Green 1986;Kamp et al 1992). The three samples from the Fraser Complex all yielded dates of between 8 and 10 Ma and were inferred by White & Green (1986) and Kamp et al (1992) to represent late Cenozoic uplift along the Fraser Fault.…”

“…Geology between Urquhart Creek and the Fraser Fault is compiled from Young (1968) and Green (1982). Geology east of the Waitaha River is compiled from Warren (1967), Young (1968), and Green (1992). Rb-Sr geochronology locations of Aronson are derived from Nathan (1967).…”

“…Rb-Sr geochronology locations of Aronson are derived from Nathan (1967). Fission track geochronology locations are derived from Kamp et al (1992). Base map and grid ticks are from NZMS 260, Sheet I34.…”