Thermal history analysis by integrated modelling of apatite fission track and vitrinite reflectance data: application to an inverted basin (Buller Coalfield, New Zealand)

Abstract

Integrated analysis and modelling of apatite fission track with vitrinite reflectance (VR) data allows the timing, magnitude and pattern of Palaeogene subsidence and Neogene inversion to be established for an uplifted and largely denuded basin: the Buller Coalfield, New Zealand. At the time of maximum subsidence in the late Oligocene, the basin consisted of an extensional half graben, bounded to the west by the Kongahu Fault Zone (KFZ), with up to 6 km of upper Eocene to Oligocene section adjacent to it; currently, only a few tens of metres of basal coal measures on basement are preserved on top of a range 800-1000 m above sea level. Integrated modelling of the VR and fission track data show that the deepest parts of the basin were inverted during two Miocene compressional phases (24-19 Ma and 13-8 Ma), and are consistent with a further phase of inversion during the Quaternary that formed the present topography. Palinspastic restoration of the three phases of inversion shows that the basin was not inverted in a simple way: most of the rock uplift/denudation adjacent to the KFZ occurred during the early Miocene phase, and at the same time burial occurred in the south-eastern part of the basin (maximum temperatures were experienced at different times at different places in the basin); during the middle to late Miocene there was broad uplift in the central and eastern parts of the coalfield. Because the timing and magnitude of uplift have been derived from the zone of inversion, they can be compared independently with the timing of unconformity development and rapid subsidence in the adjacent foredeeps, particularly the Westport Trough. For the middle to late Miocene phase of inversion, we show that during the first 1-2 million years of compression, the uplift within the coalfield also involved the margins of the Westport Trough, contributing to unconformity development; subsequently, uplift continued on the inversion structure but the margins of the Westport Trough subsided rapidly. This is explained by a model of stick slip behaviour on the boundary faults, especially for the KFZ. When compression started the fault zone has locked and uplift extends into the basin, whereas subsequently the fault zone unlocks, and the inversion structure overrides the basin margin, thereby loading it and causing subsidence. through burial, in sedimentary basins. Apatite fission I N TR OD UCTI ON track analysis, by comparison, is a new technique for assessing the thermal history of apatite-bearing host rocks Vitrinite reflectance (Ro%) is a well-established means in sedimentary basins (Gleadow et al., 1983; Green et al., of assessing the maximum temperature experienced by 1989a; Naesar et al., 1989). It takes advantage of the time organic-bearing sediments as a result of heating, normally and temperature dependence of the inherent instability (open-system behaviour) of the crystallographic radiation damage (latent fission tracks) generated by the spon-Correspondence: Peter Kamp.

Tectonic reconstructions of New Zealand: 40 Ma to the Present

“…Minor conglomerates were deposited adjacent to the proto-Alpine Fault in the upper Grey valley and east Murchison areas, and mudstones and flysch were deposited in the Inangahua-Grey valley trough and areas to the north. Inversion of the Buller coalfield region began c. 24-23 Ma (Kamp et al 1996).…”

Heat flow through the West Coast, South Island, New Zealand

“…Subsequent heat flow calculations incorporating new BHT data from the Taranaki and Great South Basins, north and southeast of the West Coast, respectively, have suggested that the conductivities used by Pandey (1981) were too high . Kamp et al (1996) studied in detail the thermostratigraphic history of the Buller Coalfield, an inverted basin northwest of Westport, using integrated vitrinite reflectance and apatite fission track data. However, Kamp et al did not address the transient thermal history of the basin specifically, but instead assumed a constant geothermal gradient and surface temperature for the 40 m.y.…”

Tectonic controls on Miocene sedimentation in the Southern Taranaki Basin and implications for New Zealand plate boundary deformation

“…23–20 Ma, is consistent with the change in convergence rates at ca. 20 Ma (Figure b), and has been observed in northwestern South Island, west of the Alpine Fault (Ghisetti et al, ; Kamp, Webster, & Nathan, ; Lihou, ; Nathan et al, ). Despite this contraction, marine facies persisted through many of the basins west of the Alpine Fault during the earliest Miocene, with limited supply of terrigenous sediment to most West Coast basins and the STB at this time (Figure a).…”

“…13 and ca. 7 Ma (Reilly et al, ; Figure c), eastern North Island (Field et al, ; Nicol et al, ), the West Coast region (Ghisetti et al, ; Kamp et al, ) and in the hanging‐wall of the Alpine Fault (Tippett & Kamp, ).…”