THERMAL HISTORY ANALYSIS BY INTEGRATED MODELING OF APATITE FISSION-TRACK AND VITRINITE REFLECTANCE DATA - APPLICATION TO AN INVERTED BASIN (BULLER-COALFIELD, NEW-ZEALAND)

Integrated analysis and modelling of apatite fission track with vitrin ite reflectance (VR) data allows the timing, magnitude and pattern of Palaeogene subsidence and Neogene inversion to be established for an u plifted and largely denuded basin: the Buller Coalfield, New Zealand. At the time of maximum subsidence in the late Oligocene, the basin con sisted of an extensional half graben, bounded to the west by the Konga hu Fault Zone (KFZ), with up to 6 km of upper Eocene to Oligocene sect ion 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 pres ent topography. Palinspastic restoration of the three phases of invers ion 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-easte rn part of the basin (maximum temperatures were experienced at differe nt 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 deriv ed from the zone of inversion, they can be compared independently with the timing of unconformity development and rapid subsidence in the ad jacent foredeeps, particularly the Westport Trough. For the middle to late Miocene phase of inversion, we show that during the first 1-2 mil lion years of compression, the uplift within the coalfield also involv ed the margins of the Westport Trough, contributing to unconformity de velopment; subsequently, uplift continued on the inversion structure b ut the margins of the Westport Trough subsided rapidly. This is explai ned by a model of stick slip behaviour on the boundary faults, especia lly for the KFZ. When compression started the fault zone has locked an d uplift extends into the basin, whereas subsequently the fault zone u nlocks, and the inversion structure overrides the basin margin, thereb y loading it and causing subsidence.