Constraints on the thermal and tectonic evolution of Greymouth coalfield

Abstract

The southern end of the Paparoa Range in Westland, South Island, New Zealand, comprises an asymmetrical, southward plunging, faulted (Brunner-Mt Davy) anticline, the eastern limb of which is common with the western limb of an asymmetrical (Grey Valley) syncline forming a Neogene foreland basin (Grey Valley Trough). The faulted anticline is a classic inversion structure: compression during the Neogene, associated with the development of the modern Australia-Pacific plate boundary, caused a pre-existing normal fault zone, about which a late Cretaceous-Oligocene extensional half graben had formed (Paparoa Trough), to change its sense of displacement. The resulting basement loading formed the foreland basin, containing up to 3 km of mainly marine sedimentary section. Fission track results for apatite concentrates from 41 shallow drillhole and outcrop samples from the Greymouth Coalfield part of the Brunner-Mt Davy Anticline are reported and interpreted, to better establish the timing and amount of inversion, and hence the mechanism of inversion. The fission track results integrated with modelling of vitrinite reflectance data, show that the maximum paleotemperatures experienced during burial of the Late Cretaceous and mid-Eocene coal-bearing succession everywhere exceeded 85deg.C, and reached a peak of 180deg.C along the axis of the former basin. Cooling from maximum temperatures occurred during three discrete phases: 20-15 Ma, 12-7 Ma, and c. 2 Ma to the present. The amount of denudation has been variable across the inverted basin, decreasing westward from a maximum of c. 2.5 km during the first deformation phase, c. 1.2 km during the second phase, and 1.4 km during the third phase. It appears that exhumation over the coalfield continued for about 2 m.y. beyond the biostratigraphically determined time ranges of each of two synorogenic unconformities along the western limb of the Grey Valley Syncline. Stick-slip behaviour on the range front fault that localised the inversion is inferred. The tectonic evolution of the anticline-syncline pair at the southern end of the Paparoa Range, is therefore identical in style, and similar in timing, to the development of the Papahaua Range-Westport Trough across the Kongahu Fault Zone, in the vicinity of Buller Coalfield.