Constraints on the thermal and tectonic evolution of Greymouth coalfield

The southern end of the Paparoa Range in Westland, South Island, New Zealan d, 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: co mpression 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 basem ent loading formed the foreland basin, containing up to 3 km of mainly mari ne sedimentary section. Fission track results for apatite concentrates from 41 shallow drillhole an d 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 trac k results integrated with modelling of vitrinite reflectance data, show tha t the maximum paleotemperatures experienced during burial of the Late Creta ceous and mid-Eocene coal-bearing succession everywhere exceeded 85 degrees C, and reached a peak of 180 degrees C along the axis of the former basin. Cooling from maximum temperatures occurred during three discrete phases: 2 0-15 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 seco nd phase, and 1.4 km during the third phase. It appears that exhumation ove r 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 evoluti on 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 th e vicinity of Buller Coalfield.