New Zealand Journal of Geology and Geophysics abstracts
Constraints on the thermal and tectonic evolution of Greymouth coalfield
PETER J. J. KAMP
IAN W.S. WHITEHOUSE
Department of Earth Sciences
The University of Waikato
Private Bag 3105
Hamilton, New Zealand
JANE NEWMAN
Newman Energy Research
2 Rose Street
Christchurch, New Zealand
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.
Keywords Greymouth Coalfield; inversion; foreland; fission
track; vitrinite reflectance; thermochronology
New Zealand Journal of Geology & Geophysics, 1999, Vol. 42:
447-467
0028-8306/99/4203-0447 $7.00/0 (c) The Royal Society of New Zealand
1999
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