Kevin C. Hill

Mesozoic-Cenozoic evolution of Australia's New Guinea margin in a west Pacific context

The northern Australian margin includes the island of New Guinea, which records a complex structural and tectonic evolution, largely masked by Mio-Pliocene orogenesis and the Pleistocene onset of tectonic collapse. In thePalaeozoic, New Guinea contained the boundary between a Late Palaeozoic active margin in the east and a region of extension associated with Gondwana breakup along the western margin of Australia. In the Permian and Early Triassic, New Guinea was an active margin resulting in widespread Middle Triassic granite intrusions, The Mesozoic saw Triassic and Jurassic rifting followed by Cretaceous passive margin subsidence and renewed rifting in the Late Cretaceous and Paleocene. Since the Eocene, New Guinea tectonics have been driven by rapid northward movement of the Australian Plate and later sinistral oblique convergence with the Pacific Plate, resulting in Mio-Pliocene arc-continent collision. Neogene deformation along the margin, however, has been the result of direct interaction with the Philippine and Caroline Plates. Collision with the Philippine-Caroline Arc commenced in the Late Oligocene and orogenesis continues today. We suggest that the New Guinea Mobile Belt comprises a collision zone between a north-facing Cretaceous indented margin and a south-facing Palaeogene accretionary prism, subsequently cut by a Neogene strike-slip fault system with well over 1000 km sinistral displacement that has alternated between extension and compression. The change in character of the lithosphere in New Guinea, from thick and strong in the west to thin and weak north and east of the Tasman Line, was also an important influence on the style and location of Mesozoic and Cenozoic deformation.

Mid-Cretaceous uplift and denudation of the Bowen and Surat Basins, eastern Australia: relationship to Tasman Sea rifting from apatite fission-track and vitrinite-reflectance data

Interpretation of apatite fission-track and vitrinite-reflectance data for samples from nine petroleum-exploration wells in the eastern part of the Bowen and Gunnedah Basins, eastern Australia, indicates that peak paleotemperatures were reached during the Early Cretaceous, through progressive exposure to higher temperatures due to increased depth of burial. The paleotemperatures were 28–58°C higher than at present. Cooling from the peak temperatures took place in the mid-Cretaceous, at some time during the interval 100–80 Ma, with the greatest amount of cooling occurring in the northern part of the study area. Paleogeothermal gradients were generally in the range 21–35°C/km, similar to present-day geothermal gradients in the region. The estimated maximum amount of denudation in the study area is ∼1.9 km, with a significant portion of the eroded succession being Jurassic to Early Cretaceous in age. The synchronicity between the times of cooling inferred from all the sampled wells, regardless of their location with respect to the fault system near the present eastern margin of the Bowen Basin, suggests that the uplift was widespread, rather than being localised by faults during the mid-Cretaceous. This can be correlated with uplift along much of the eastern margin of Gondwanaland at the same time, including all of eastern Australia, in New Zealand and in Marie Byrd Land, Antarctica. The onset of this mid-Cretaceous regional cooling and denudation coincided with a period of continental extension after the cessation of volcanism and subduction at about 95 Ma, and prior to the initiation of seafloor spreading at about 84 Ma and formation of the current passive margin.

Thermochronological and geochemical constraints on the tectonic evolution of northern Papua New Guinea

Abstract The Bewani-Torricelli-Prince Alexander Mountains, along the northern margin of Papua New Guinea, probably formed as a tholeiitic island arc in the Late Eocene-Early Oligocene. The arc is interpreted to have accreted to the margin by the Late Oligocene, but may have formed on a ribbon of extended continental crust along the New Guinea margin. Inferred roll-back of the subducting slab beneath New Guinea in the Early Miocene placed the margin into extension, creating starved graben in northern New Guinea and causing regional subsidence. Near the graben, metamorphic core complexes resulted as the upper crust was pulled off the lower crust along low angle detachments such that lower crustal rocks cooled rapidly from temperatures >500°C. The two inferred core complexes that have been dated show rapid cooling from 27-23 Ma and 20-18 Ma. Continued subduction beneath New Guinea resulted in formation of the Maramuni arc in the Middle Miocene and the end of extension. In the Late Miocene, collision of the Melanesian arc caused regional uplift of all basement of northern Papua New Guinea, mainly from 8-5 Ma, causing at least 3–4 km of denudation. The compressional deformation propagated south causing uplift, denudation and cooling in the Papuan Fold Belt at c. 4 Ma, but is continuing at the present.

Sediment supply to the Gippsland Basin from thermal history analysis: constraints on Emperor-Golden Beach Reservoir Composition

The Emperor and Golden Beach Subgroups are becoming the focus of Gippsland Basin exploration, yet little is known about their composition and distribution. Regional modelling of over 400 apatite fission track analyses in the hinterland constrains the timing, magnitude and distribution of uplift and denudation and hence sediment supply to the basin. The study yielded regional maps through time of palaeotemperature, overburden, denudation rate and palaeotopography, with increasing assumptions and hence uncertainty. Regionally the >60,000 km3 of Strzelecki Group comprises ~90% volcanoclastic detritus and coal with only ~10% basement-derived sediment, but the northern margin of the basin, near Lakes Entrance, is likely to have a higher basement-derived portion resulting in better reservoirs. The basement-derived sediments are probably largely granitic as the Devonian granites were exposed during the Permo-Triassic Hunter-Bowen Orogeny. Regional mid-Cretaceous uplift resulted in increased denudation of basement, but inversion of the basin margins resulted in denudation of the onshore Strzelecki Group sediments. Emperor and Golden Beach Subgroup sediments deposited in the subsiding Central Graben were at least 50% basement-derived, again with higher quality reservoirs predicted near the Lakes Entrance area and poorer reservoirs near to Wilson’s Promontory. The Latrobe Group siliciclastics were at least 80% derived from basement with a substantial portion from northern Tasmania and the Furneaux Islands around 60-50 Ma.

The Muller anticline, Papua New Guinea. Basement-cored, inverted extensional fault structures with opposite vergence

Abstract Throughout the NW-SE trending Papuan Foldbelt, two dominant structural styles are present. In the northeast the 1 km thick, thrust-imbricate slices of Miocene limestone attest to the classical thin-skinned nature of the deformation. In the southwest, adjacent to the undeformed foreland, fold structures are 1 or 2 orders of magnitude greater in size, are strongly asymmetrical and commonly have relatively thicker stratigraphie sections on the crests. Such structures are the result of Mio-Pliocene compression that caused thrusting along faults that are hypothesized to have been extensional in the Mesozoic and Palaeogene. Geological traverses from the undeformed foreland heading northeast across the 150 km by 50 km Muller Range in western Papua New Guinea, provided detailed surface structural data. When combined with analysis of existing geological maps, exploration wells, regional gravity surveys and extensive seismic in the foreland, moderately constrained balanced and restorable cross-sections through the Muller anticline could be constructed. Interpretations of gravity data are consistent with the Muller anticline, being cored by basement and seismic data extrapolated beneath the mountains, show the outcrop of basement in the centre of the Muller Range to be elevated 8 km above regional. The eastern Muller anticline (EMA) has a shallow northeast limb and steeper southwest limb, suggesting a NE-dipping fault below, such that basement was thrust to the southwest. The Cecilia, Wai Asi, Juha and Lavani anticlines to the southwest of the EMA are interpreted to be formed by shortening of the whole sedimentary section to balance the shortening in basement. There are no such structures to the southwest of the western Muller anticline (WMA) where the southwest limb is gently dipping and the northeast limb is steep. Hence, the WMA is interpreted as having formed above a SW dipping fault along which basement was thrust to the northeast. The marked difference in shortening of the southwest limbs requires a tear fault between the EMA and WMA. The tear fault is here interpreted to coincide with the northwestern end of the Cecilia and Lavani anticlines and the marked change in strike of the Muller anticline. By analogy with other very large, asymmetrical, basement-involved structures in the Papuan Foldbelt it is suggested that the faults underlying the Muller anticline were active as extensional faults in the Mesozoic, soling at a mid-crustal detachment. The extensional faults beneath EMA and WMA had opposite vergence and were separated by a transfer zone. Subsequent compressional reactivation of the extensional faults in the Pliocene gave rise to the present Muller anticline.