The 1st AAPG/EAGE PNG Geosciences Conference, PNG’s Oil and Gas Industry:
Maturing Through Exploration and Production

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Compressional Evolution of the PNG Margin; A Tale of Two Collisions


New and existing thermochronology data across the Muller Range and north to the Sepik Terrane have been analysed to determine the time temperature history, particularly burial and erosion events. This was to investigate the large regional angular unconformity between mid-Late Cretaceous beds and the Miocene Darai Limestone. QTqt modelling of four profiles reveals that the Muller area was buried by >1.5 km of sediment during the Late Cretaceous to Eocene, presumably by the Chim, Moogli and Mendi beds as preserved near the Porgera Mine. The area was then uplifted and eroded during the Oligocene due to docking of the Sepik Terrane such that an Eocene accretionary prism over-rode the margin. During the Early Miocene there was regional extension and subsidence as exemplified by metamorphic core complexes in northern PNG and grabens in the Darai on the Fly Platform. This led to deposition of the Darai Limestone and distal marls that cover the suture between the Sepik Terrane and mainland PNG. A crustal-scale, fully restored section across the PNG orogenic belt reveals the Oligocene to Recent compressional deformation of the margin. The northern end of the section comprises the Landslip Metamorphics, the accreted continental terrane, separated from the main part of the Fold Belt by the Eocene accretionary prism.. Neogene compression commenced around 12 Ma with ~70km shortening in the Om terrane and ~38km shortening in the Fold Belt. Existing thermochronology data indicate shortening of ~12mm/year from 12-4 Ma, but only 2.5mm/year from 4-0 Ma, consistent with a change in structural style in the Fold Belt from thrust to more ductile, fold-dominated deformation. The model also requires substantial thickening of the continental crust beneath the Muller Ranges, here represented by ‘basement’ underthrusting. Gravity modelling indicates the presence of sedimentary graben up to 10 km deep beneath the Fold Belt, which were strongly inverted, such as beneath the Lavani Valley. A key issue is when this inversion occurred, in the Oligocene or Pliocene, as this has a significant influence on the timing of hydrocarbon generation and migration.