Hunter-Bowen orogeny

The Hunter-Bowen Orogeny was a significant arc accretion event in the Permian and Triassic periods affecting approximately 2,500 km of the Australian continental margin.

The Hunter-Bowen Orogeny occurred in two main phases, a Permian accretion of previously formed passive-marginal Devonian and Carboniferous sediments in the Hunter region and mid-west region of what is now New South Wales, separated by rifting, back-arc volcanism and a later Permian to Triassic event resulting in arc accretion and metamorphism during a subduction event.

The Hunter-Bowen Orogeny has resulted in the New England Fold Belt, a tectonic accretion of metamorphic terranes and mid-crustal granitoid intrusions, flanked by Permian to Triassic sedimentary basins which were formed distally to the now-eroded orogenic mountain belt.

While the Great Dividing Range north of Sydney is a prominent landform, this is more the result of Cainozoic volcanism and crustal uplift since the Jurassic than the result of the original orogenic belt which is essentially mimics. Gravity, magnetics and bathymetry indicate that several slivers of crust formerly from the Hunter-Bowen orogen are now spread out across the Indo-Australian plate east of the Australian continental landmass, forming some isolated submerged ocean plateaux and islands, notably Lord Howe Island.

The Sydney-Gunnedah-Bowen Basin

The Hunter-Bowen event produced a ~3,000 km long structural foredeep above a Late Carboniferous and Palaeozoic margin to the weakly consolidated Australian continental mass which was part of the Gondwana Supercontinent at this time; the orogen developed to the east of the Palaeozoic Lachlan Orogen and the Proterozoic terranes of the Mount Isa Inlier.

This structural foredeep filled with marine deepwater sediments and later fluviatile sandstones, which during the Permian and Triassic formed vast accumulations of coal. The Sydney and Bowen Basins were flanked by an offshore island arc system during continued accretion and subduction during the Permian.

Thrusting of the Permian sequences westward in a Rocky Mountains style foreland basin system continued as metamorphism began affecting the lower parts of the offshore island arcs, composed primarily of Devonian marine sediments of continental origin, and Carboniferous flysch. Metamorphism resulted in the generation of S-type and I-type granites which intruded this Palaeozoic sedimentary sequence in the New England Fold Belt. To the north, significant deformation affected the Carboniferous Marlborough and Yarrol Terranes, resulting in magmatism and restricted granite emplacement.

The results of the Hunter-Bowen event were:

• Deformation
  • Metamorphism of both greenschist and rare blueschist facies
  • thrusting
  • Transtension pull-apart basins such as the
    • Esk Trough, a thin pull-apart rift infill fluvial coal-bearing basin
    • Clarence-Moreton Basin, including the Ipswich Basin
  • Transpressional faulting and widespread deformation
• Arc volcanism, namely in the Gympie province, offshore of the Hunter valley and Sydney Basin
• Back-arc basin formation,
  • Gunnedah Basin
  • Sydney basin
  • Bowen Basin
• Granite emplacement
  • New England I-type and S-type suites during accretion (330-260Ma)
  • Gympie M-type and I-type suites in back-arc position (4 suites; 260-245; 240-235, 231-225 and 220-215Ma)
• Andesite, rhyolite and basalt volcanism in the
  • In-arc at the Permo-Triassic boundary
  • Back-arc in the early to mid Triassic
• Gold, tin, tungsten mineralisation in the Gympie Block, the New England orogen and throughout the Queensland hinterlands
• Coal formation in the Sydney basin, Esk Trough, Gunnedah Basin, Bowen Basin and Ipswich Basin

Geochronology

Geochronology has identified several episodes of deformation, accretion, subduction and magmatism.

Gympie Block

• Deformation occurred at 250-240 Ma
• Tholeiitic suite of magmatism, including M-type and I-type granites at ~250-245 Ma
• Transitional tholeiitic to calc-alkaline granite and andesite suite ~245-240 Ma
• I-type granite suite and basalt suite ~229 Ma associated with gold mineralisation
• Late I-type and S-type high-level caldera complexes 225-221 Ma

See also

• Geology of Australia
• Orogeny
• Subduction zone
• Shear (geology)
• Granite; particularly the section on S-type and I-type