Hunter-Bowen orogeny

TheHunter-Bowen Orogenywas a significantarc accretionevent in thePermianandTriassicperiods affecting approximately 2,500 km of theAustraliancontinental margin.

The Hunter-BowenOrogenyoccurred in two main phases: The first being Permian accretion ofpassive-marginalDevonian- andCarboniferous-age sediments, occurring in theHunter region,as well as the mid-west region of what is nowNew South Wales,followed by rifting, back-arc volcanism. The later Permian to Triassic event consisted of arc accretion andmetamorphismrelated to ongoingsubduction.

The Hunter-Bowen Orogeny is today represented by a geological structure known as the New EnglandFold Belt,atectonicaccretion of metamorphicterranesand mid-crustalgranitoidintrusions,flanked by Permian to Triassicsedimentary basinswhich were formed distally to the now-erodedorogenic mountain belt.

While theGreat Dividing Rangenorth ofSydneyis a prominent landform, it is more the result ofCenozoicvolcanism andcrustal upliftsince theJurassic,which broadly affects the same area as the Hunter-Bowen Orogeny did.Gravity,magneticsandbathymetryindicate that several slivers of crust formerly from the Hunter-Bowen orogen are now spread out across theIndo-Australian Plateeast of the Australian continental landmass, forming some isolated submerged ocean plateaux and islands, notablyLord Howe Risewhich includesLord Howe Island.Lord Howe Rise has a total area of about 1,500,000 square km.

Prior tectonics

The Hunter-Bowen Orogeny produced a ~3,000 km long structuralforedeepabove a LateCarboniferousandPalaeozoicmargin, adjacent to the weakly consolidated Australian continental crust which at this time was part ofGondwana.The orogen developed to the east of the PalaeozoicLachlan Orogenand the Proterozoic terranes of theMount Isa Inlier.

Before the orogeny the rocks of the coastal area were formed. During the Late Carboniferous there was a continental margin defined by a subduction zone off the coast. The oceanic plate that was being subducted hosted a series ofmagmatic arcs;now represented by the Connors Arch, Auburn Arch, Combarrago Volcanics and the Bathurst Batholith. Aforearc basinis preserved in the Tamworth Belt and Yarrol Belt. Subduction resulted inblueschist metamorphismof the subducted slab. At 309Maanoceanic ridgeencountered the subduction boundary at two points, near what is nowTin Can BayandCoffs Harbour.These encounters interrupted subduction at those locations and resulted in the formation oftriple junctions,which began moving apart north and south along the trench, eventually being replaced by a dextraltransform fault.

The northernmost triple junction, a ridge-trench-fault junction, moved up theQueenslandcoast at 28 mm per year, reachingTownsvilleabout 290 Ma. As it moved, the magmatism inland reduced drastically due to interruption of subduction. The other triple junction, a fault-trench-fault junction, moved south fromBrisbaneat about 12 mm per year. The triple junctions in the New England region stopped subduction very quickly because the mid ocean ridge was almost parallel to the trench. The merging of the subduction zone and the ridge converted the trench to a transform fault, ceasing subduction-related volcanism. Between these two ridge-trench encounter points, a small triangular shaped plate continued to subduct between Brisbane andCoffs Harbour.Thus time was available to build up a thicksubduction wedge.

Megafold

The Texas-Coffs Harbour megafold resulted from dextral motion (clockwise) along a major fault in eastern Queensland. The hypothetical fault, which is not exposed at the surface, is called the Gogango-Baryulgil fault zone. The coastal terrane which stretches from Coffs Harbour in the south toBroad SoundnearSt Lawrence, Queenslandin the north, underwent 500 km of southerly displacement. This brought it into its current position relative to the rest of Australia, and formed a major structural fold inland from Coffs Harbour. The movement may have been caused by absorption of lateral motion between the northern pair of triple junctions. The megafold formed around 290 Ma (early Permian) and took from 10 to 20 million years.

Around this terrane some areas of extension happened on the main craton, creating several basins up to 2 km deep. The Texas region is one such sedimentary basin.

Around 280 Ma in the early Permian, the relative oceanic plate movement changed direction and the continental margin again became convergent. This may have been due to distant continental collisions in the buildup ofPangea.Arc volcanism occurred in central Queensland, forming the Lizzie Creek Volcanics and the Camboon Volcanics. Minor extension in these units formed the Grantleigh Trough, anintra-arc rift.The subduction zone was aligned with the north-east coast of Queensland, with a back arc basin formed to the east of the Hunter region, called the Barnard Basin.Depositioncontinued in basins around 270 Ma into the Permian, but volcanic activity was reduced.

The Sydney-Gunnedah-Bowen Basin

This structural foredeep filled withmarine deepwater sedimentsand laterfluviatile sandstones,which during the Permian and Triassic formed vast accumulations ofcoalfrom entrapped organic matter. TheSydneyandBowen Basinswere flanked by an offshoreisland arcsystem during continued accretion and subduction during the Permian.

Thrusting of the Permian sequences westward in aRocky 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 Carboniferousflysch.Metamorphism resulted in the generation ofS-typeandI-typegranites,which intruded the Palaeozoic sedimentary sequence in the New England Fold Belt. To the north, significantthin-skinned deformationaffected the Carboniferous Marlborough and Yarrol Terranes, resulting in magmatism and restricted granite emplacement.

The subduction zone was curved in an arc, resulting in compression in the west-southwest east-northeast direction as well as sinistralshearin the New England district. A continental fragment may have collided with the area, pushing off the Hastings Block and fracturing the Barnard Basin.

The results of the Hunter-Bowen event were:

• Deformation,
  • Metamorphismof bothgreenschistand rareblueschistfacies
  • Thrust faulting
  • Transtensionpull-apart basinssuch as the
    • Esk Trough, a thin pull-apart rift infillfluvialcoal-bearing basin
    • Clarence Moreton Basin,including theIpswich Basin
  • Transpressionalfaulting and widespread deformation
• Arcvolcanism,namely in theGympieprovince, offshore of the Hunter valley andSydney Basin,
• Back-arc basin formation,
  • Gunnedah Basin
  • Sydney basin
  • Bowen Basin
• Graniteemplacement,
  • New England I-type and S-type suites during accretion (330–260 Ma)
  • Gympie M-type and I-type suites in back-arc position (4 suites; 260–245; 240–235, 231–225 and 220–215 Ma)
• Andesite,rhyoliteandbasaltvolcanism,
  • In-arc at the Permo-Triassic boundary
  • Back-arc in the early to mid Triassic
• Gold,tin,tungstenmineralisationin the Gympie Block, the New England orogen and throughout the Queensland hinterlands,
• Coalformation in theSydney basin,Esk Trough,Gunnedah Basin,Bowen BasinandIpswich Basin

Geochronology

Geochronologyhas identified several episodes of deformation, accretion, subduction and magmatism within the Gympie Block:

• Deformation occurred at 250–240 Ma
• Tholeiiticsuite of magmatism, including M-type and I-type granites at ~250–245 Ma
• Transitional tholeiitic tocalc-alkalinegranite 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-levelcalderacomplexes225–221 Ma

See also

• Geology of Australia
• Orogeny
• Subduction zone
• Shear (geology)
• Bundook beds