The Hunter-Bowen Orogeny was a significant arc accretion event in the

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleoz ...

and

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

periods affecting approximately 2,500 km of the

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

continental margin A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margin ...

. The Hunter-Bowen Orogeny occurred in two main phases, a Permian accretion of previously formed passive-marginal

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

and

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...

sediments in the

Hunter region The Hunter Region, also commonly known as the Hunter Valley, is a region of New South Wales, Australia, extending from approximately to north of Sydney. It contains the Hunter River and its tributaries with highland areas to the north and so ...

and mid-west region of what is now

New South Wales ) , nickname = , image_map = New South Wales in Australia.svg , map_caption = Location of New South Wales in AustraliaCoordinates: , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , es ...

, 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 A granitoid is a generic term for a diverse category of coarse-grained igneous rocks that consist predominantly of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quartz ...

intrusions, flanked by Permian to Triassic sedimentary basins which were formed distally to the now-eroded orogenic mountain belt. While the

Great Dividing Range The Great Dividing Range, also known as the East Australian Cordillera or the Eastern Highlands, is a cordillera system in eastern Australia consisting of an expansive collection of mountain ranges, plateaus and rolling hills, that runs rough ...

north of

Sydney Sydney ( ) is the capital city of the state of New South Wales, and the most populous city in both Australia and Oceania. Located on Australia's east coast, the metropolis surrounds Sydney Harbour and extends about towards the Blue Mountain ...

is a prominent landform, this is more the result of

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...

volcanism and crustal uplift since the Jurassic than the result of the original orogenic belt which is essentially mimics. Gravity, magnetics and

bathymetry Bathymetry (; ) is the study of underwater depth of ocean floors (''seabed topography''), lake floors, or river floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of water de ...

indicate that several slivers of crust formerly from the Hunter-Bowen orogen are now spread out across the

Indo-Australian Plate The Indo-Australian Plate is a major tectonic plate that includes the continent of Australia and the surrounding ocean and extends northwest to include the Indian subcontinent and the adjacent waters. It was formed by the fusion of the Indian an ...

east of the Australian continental landmass, forming some isolated submerged ocean plateaux and islands, notably

Lord Howe Rise The Lord Howe Rise is a deep sea plateau which extends from south west of New Caledonia to the Challenger Plateau, west of New Zealand in the south west of the Pacific Ocean. To its west is the Tasman Basin and to the east is the New Caledonia ...

which includes

Lord Howe Island Lord Howe Island (; formerly Lord Howe's Island) is an irregularly crescent-shaped volcanic remnant in the Tasman Sea between Australia and New Zealand, part of the Australian state of New South Wales. It lies directly east of mainland P ...

. Lord Howe Rise has a total area of about 1,500,000 square km,

Prior tectonics

The Hunter-Bowen event produced a ~3,000 km long structural

foredeep A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithosphere ...

above a Late

and

Palaeozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and '' ...

margin to the weakly consolidated Australian continental mass which was part of the

Gondwana Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...

Supercontinent at this time; the orogen developed to the east of the Palaeozoic

Lachlan Orogen The Lachlan Fold Belt (LFB) or Lachlan Orogen is a geological subdivision of the east part of Australia. It is a zone of folded and faulted rocks of similar age. It dominates New South Wales and Victoria, also extending into Tasmania, the Australia ...

and the Proterozoic terranes of the Mount Isa Inlier. Before the orogeny the rocks of the coastal area were formed. During Late Carboniferous there was a continental margin with an oceanic trench subducting the ocean floor off the coast. To the east there was a

magmatic arc Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

. This includes Connors Arch, Auburn Arch, Combarrago Volcanics and the Bathurst Batholith. A

forearc basin Forearc is a plate tectonic term referring to a region between an oceanic trench, also known as a subduction zone, and the associated volcanic arc. Forearc regions are present along a convergent margins and eponymously form 'in front of' the vol ...

in between included the Tamworth Belt and Yarrol Belt. Subduction resulted in

blueschist metamorphism Blueschist (), also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures (), approximately corresponding to a depth of . The blue co ...

. At 309 Ma a mid oceanic ridge plate boundary encountered the subduction boundary near

Tin Can Bay Tin Can Bay is a coastal town and locality in the Wide Bay–Burnett region in Queensland, Australia. The locality is split between the Fraser Coast Region (the northern part of the locality) and the Gympie Region (southern part of the locality) ...

south of

Fraser Island Fraser Island (Butchulla: ) is a World Heritage-listed island along the south-eastern coast in the Wide Bay–Burnett region, Queensland, Australia. The island is approximately north of the state capital, Brisbane, and is within the Fraser ...

on the North D'Aguilar block. Another encounter point was near what is now

Coffs Harbour Coffs Harbour is a city on the Mid North Coast of New South Wales, Australia, north of Sydney, and south of Brisbane. It is one of the largest urban centres on the North Coast, with a population of 78,759 as per 2021 census. The Gumbaynggirr ...

. These ridge encounters with the trench stopped the subduction at those points and resulted in

triple junction A triple junction is the point where the boundaries of three tectonic plates meet. At the triple junction each of the three boundaries will be one of three types – a ridge (R), trench (T) or transform fault (F) – and triple junctions can b ...

s moving north and south up the trench, and being replaced by a dextral transform fault. The northernmost triple junction, a ridge-trench-fault junction moved up the Queensland coast at 28 mm per year. As it moved the magmatism inland stopped and it reached Townsville about 290 Ma. The paired triple junction with this, a fault-trench-fault junction, moved south from Brisbane at 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 merger converted the trench to a transform fault and turned off the volcanoes to the west. In between these two ridge trench encounter points, a small triangular shaped plate continued to subduct between Brisbane and Coffs Harbour. More time was available to build up a wide subduction complex.

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 was called Gogango-Baryulgil fault zone. 500 km southerly displacement of the coastal terrane which stretches from Coffs Harbour in the south to Broad Sound near

St Lawrence, Queensland St Lawrence is a rural town and locality in the Isaac Region, Queensland, Australia. In the the locality of St Lawrence had a population of 235 people. Geography St Lawrence is located north of Brisbane and off the Bruce Highway.The town is ...

in the north, 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 01 or '01 may refer to: * The year 2001, or any year ending with 01 * The month of January * 1 (number) Music * '01 (Richard Müller album), 01'' (Richard Müller album), 2001 * 01 (Son of Dave album), ''01'' (Son of Dave album), 2000 * 01 (Urban ...

) 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 be due to far distant continental collisions in the buildup of

Pangea Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...

. Volcanoes appeared in central Queensland forming the Lizzie Creek Volcanics and the Camboon Volcanics. Extension in these formed the Grantleight Trough, an intra-arc rift. The subduction zone was in a line with the north-east coast of Queensland, with a back arc basin formed to the east of the Hunter region, called the Barnard Basin. Deposition continued in basins around 270 Ma into the Permian, but volcanic activity was reduced.

The Sydney-Gunnedah-Bowen Basin

This structural foredeep filled with marine deepwater sediments and later fluviatile sandstones, which during the Permian and Triassic formed vast accumulations of

coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when dea ...

. 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 Flysch () is a sequence of sedimentary rock layers that progress from deep-water and turbidity flow deposits to shallow-water shales and sandstones. It is deposited when a deep basin forms rapidly on the continental side of a mountain building ep ...

. 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

thin-skinned deformation Thin-skinned deformation is a style of deformation in plate tectonics at a convergent boundary which occurs with shallow thrust faults that only involves cover rocks (typically sedimentary rocks), and not deeper basement rocks. The thin-skinned s ...

affected 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 sinistral shear in 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 **

Metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chem ...

of both

greenschist Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically and 2–10 kilobars (). Greenschists commonly have an abundance of green minerals such as chlorite ...

and rare

blueschist Blueschist (), also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures (), approximately corresponding to a depth of . The blue co ...

facies **

thrusting Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that sys ...

** Transtension pull-apart basins such as the ***Esk Trough, a thin pull-apart rift infill

fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluviog ...

-bearing basin ***

Clarence Moreton Basin The Clarence Moreton Basin is a Mesozoic sedimentary basin on the easternmost part of the Australian continent. It is located in the far north east of the state of New South Wales around Lismore and Grafton and in the south east corner of Q ...

, including the

Ipswich Basin Ipswich () is a port town and borough in Suffolk, England, of which it is the county town. The town is located in East Anglia about away from the mouth of the River Orwell and the North Sea. Ipswich is both on the Great Eastern Main Line railw ...

** Transpressional faulting and widespread deformation * Arc

volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called ...

, namely in the

Gympie Gympie ( ) is a city and a Suburbs and localities (Australia), locality in the Gympie Region, Queensland, Australia. In the Wide Bay-Burnett District, Gympie is about north of the state capital, Brisbane. The city lies on the Mary River (Queen ...

province, offshore of the Hunter valley and

Sydney Basin The Sydney Basin is an interim Australian bioregion and is both a structural entity and a depositional area, now preserved on the east coast of New South Wales, Australia and with some of its eastern side now subsided beneath the Tasman Sea. ...

* Back-arc basin formation, **

Gunnedah Basin Gunnedah is a town in north-eastern New South Wales, Australia and is the seat of the Gunnedah Shire local government area. In the the town recorded a population of 9,726. Gunnedah is situated within the Liverpool Plains, a fertile agricultu ...

Sydney basin The Sydney Basin is an interim Australian bioregion and is both a structural entity and a depositional area, now preserved on the east coast of New South Wales, Australia and with some of its eastern side now subsided beneath the Tasman Sea. ...

Bowen Basin The Bowen Basin contains the largest coal reserves in Australia. This major coal-producing region contains one of the world's largest deposits of bituminous coal. The Basin contains much of the known Permian coal resources in Queensland includ ...

Granite Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...

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 Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomi ...

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...

and

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

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 Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when dea ...

formation in the

, Esk Trough,

and

Geochronology

Geochronology Geochronology is the science of determining the age of rocks, fossils, and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, whereas relative geochronology is pr ...

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

References

{{DEFAULTSORT:Hunter-Bowen Orogeny Orogenies of Australia Geology of New South Wales Permian orogenies Triassic orogenies

Prior tectonics

Megafold

The Sydney-Gunnedah-Bowen Basin

Geochronology

See also

References