Statewide tectonics

The break-up of Rodinia is recorded in the rocks that now form the Late Proterozoic to Cambrian (1000 to 490 million years old) Delamerian Orogen. This orogenic belt mainly occupies the eastern half of South Australia where the Adelaide Rift Complex contains mixtures of sandstone, conglomerate, shale, limestone, glacial deposits and local volcanic rocks that record rift and sag phases of crustal extension.

The eastern part of the Delamerian Orogen extends into far western New South Wales where rocks north, south and east of Broken Hill form part of the Adelaide Rift Complex (the Koonenberry Belt and the Loch Lilly-Kars Belt respectively). These rocks were deposited in shallow water while to the east there was deeper water in which felsic and mafic volcanic lavas accumulated at the edge of the Australian plate. Similar rocks occur to the south in western Victoria and are also found in parts of Antarctica, which at that time lay south of Kangaroo Island.

The break-up of Rodinia took around 200 million years (from 800 to 600 million years ago). Stretched crust thinned to form rift basins, some with volcanics and glacial deposits in them, and thinned even more to give way to oceanic crust, resulting in sea-floor spreading.

Sea floor spreading changed to subduction about 530 million years ago and a convergent plate boundary formed with development of a west-dipping subduction zone and formation of Cambrian island arc volcanoes and associated rocks. Although these are best represented in western Victoria, they are also inferred from aeromagnetic data and drilling to underlie the Bancannia Trough north-east of Broken Hill and the Murray Basin south-west of Broken Hill.

Rocks of the Delamerian Orogen were deformed (the mountain-building phase of the Orogen took place) near the end of Cambrian around 500 million years ago when the volcanic arc collided with the older Broken Hill rocks forming the eastern margin of the Australian continent. This deformation caused the shovelling together of rocks.

Mountains formed by the Delamerian deformation event extended from western New South Wales westwards into South Australia and then southwards into Antarctica. Their erosion shed vast amounts of mud and quartz-rich sand into ocean basins to the east where they covered Cambrian basalts and other rocks that formed the oceanic igneous crust. These sediments are now preserved as the widespread Early to Middle Ordovician (490-460 million year old) turbidites that occupy much of the Lachlan Orogen.

Andesite and basalt (volcanic rocks) are part of the Ordovician story too. Destruction of the Cambrian subduction zone caused a new subduction zone to form hundreds of kilometres to the east and a new island arc system (the Macquarie arc), best seen in central and southern New South Wales, developed above a west-dipping subduction zone. Several phases of volcanism are present in the arc from earliest to latest Ordovician (490-440 million years ago). Breaks or quiet periods in volcanism are marked by the formation of tropical limestone reefs. Arc volcanism died out at the end of the Ordovician with the intrusion of monzonitic plutonic rocks ('granodiorite') before plate tectonic movements in the Early Silurian (around 435 million years ago) caused the arc to collide with the back arc basin turbidites, resulting in the major Benambran deformation that caused the folding and faulting of older rocks and generation of new granite magmas.

The Macquarie arc is a world-class porphyry copper-gold province. It hosts gold-copper deposits at Cadia, Northparkes, Lake Cowal, Browns Creek and major mineral accumulations at Cargo and Copper Hill. There is considerable potential for more discoveries.

The Benambran deformation ended the first stage in the development of the Lachlan Orogen.

Because the Australian and proto-Pacific plates were still converging, a new subduction zone was formed several hundred kilometres to the east (off the present coastline) after the Benambran deformation event. For the remainder of its history, the Lachlan Orogen was in a back arc position. (Note that a different story took place in the New England area.)

Back arc areas commonly undergo extension, and from the Middle Silurian (425 million years ago) to middle Devonian (385 million years ago), the Lachlan Orogen was largely in extension except for small periods of local convergence and deformation.

Rifling of the old crust led to formation of sedimentary basins and mixed sedimentary plus volcanic filled basins, as well as the emplacement of large amounts of granite, some hosting tin and gold deposits. The Macquarie arc was rifled into several belts, separated by rift-sag basins. These rifts were closed by the mid-Devonian deformation (the Tabberabberan) which marks the end of the second stage in development of the Lachlan Orogen.

Mid Silurian to Mid Devonian extension in the Lachlan Orogen also coincides with two major metallogenic episodes. Mixed sedimentary-volcanic filled basins formed a major volcanic province with numerous volcanic hosted metal sulfide deposits. Deposits with strong structural controls occur at Captains Flat, Woodlawn, and nearby Currawang, Mt Bulga and Lewis Ponds. Other deposits occur in the Hill End, Mt Hope and Rast troughs. Elsewhere, sediment-rich basins, such as the Cobar Basin in the central part of the state, also contain several large, structurally controlled deposits.

The Tabberabberan deformation was followed by the localised development of rift basins with volcanic fill. However, the remainder of the Devonian period was mainly typified by the deposition of fluviatile (river and floodplain) sedimentation – mapped as the Western Devonian Basins in the NSW Seamless Geology. The enigmatic but widespread early Carboniferous Kanimblan deformation event, possibly related to events in the New England Orogen to the north, followed, and was itself followed by emplacement of the post-tectonic granite plutons of the Bathurst Batholith.

The Thomson Orogen lies north of the Lachlan Orogen and extends north into central Queensland. In NSW the orogen has a broadly east-west strike and although most of it is covered by younger sedimentary sequences approximately 30% of units making up the orogen are less than 300 metres from the surface. The Thomson Orogen is currently considered a NSW new frontier.

Recent work on the Thomson Orogen suggests that it may have had a similar tectonic history to that of the Lachlan Orogen to its south, as it also contains Ordovician oceanic island basalts and turbidites, Mid Silurian to Mid Devonian rock packages and possible Late Devonian basins. Recent drilling has found calc-alkaline andesitic rocks in the Bourke region with a subduction-related geochemical signature. Airborne magnetic data and ground acquired gravity data are suggestive of an arcuate amalgamation of these rocks divided into smaller geophysically distinct domains interpreted as deformed volcanic belts which are variably crosscut by elliptical domains interpreted as younger intrusive complexes. Seismic reflection data suggests that the faulted contact of the Lachlan and Thomson orogens extends to, and significantly offsets the Mohorovicic discontinuity. The seismic data shows a north-dipping fault with the Thomson Orogen to the north overriding the Lachlan Orogen to the south. The fault also truncates a Late Devonian sequence hence it can be concluded that the fault was active since Late Devonian times.

The multi-phase history of the New England Orogen began with the fragmentary Cambrian to Ordovician convergent margin volcanic and volcaniclastic rocks as well as disrupted Cambrian ophiolites and Ordovician blueschists. The second, Silurian to Early Devonian phase was marked by plate convergence between the Australian plate with the proto-Pacific plate, causing the mix of intra-oceanic arc and accretionary prism rocks (a block of rocks trapped between the plates, in the subduction zone).

More is known about the third phase, when the New England Orogen was the site of a Late Devonian (370-355 million year old) continental margin arc of mafic character, above a west dipping subduction zone, passing eastwards into a forearc basin and accretionary prism. The arc continued into the Carboniferous (to about 325 million years old), but changed in composition to produce more felsic lava deposits and volcaniclastic rocks.

Multiple deformation, metamorphism, and emplacement of granites (some antimony-bearing) occurred in the accretionary complex rocks in the Late Carboniferous to Early Permian, and the forearc belt started to be deformed into a fold and thrust belt. The Peel Fault System, between the subduction complex rocks and the forearc basin, became a major fault containing Cambrian ultramafic rocks, with scattered occurrences of asbestos, chromite and gold.

In the Early Permian (around 300 million years ago), convergence along this plate margin changed into extension, coupled with strike-slip faulting. This led to the formation of small rift basins (some with felsic volcanic-hosted mineralisation, such as at Halls Peak), and formation of a major back arc rift basin that formed the early stage of the Sydney and Gunnedah basins. Renewed plate convergence in the Late Permian to Triassic led to volcanism, formation of epithermal gold and base metal deposits and emplacement of more granite.

The Sydney and Gunnedah basins lie between the Lachlan and New England Orogens, forming part of the Permo-Triassic Basin province. Starting as back-arc rifts in the earliest Permian, they developed into foreland basins. Most of their fill was generated by uplift in the New England Orogen and this alternated with lesser fill coming from the Lachlan Orogen to the (south)west. Westward migrating deformation persisting until the mid-Triassic gradually converted these basins into west-verging foreland fold and thrust belts. Coal deposition was associated with fluviatile events and led to the formation of thick Permian seams of bituminous black coal.

Permian coal deposits also occur in the southern part of the state, in successor basins developed above the Delamerian and Lachlan Orogens, and preserved as infra basins below the younger Cenozoic Murray Basin.

The Mesozoic Era lasted from 250 to 65 million years and was a period of worldwide rifting and basin formation as the supercontinent Pangaea broke up.

In the north-eastern corner of New South Wales, the Clarence-Moreton Basin developed in several stages - from Triassic oblique extension - to a Jurassic sag basin - into an Early Cretaceous half rift. The movements resulting from extension associated with early stages of opening of the Tasman Sea. Basin inversion (uplift and closure) occurred in the mid-Cretaceous. Triassic and Jurassic coal, oil and gas shows are present.

Further west, Jurassic and Cretaceous rocks were deposited in the Great Australian Basin which covers older rocks in the northern inland of New South Wales, and adjoining areas of Queensland and South Australia. Two subdivisions of the basin, the Eromanga and the Surat basins, lie partly in New South Wales. Both range in age from Early Jurassic to (Late) Cretaceous and contain low grade coal deposits and rich oil and gas fields in the adjacent states. Cenozoic weathering led to formation of valuable deposits of opal.

Collectively these basins form the Great Australian Bain in the NSW Seamless Geology. Magmatic activity associated with the opening of the basins and related to other tectonic processes are mapped as the Permian-Mesozoic Igneous Province.