Australasian and Oceanian geology

The continental mass of Australasia is composed of five geologically distinct units; the oldest of these is the Precambrian craton of central and western Australia. The southeastern margin of the craton adjoins the younger Adelaidean mobile belt, of late Precambrian and early Palaeozoic age. The eastern margin of the Australian land mass is occupied by the Tasman mobile belt, a long-lived belt active from Palaeozoic to Cretaceous times. Within the craton are a number of large Phanerozoic sedimentary basins, such as the Great Artesian Basin. The youngest province contains the still-active mobile belts of the Indonesian Islands and New Zealand.

The Precambrian Craton The ancient shield area of Western Australia has a cover of younger rocks over much of its area; Precambrian rocks are only exposed in a number of large isolated blocks. The Yilgam and Pilbara blocks are areas of middle Precambrian rocks, composed of a highly metamorphosed granitic basement complex containing greenstone belts. The greenstone belts contain metamorphosed volcanic and sedimentary rocks, and the whole complex was intruded by a swarm of basic dykes about 2,400 million years ago. The greenstone belts also contain layered basic intrusions associated with the volcanics, and these contain important nickel ores. The gold ores of this area are also found associated with mineralization of the basic rocks in the greenstone belts.

The Musgrave and Arunta blocks in central Australia are composed of high-grade metamorphic Precambrian complexes. The Gawler block in South Australia is composed of granitic basement with remnants of a greenstone-belt succession, containing high-grade iron ores in the Middleback Ranges.

In late Precambrian times a number of mobile belts became established; a new sedimentary basin was developed in the Hamersley belt, and cover sediments were laid down on the established craton.

The mobile belts are the Halls Creek belt, East Kimberleys, and the West Kimberleys in Western Australia, the Pine Creek belt in the Northern Territories, and the Willyama Complex in South Australia. The latter belt is of economic importance as it contains the Broken Hill mining district, well known for its silver-lead-zinc mineralization.

The Hamersley belt of Western Australia is a large area containing a remarkably well-preserved and thick accumulation of late Precambrian sediments. The Mount Bruce supergroup at the base is a largely unmodified sequence, 10 km thick, containing banded iron formations. These were gently folded and then a further thick sequence of late Precambrian sandstones, lavas, and dolomites were deposited. At the same time thick sedimentary successions were laid down on the southwestern side of the Gulf of Carpenteria, the Kimberley basin, and on the Gawler block of South Australia.

The Adelaidean mobile belt This belt stretches over 1,100 km following the southeastern margin of the Precambrian craton, and marking the site of an extensive late Precambrian and early Palaeozoic mobile belt. It contains a 16-km thick geosynclinal sequence of mainly shallow-water sediments; there are well-preserved, thick glacial deposits, and these are also found extensively outside the geosynclinal area resting directly on the craton. In the uppermost Precambrian rocks is an important group of fossils, the Ediacara Fauna, a remarkably well-preserved collection of soft-bodied organisms. The youngest sediments in the belt are of Cambrian age, and orogeny began in the Cambrian, resulting in low-grade regional metamorphism and folding of the rocks.

The Tasman mobile belt This 3,000-km belt occupies the whole east coast of Australia. It initiated in the Lower Palaeozoic and continued for some 400 million years. Thick sequences of sediments and volcanics accumulated in a number of north-south troughs, and suffered a number of orogenic disturbances culminating in major orogeny in Silurian times. Sedimentation continued in some areas until Jurassic times. The belt is notable for the amount and variety of the igneous rocks produced over its long history. Large volumes of granite and acid volcanics were produced over a period of some 200 million years, from Devonian to Permian times. Mineralization is associated with some of the granites and volcanics; gold, lead-zinc, copper, and tin are all important economic deposits. Late movements were associated with widespread post-orogenic volcanism in Tertiary times, continuing into the Plio-Pleistocene.

The intra-cratonic basins The Carnarvon, Canning, and Bonaparte Gulf basins near the west and northwest borders of the craton came into existence in mid-Palaeozoic times, and contain marine sandstones, shales, and limestones mainly of Devonian and Lower Carboniferous age. These were followed by glacial deposits in Permian times, evidence of the widespread Permian glaciation of Gondwanaland, of which Australia was still a part. Many of these glacial deposits spread outside the basins and onto the old craton. These were followed by thick Jurassic and Cretaceous sequences. The Great Artesian Basin came into existence in Jurassic times, and contains a thin continental succession of Mesozoic rocks.

The Indonesian island arcs The islands of Southeast Asia form a number of seismically and volcanically active island arcs. This area is one where three crustal plates meet, with complex geological results. The area has been active since the late Palaeozoic, and activity continues to the present day.

The New Zealand island arc system The earliest-formed rocks in New Zealand are those of the Palaeozoic metamorphic complex in western New Zealand. These consist of metamorphosed Palaeozoic volcanics and greywackes. Much of the rest of the country is occupied by the New Zealand geosyncline, which began forming in Permian times and contains thick sediments of Permian to Cretaceous age. These were deformed during the Rangitata orogeny in the Cretaceous, and suffered low-grade regional metamorphism. The Alpine fault of New Zealand is an important fault which runs northeast through the geosyncline, and has been active since late Mesozoic times up to the present day.

The present land mass of New Zealand came into being with the uplift of new mountain ranges during the Rangitata orogeny. Volcanic activity continued through the Tertiary along the east side of South Island. In North Island acid volcanics erupted in Neogene and recent times along the still active central volcanic belt.