Phanerozoic Orogens And Basins

The main area of Phanerozoic deformation and activity in Australia is along the east coast, in the Lachlan fold belt and Tasman orogen. The Lachlan fold belt contains Cambrian ophiolitic sequences that were thrust on top of the Australian continent in the Ordovician in the Lachlan Orogeny. This orogeny was associated with many classical Alpine-type events including the formation of flysch and molasse belts, strongly deformed zones with serpentinitic and ophiolitic mélange, and affected a large part of the New South Wales region of Australia. Tectonic activity continued in this belt through the silurian with the formation of volcanic arcs in the New England orogen and the intrusion of belts of granitic batholiths. The high topography formed in the east during the Early Paleozoic was significantly eroded in the Devonian, with thick clastic sequences reaching into the continental interior.

In the Carboniferous eastern Australia collided with parts of South America and New Zealand as part of the amalgamation of the Gondwanan supercontinent; this collision formed high, Tibetan-style mountain ranges on the east coast. These ranges have since been nearly completely eroded, and just their deeper-level roots remain as testimony to this event.

The Permian-Triassic saw the establishment of major subduction zones along the east coast in the Hunter-Bowen Orogeny, which was initiated as an arc colliding with Australia and then conversion of this margin to a convergent tectonic setting, with related deformation continuing until the Middle Tri-assic at 230-225 million years ago. A major glaciation event in the Permian caused accelerated erosion of these mountain ranges, particularly in central and western Australia. Glacial tillite deposits from this event cover large parts of central Australia.

The environment of the Jurassic changed such that most of western Australia experienced tropical weathering in a savanna to jungle setting, and several offshore oil basins formed including the Gippsland, Bass, and Otway basins in Victoria. Coal-bearing strata were laid down across northern Australia, while passive margin sedimentation continued in the Perth basin in the west.

Antarctica rifted from Australia in the Jurassic. Rift-sedimentation and subsidence continued in the Cretaceous and developed into seafloor spreading and the separation of Tasmania from the Australian mainland. These rifted to passive margins, then developed extensive coral reefs in the northeast, and rare intraplate volcanic centers formed through the Tertiary.

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