Lachlan Fold Belt - History

History

The basement of the belt is Cambrian oceanic crust which was formed in a back arc basin or a fore arc basin. An ancient shoreline of Australia, called the cratonic margin existed off the east coast of the Delamerian Orogen in western New South Wales, Western Victoria and Western Tasmania. In the early Ordovician there was a shallow marine shelf called the Gnalta Shelf, over the top of the Koonenberry Belt near Broken Hill. At the same time there were marine conditions in Amadeus Basin where Horn Valley Siltstone was formed, and Georgina Basin where the Coolibah Formation deposited. The Larapintine Sea formed a shallow marine connection through central Australia to the Canning Basin in Western Australia. Sediments formed on the continental shelf of the continent from this time appear in western Tasmania and north west New South Wales. A subduction zone consumed the Pacific crust below the Delamerian coast, but this subduction moved 1000 km oceanwards. An island arc was formed on the northern end of the 900 km long trench. This was the Macquarie Volcanic Arc.

Between the continent and the island arc or trench, deep water sediments in the form of turbidites appeared in the Central Lachlan Orogen. These were derived from the western Delamerian Orogen and from the south west Ross Orogen, which is now left behind in Antarctica.

The Ordovician volcanoes of the arc are now found around Parkes, Wellington, Molong and east of Condobolin, Cowra and Boorowa. The east Lachlan Orogen containing Adaminaby Group Turbidites is now to the east and south of the Macquarie Arc. All its boundaries with the Macquarie Arc are faults, indicating that this is a separate terrane, also known as the Adaminaby Superterrane. In the Late Ordovician the turbidites were overlaid by a black shale. The back arc region was extended at this time.

Between 447 to 443 million years ago in the Bolindian to Llandovery, the arc collided with the back arc, ending the Benambran Cycle. The turbidites were deformed, biotite formed, and the arc was thrust over or under the turbidites.

In the Middle Llandovery, the Pacific plate boundary moved a few hundred km to the east. A new subduction zone dipping westwards lasted from the end of the Silurian into the Late Devonian. The whole of the Lachlan Fold Belt became a back arc area with a new volcanic arc formed to the east in what is now the New England Orogen. Extension stretched the LFB forming rifts, and shelves, along with intrusion of granites and volcanism. This was the Tabberabberan Orogeny.

Deformation happened in the west 450 to 395 Mya and in the east 400 to 380 Mya. Extensional basins occur in the central and east parts of the fold belt. Oceanic subduction (or underthrusting) is evidenced for the western and central parts by slivers of ophiolite and blueschist metamorphism. These are known as Coolac serpentinite, and Honeysuckle Beds east of Tumut; The Kiandra Beds north of Batlow and the Tumut Pond Serpentinite Belt on the west side of Talbingo Dam.

The Narooma Terrane migrated 2500 km westwards on the moving Pacific plate and became attached to the Adaminaby Superterrane in Silurian times. Conodont fossils in the Narooma Chert prove the age of the terrane to be from late Cambrian to middle Ordovician. The Narooma Terrane exposure is between Narooma and Eurobodalla and also between Burewarra Point and Durras around Batemans Bay on the south coast of New South Wales.

The western parts under New South Wales and Queensland are mostly heavily weathered and or covered in younger sediments of the Great Artesian Basin or Great Australian Basin and Murray-Darling Basin. The underlying structure can still be explored through magnetic, gravity and seismic geophysical measurements.

About 84 million years ago The Tasman Sea started to form by seafloor spreading. This split off a segment of the coast to form the Lord Howe Rise, part of Zealandia. Around then, Bass Strait was extended moving Tasmania away from the rest of Australian Mainland.