Hunter–Bowen orogeny

The Hunter–Bowen orogeny was a big event that changed the shape of the land in ancient Australia. It happened during the Permian and Triassic periods, affecting about 2,500 kilometers of Australia's continental margin.

This event happened in two main steps. First, rocks from the Permian time were added to the land in places like the Hunter region and parts of what is now New South Wales. Later, from the late Permian into the Triassic, more changes happened as the Earth's plates moved and pushed together.

Today, the Hunter–Bowen orogeny is seen in a place called the New England Fold Belt. This area includes many different types of rocks that were changed by heat and pressure, along with some older rocks pushed up into the crust. Nearby, there are also rock layers formed from ancient sediments.

Although the Great Dividing Range near Sydney looks like it might be connected to this old event, it was mostly formed much later by volcanic activity and uplift of the Earth's crust. Some pieces of the Hunter–Bowen rocks are now far out in the ocean, part of the Indo-Australian plate, including places like the Lord Howe Rise and Lord Howe Island.

Prior tectonics

The Hunter–Bowen orogeny created a long line of rock formations above an old edge of the Earth’s crust. At that time, this area was part of a huge landmass called Gondwana. Before this event, the rocks along the coast were already forming. There was a deep ocean plate moving under the continent, creating lines of volcanic activity. These old volcanic areas are now known as the Connors Arch, Auburn Arch, Combarrago Volcanics, and the Bathurst Batholith.

When an underwater mountain range met the edge where the ocean plate was moving down, it changed how the Earth’s plates moved. This created special points where three pieces of the Earth’s crust met. These points moved along the coast, changing the way the plates interacted and stopping some of the volcanic activity. Over time, this led to the building up of thick layers of rock in certain areas.

Megafold

The Texas-Coffs Harbour megafold happened because of movement along a big underground break in eastern Queensland called the Gogango-Baryulgil fault zone. A long stretch of land from Coffs Harbour to Broad Sound near St Lawrence, Queensland moved 500 km south, forming a big fold inland from Coffs Harbour. This happened around 290 million years ago in the early Permian and lasted for about 10 to 20 million years.

Some areas near this land stretched out, creating deep basins. One of these basins is in the Texas region. Around 280 million years ago, the movement of ocean plates changed, and volcanic activity happened in central Queensland, forming the Lizzie Creek and Camboon Volcanics. A small rift called the Grantleigh Trough formed, and a basin called the Barnard Basin appeared east of the Hunter region. Deposition kept happening in these basins until about 270 million years ago.

Sydney-Gunnedah-Bowen Basin

This area filled up with deep water sediments and later sandstones, which during the Permian and Triassic periods created large amounts of coal from trapped plant material. The Sydney and Bowen Basins were next to an underwater island chain that grew as the Earth's plates moved.

The land pushed westward, changing the rocks and creating new types of granite. The ground moved in many ways, forming valleys and basins such as the Esk Trough and Clarence Moreton Basin. Volcanoes erupted, adding layers of rock. Valuable minerals like gold and tin formed, and more coal built up in several basins.