The Rift That Almost Split New South Wales From Australia

Setting the Stage: Gondwana’s Restless East

In the late Carboniferous, around 320 million years ago, the land that is now eastern Australia lay deep within the supercontinent Gondwana. The climate was harsh and cool, even glacial in places – a far cry from the sunbaked Outback of today. Beneath grey Permian skies, immense tectonic forces were building. Along Gondwana’s eastern margin, an oceanic plate was sinking beneath the continent in a subduction zone, spawning a line of volcanoes and island arcs offshore. These fiery arcs marked the edge of an ancient ocean (the proto-Pacific), and behind them, the continental crust of Gondwana began to stir. The collision and accretion of crustal fragments in the New England Orogeny (circa 300–250 Ma) had left eastern New South Wales compressed and heated. As the subduction continued, something changed – the tectonic regime shifted from compression to extension. The earth under Gondwana started pulling apart at the seams.

It was a perfect recipe for rifting: the crust was hot and weakened by recent arc volcanism, and far to the east, the subducting slab began to roll back (retreat eastward) beneath the ocean. This slab rollback acted like a giant winch tugging on the edge of the continent, creating extensional forces. Gondwana’s crust stretched and thinned in response, foreshadowing a tectonic drama. In this moment of continental restlessness, a rift was about to be born – one that would cut deep into the heart of what is now New South Wales.

This is the story of the rift zone that almost tore New South Wales away from Australia.

The Birth of a Rift Basin (Late Carboniferous–Permian)

In a profound geologic instant (measured in millions of years), the earth’s crust cracked open. Faults propagated for hundreds of kilometers, and blocks of crust sank along great faults, forming a chain of sunken valleys known as half‐grabens. Geologists identify this event as beginning around 305–300 Ma with an initial pulse of rifting. This first stage tore a series of asymmetric basins into Gondwana’s eastern side. Huge slabs of rock slumped downward along east-dipping fault planes, creating a trough that trended roughly north–south. Molten rock welled up through the fractures: dark basalt lava and ash from fissure eruptions spread across the forming valleys. These volcanic outpourings, chemically akin to mid-ocean ridge basalts, were a clear sign that the crust was under intense tension. A volcanic arc still rumbled to the east, but now behind it the continent was rifting – a process geologists refer to as back-arc extension.

By about 285 Ma, rifting leapt further inland. A second phase of faulting jolted regions further west, expanding the rift basin’s width. This later phase created additional half-grabens, though with less volcanism than before – suggesting that while the crust was still pulling apart, the earlier volcanic fury was waning. At the surface, the landscape must have been otherworldly. Imagine vast rift valleys dropping along fault scarps; earthquakes would have rattled the young sedimentary basins, and volcanoes on the horizon would paint the sky with ash. Low areas between uplifted blocks likely became swamps and shallow lakes, even as an inland sea flirted with the edges of the basin. Sediments from both the crumbling highlands and the encroaching sea began to fill the depressions. In some places, glacial meltwater and rivers carried sand and mud; in others, tropical forests of Glossopteris ferns and giant horsetails carpeted coastal swamps. These lush swamp forests would become the stuff of legend – or at least of coal – as layer upon layer of fallen vegetation built up in waterlogged bogs. Deprived of oxygen, the plant matter turned to peat and eventually hardened into thick black coal seams. Indeed, the low sediment input during this rifting allowed peats to accumulate into coal seams tens of meters thick, a carbon-rich bounty that modern Australia still mines from the earth.

The newly formed Sydney–Gunnedah–Bowen Basin was truly colossal. Stretching over 1,200 km from the south coast of New South Wales to central Queensland, it was the most extensive of several rifts to gouge Eastern Gondwana. In essence, a huge gash now cut across what would become the Sydney region up through the Hunter Valley and further north. To the east of this gaping rift lay the volcanic arc and the deep Paleo-pacific ocean; to the west lay the ancient cratonic heart of Australia. The rift basin’s floor sagged under its own weight, and portions likely foundered below sea level. Geological evidence shows the basin filled first with river deltas and coastal plains, then was inundated by shallow seas. Marine shales and deepwater sediments blanketed parts of the basin floor, while nearer to the coasts, broad delta plains accumulated sandstones. As the Permian period progressed, the basin continued to subside gently under its own thermal and tectonic sag. By the early Triassic, a thick sedimentary blanket – up to 5 kilometers in places – had accumulated in this trough. Entombed in those layers were the remains of the Permian world: pebbles from long-eroded mountains, volcanic ash from distant eruptions, and extensive coal beds formed from Glossopteris forests. It seemed as if a new ocean might be born in the rift – a split that would sever New South Wales from the mainland. The stage was set for a continental breakup… but fate had other plans.

The Rift That Failed: A Dramatic Tectonic U-Turn

Around 270 million years ago, the rifting drama took an abrupt twist. The expansive gulf threatening to cleave Gondwana began to close almost as quickly as it had opened. The root cause was the shifting ballet of tectonic plates at Earth’s surface. The subduction zone outboard of the rift changed its character: the oceanic plate’s retreat slowed and then halted. Instead of pulling the continental margin apart, the forces began to push it together. Gondwana’s eastward motion and a possible collision of a microcontinent or arc caused the trench to advance toward the continent, inducing powerful compression on the once-extending crust. In geological terms, the basin experienced inversion – regions that had been subsiding now were squeezed upward. This marked the onset of the Hunter–Bowen Orogeny, a great mountain-building episode that signalled the rift’s demise.

The reversal was cataclysmic. Over the Late Permian and into the Triassic (about 265–230 Ma), immense tectonic forces crumpled and uplifted the sedimentary strata that had accumulated in the rift. Picture the formerly tranquil swamps and shallow seas of the Sydney–Gunnedah–Bowen Basin being buckled like a giant rug on a hardwood floor. Thrust faults ramped westward, stacking slices of the basin’s sedimentary rocks atop one another in a style reminiscent of the Rocky Mountains’ formation. Along what is now the New England region and the east coast of Australia, rocks that had been deposited in quiet marine settings were smashed and deformed by east-to-west compression. Some portions of the basin became a foreland basin – a downwarp formed inland of a rising mountain belt – as weighty thrust sheets to the east caused the crust to flex downward. Thick molasse (continentally derived sediment) and fresh water fluvial sands poured in, replacing the earlier coal swamps and marine deposits with braided river systems. The once gaping rift was effectively clamped shut by Gondwana’s mighty grasp.

By the early Triassic, the continent had won its tug-of-war with the ocean. The would-be oceanic rift failed to penetrate further; New South Wales remained firmly sutured to Australia. The Hunter–Bowen Orogeny left a legacy of mountains and metamorphism along 2,500 km of Australia’s eastern margin. Though time has worn them down, back then these mountains may have been formidable, shedding sediments that would later become part of younger basins (like the Surat Basin to the west). The end of this orogeny around 230 Ma effectively sealed the ancient fracture. What had nearly become a new ocean basin was now a giant, fossilized rift — a failed rift encased in layers of rock and hemmed in by fold belts. The dramatic tectonic near-miss was over, and Gondwana sailed on, intact, into the rest of the Mesozoic era.

Legacy in the Landscape: An Ancient Scar Remembered

Though 250 million years have passed, the story of this failed rift is written into Australia’s geology in grand and subtle ways. In eastern New South Wales and Queensland, the scar of the Sydney–Gunnedah–Bowen rift remains visible to those who know where to look. It lies in the arc of sedimentary basins and ranges that trace the rift’s path, in the coal mines and escarpments that have shaped industry and scenery. Indeed, the very placement of certain rivers, ridges, and cliffs in NSW owes itself to this deep geologic structure. The basin system today forms a geological divide — separating older, hardened Paleozoic rocks of the inland Lachlan Fold Belt from the likewise ancient New England rocks nearer the coast. This linear depression, now mostly filled by Permian and Triassic strata, delineates where the continent was once stretched thin.

One of the most striking remnants is the Illawarra Escarpment south of Sydney – a towering line of cliffs that overlooks the coastal plain. Here, nearly flat-lying sandstones and coal seams form sheer walls up to 500 m high. These rocks are part of the Permian Illawarra Coal Measures and overlying Triassic sandstones, deposited in the ancient rift basin. As the Pacific waves later gnawed at Australia’s edge (during the breakup of Gondwana and opening of the Tasman Sea in the Cretaceous), the coastal side of the old rift was eroded into a dramatic escarpment. When you stand on those cliffs today, you are essentially standing on the eroded edge of the failed rift – looking out toward the sea that much later finally succeeded in splitting Australia from Zealandia. Similarly, the Blue Mountains west of Sydney form another grand escarpment: their flat-lying sandstone plateau ends abruptly in cliffs that drop into the valleys of the Hawkesbury–Nepean. Those sandstones were once sediment at the northwestern edge of the rift basin. The uplift and gentle westward tilt of the basin during and after the orogeny set the stage for such escarpments to develop through erosion. In essence, the basin’s margins became nature’s canvas for cliffs and waterfalls.

Reflections on a Near-Miss in Earth’s History

In the grand history of continents, rifts come and go like fleeting breaths – but few leave such an indelible mark as the one that attempted to split New South Wales from Australia. The Sydney–Gunnedah–Bowen Basin is a story of a near-miss: had the rifting gone just a bit further, the map of Gondwana (and today’s Australia) would look very different. Instead of one land, there might have been two – with an ancient sea (a precursor to today’s Tasman) lapping at the shores of a separated New England microcontinent. But the Earth’s dynamic plates had other ideas, and the rift was clamped shut, frozen in time.

Here's the video we made on this on the OzGeology YouTube channel: