Australia: The Land Where Time Began
Australia Separates from Antarctica
The Rifting Begins
The Jurassic was the period when the process of separation of Australia by passive margin rifting began, eventually giving rise to the isolated continent of today. This process gave rise to many of the geological features and landforms of the continent.
At the start of the formation of Gondwana Australia collided with Antarctica and no other major landmasses followed it, the other collisions occurred on other edges of Antarctica. So Australia spent its whole time with Gondwana on the outer edge of the supercontinent.
Early in the Jurassic, a wide belt of continental crust was uplifted along the east coast. Comprising this raised crust are the now mostly submerged Lord Howe Rise & Norfolk Rises, the submerged Queensland Plateau, the islands of New Caledonia and New Zealand and the submerged Campbell Plateau to the south of New Zealand. New Zealand is actually more like a sub-continent, much of its area being submerged.
It was previously believed that the pre-sea floor spreading continents could be fitted together at the edges of their continental shelves. It has now been found that stretching of the continental crust preceded the start of rifting that separated the land masses. Based on this improved knowledge of how the landmasses fitted together in Gondwana, it places Australia adjacent to the present Wilkes Land of Antarctica. India, a much larger landmass than present-day India, including the crust folded up in the Himalayas and subducted under them, was connected to Antarctica adjacent to the present West Australian coast.
Beginning 160 million years ago stretching of the continental crust began. The first upwelling of sea floor crust, basalt, is estimated to have occurred when the rift valley was about 600 km wide and the inner 360 km of crust had stretched and thinned. Because of the thinning of the crust it becomes lighter and isostatic adjustment caused the thinned crust to rise, allowing the underlying hot lithosphere to rise closer to the surface. When the thickness of the crust was reduced from about 40 km to 10 km, the pressure changes caused the upper lithosphere to become molten and the magma was forced up at the thinnest point in the stretched crust. The continued emergence of basalt pushed the separated sections of crust apart, as sea floor continues to be added at mid-ocean ridges.
The first part of the continent to begin separation at this time was the West Australian end. About 20 % of the continent is thought to have separated during the first phase. .
During this first phase of rifting, from 160 to 132 million years ago, Middle Jurassic to Early Cretaceous, a small fragment of crust, Argoland, moved away from a section of the northwest coast of Western Australia occupied by the Canning Basin, by active seafloor spreading, allowing the sea to flood the basin. The seafloor that resulted from this spreading is known as the Argo Abyssal Plain. Plant material was deposited in the troughs associated with the rifting and was the source of the Northwest Shelf's oil and gas deposits.
Early in the Cretaceous, India rotated away from Australia, the sea entered the proto-Indian Ocean rift. At this time the Darling Scarp, the edge of the Yilgarn Block, becomes a prominent feature of Western Australia. This is partly because there was some crustal uplift, but also because the crust seaward of the Scarp was warped down.
By about 150 Ma the coastline of Antarctica was close to the South Pole. At this time the western end of Tethys was connected to the proto-Pacific Ocean. From the northeast margin of Gondwana slivers of continental crust in the area of Australia's Northwest Shelf, and the New Guinea region, then the eastern tip of Gondwana, are believed to have been rifted to the northwest, beginning in about the Permian. These fragments are thought to possibly be in South Tibet and a number of places in Southeast Asia. There is very little evidence known of their journey away from Australia across Tethys. (Hill, 1994). Movement of these fragments has been suggested to have occurred later rather than earlier, maps in Hill (1994) show them as possible islands, and it has been assumed that most of them started rifting about 167 Ma, after the Callovian seafloor spreading event along that part of the coast of Gondwana. It is believed the fragments attached to the southeastern section of what was then the Asian continent to form Sundaland, which was a promontory. It is still in the equatorial zone. Possible 'stepping-stone' links connecting the Asian mainland with the Australian section of Gondwana are indicated on the maps in Hill (1994) as a group of hypothetical islands. The palaeogeography of the area is believed to have probably changed dramatically since then.
At about 150 Ma a strip of terrain separated the present coast of eastern Australia from the proto-Pacific Ocean. This terrain is now marked by the Lord Howe Rise, Queensland Plateau and New Caledonia.
In the Early to Mid Jurassic the breakup of Gondwana was preceded by a major thermal event that produced basaltic magmatic activity over a wide area, comprised of the dolerites of Tasmania and southeast Australia, Karoo dolerites in South Africa and in Antarctica, the Ferrar Super Group, Dufek Intrusion. Movement between west Gondwana (Africa-South America) and east Gondwana (Madagascar, Greater India and Australia) had begun by 150 Ma. Strike-slip movement dominated the breakup, which formed narrow basins, and it is believed there may have been land connecting the 2 major fragments periodically. According to Wilcox & Stagg (1990), it was about this time that rifting along the southern margin of Australia began (Hill, 1994).
By 140 Ma much of the gap between Africa and the the block composed of Antarctica, Madagascar, Greater India and Australia was covered by a narrow expanse of ocean. The fragments of crust that had separated from the northeastern edge of Gondwana had moved towards the Equator. Rifts continued to form along the western and southern margins of Australia, separating Australia from Antarctica and Greater India. Accumulation of sediments in East Antarctica is mostly recycled palynomorphs (Truswell, 1983). Along the southern margin of Australia, as rifting progressed, there was initial extension, northwest to southeast, of about 300 km which was accomplished by 120 Ma. New Zealand was part of the landmass along the margins of East Antarctica and southeast Australia. The Rangitata Orogeny is thought to have caused this landmass to reach its greatest extent in the Early Cretaceous (Stevens, 1989), in Hill, 1994),
By 132 million years ago the position of the South Pole had shifted, bringing Australia's most southerly point at the time, somewhere near the Queensland-New South Wales border to about 80o South. By 120 million years ago the South Pole was close to Tasmania.
In the second phase of separation, 132-96 million years ago, the remainder of Australia separated. During this phase, India started rotating northwestward from Australia. As a result of this the Perth Basin subsided allowing the sea to flow in. Australia was joined to Antarctica along the eastern edge of that continent. What is now Australia's east coast was then it's southern coast.
In the first phase of rifting the West Australian end of the rift had opened up as far as the Great Australian Bight. In the second phase rifting shifted direction. It was now the eastern Australian end that opened forming the Otway Basin and Torquay Basin. About 15 million years later rifting of an arm of the Tasman Sea formed the Gippsland Basin.
During the second phase of separation Australia rotated to the northwest, so that the southern (now eastern) end of the rift opened. In the second phase of rifting, 132-96 million years ago, the remainder of the continent separated. This rotation squeezed the West Australian section, forcing thinner basin crust, with overlying sediments, against the solid Yilgarn Block. This resulted in the heaving up of the Stirling Ranges.
By 130 Ma, the last 3 continents had begun to separate. There were still no significant water gaps between them (Powell et al., 1988), and Australia and Antarctica were moving apart at a rate of only a few mm per year. The separation between Australia and Antarctica had begun about 132 Ma at the southwest margin of the Exmouth Plateau, progressing to the south then to the east along Australia's southern margin. At about this time the South Atlantic had begun to open, opening from the south to the north (Nurnberg & Muller, 1991).
At this time of great tectonic activity that was tearing apart the previously stable landmasses, as with Gondwana, the sea level was rising. Added to the easier access to the interconnected basins that characterised much of Australia from down warping and rifting, this rising sea level resulted in large areas of the continent being swamped. In the modern north of Australia the sea entered the Carpentaria Basin, starting a process of inundation that saw a vast area of the interconnected inland basins being flooded. This epicontinental sea, the Eromanga Sea, reached as far inland as the middle of the Eromanga Basin. By its peak, between 115 and 110 million years ago all the inland basins were covered by a single, huge sea. Australia then consisted of 4 islands. The part of the sea that had entered the rift forming between Antarctica and Australia was connected to the Eromanga Sea by its extension into the Eucla Basin.
By 120 Ma, there were wide expanses of water on the African east coast and between India and Antarctica and Western Australia, Australia still being attached to Antarctica. As the continental extension between Antarctica and Australia continued slowly, the direction of the extension changed from NW-SE to NNE-SSW. In the east movement of 120 km resulted in the formation of the Gippsland and Bass Basins (Wilcox & Stagg, 1990) (in Hill, 1994).
At about 110 Ma large parts of the Australian continent were submerged beneath a marine transgression, the Eromanga Sea, that peaked in the Aptian, about 116-113 Ma. Rifting along the southeastern ('Australian') edge of Gondwana, leading to the opening of the Tasman Sea (Stevens, 1989), as well as the western margin of New Zealand. The Antarctic Peninsula and adjacent areas of West Antarctica were affected by rifting and crustal blocks rotating locally. Though it continued until about 100 Ma it didn't have a significant effect on the overall geography (Storey et al., 1988). At this time the widening gap as Australia rifted from Antarctica allowed the proto-Indian Ocean to enter at the western end to begin the formation of the Southern Ocean (Hill, 1994).
After 110 Ma, the Eromanga Sea gradually receded until the continent was once again above water by the end of the Cretaceous. There is some evidence, though not conclusive, of winter ice on the Eromanga Sea during the Early Cretaceous. Drop stones, indicative of ice, have been found in fine-grained deposits in eastern parts of the Northern Territory and northwestern Queensland. These fine-grained sediments are of marine origin, deposited on a shallow marine margin of the Carpentaria Basin.
Marine sediments were deposited under the Eromanga Sea, and fresh water sediments following the return of fresh water drainage after the retreat of the Eromanga Sea, formed capping rocks above the Jurassic sandstones of the GAB.
After 160 Ma, when the crustal stretching began, and 90 Ma, when ocean floor spreading began, Tasmania moved 130 km. Tasmania was on a separate crustal plate, moving independently of the other 2 continents. The South Tasman Rise behaved like a 4th plate. When Australia began drifting, Tasmania followed.
Phase 3 - Sea Floor Spreading
By 100 Ma the sea had submerged some rift zones around New Zealand, as a result of subsidence and erosion lowering the surrounding landmasses, but in the south there was still a land connection to Antarctica (Stevens, 1989). It is thought the Eastern Highlands of Australia may have begun to be uplifted about this time, associated with the opening of the Tasman Sea (Wellman, 1987).
Before 96 Ma, India had been moving at a slow pace as it moved northwest. About 96 mya, the spreading ridges that had been pushing India's plate stopped functioning and the abandoned ridges are now the Lost Dutchman and Dirk Hartog submarine ridges. Prior to 96 mya India had been moving at only a few mm/year. After 96 Ma it picked up pace to 15-20 cm/year, and changed direction, it now headed north. About 55- 50 mya it collided with Asia. India stopped moving away from Australia at 45 Ma as the spreading ridge in the Wharton Undersea Basin powering its movement became inactive.
When India started its faster movement, sea floor spreading began in the rift between Australia and Antarctica, but moved very slowly while India was moving towards Asia. Throughout the period from 96 to 45 mya the rift grew at only 4 mm/year, Australia travelling only about 500 km. After 45 mya it moved at about 6-7 cm/year until the present.
Sea floor spreading, as opposed to stretching of the crust, began in the rift about 96 mya but the Southern Ocean connection wasn't complete because of the presence of the extension zone. The rift valley still contained river deltas, brackish swamps and fresh water lakes. Throughout the slow phase of movement, a land connection was maintained via Tasmania.
Beginning at 96 mya, rifting between Australia and the Lord Howe-New Zealand Subcontinent began. The Tasman Sea started opening from south to north, beginning from the South Tasman Rise. The New Zealand section was still connected to Australian crust by the Campbell Plateau. Tasman Sea spreading stopped during the India sprint to Asia. The Coral Sea Basin was forming between Australia and New Guinea at this time.
There was volcanic activity along the Queensland coast as it is today, the coast was much further out at the time the volcanoes were active. The now submerged Queensland Plateau was still part Queensland at the time. In the earlier part of the Cretaceous there was also volcanic activity on the Lord Howe Rise. After 96 Ma the volcanic arc moved away from Queensland until now it lies 3000 km east, passing through present-day New Zealand. In the past 96 million years the only volcanic activity n the Australian continent has been when the continent passed over "hot spots".
By 90 Ma an extension of the proto-Southern Ocean extended as far east as Tasmania, following a period of seafloor spreading between Australia and Antarctica to the NNW. The ocean to the south of New Zealand in the Tasman Sea had opened as a result of seafloor spreading. This had the result of isolating New Zealand and New Caledonia from the rest of Gondwana (Stevens, 1989). It has been suggested that there may have been a land link between New Zealand and New Caledonia that lasted until about 70 Ma, but evidence for this link has not yet been found. From about 90 Ma to about 60 Ma there was subaerial volcanism in the open ocean to the west of Australia on the northern edge of the Broken Ridge Platform (Rea et al., 1990). It has been suggested this may have provided a stepping stone for floral migration.
The seafloor was now spreading between northeastern Australia and eastern New Guinea that resulted in the formation of the Coral Sea Basin by the close of the Palaeocene. Continental fragments that were moved to the west by movements of the Pacific Plate after the close of the Palaeocene to become part of the islands of eastern Indonesia (Pygram & Symonds, 1991) are thought to have resulted from seafloor spreading in the vicinity of eastern New Guinea at this time (Hill, 1994).
By the Late Cretaceous nearly the whole of the continent was above sea level, but the sea was still present in the Perth and Carnarvon Basins. As was happening worldwide during the hot, humid Late Cretaceous, there was a great amount of erosion occurring.
96 mya was also the time when the eastern margin of the continent underwent significant uplift to form the Great Dividing Range, a reaction to pressure from the Tasman Sea Rift. A strip of crust at the edge of the continent warped down, exaggerating the height of the Divide still further. As a result of the steeper gradient erosion in the eastward flowing rivers increased, carving out great gorges.
By the Late Cretaceous, about 84 Ma, Australia was separated from Antarctica by a seaway about 100 km wide. Tasmania was still connected to Antarctica. Bass strait consisted of river flood plains, swamps and lakes. The sediment laid down in the Bass Basin at this time indicate the river and lake origin.
As the Tasman Sea widened, the New Caledonia Trough developed between the Lord Howe Rise and the Norfolk Rise. The New Zealand Subcontinent was still subsiding.
In the Late Cretaceous, 85-65 Ma, Tasmania was still connected to Antarctica by the stretched crust of the South Tasman Rise. A hot and humid climate is indicated by the presence at the time of widespread deep chemical erosion. A lot of the sediment from this period was carried to the margins of the continent, so deep sedimentary deposits didn't form to make rocks.
80 Ma. At this time India and Madagascar had been isolated by a surrounding ocean and now became separated from each other. Tasmania was still close to Antarctica, but attached to Australia by the Bassian Plain, later to become Bass Strait, that at this time was composed mostly of lowland environments where that were depocentres for the sediments derived from higher ground to the north and south, and carried there by rivers. It was eventually flooded in the Oligocene-Miocene (Hill, 1994).
70 Ma. On the Australian continent the presence of widespread, thick, weathered profiles indicate the climate was probably uniformly humid, and remained tropical for much of the Late Cretaceous (G.E. Wilford, R.P. Langford & E.M. Truswell, unpublished data). In Hill (1994).There are few clastic sediments known from 70 Ma and earlier Late Cretaceous on and around the Australian continent, an exception being the southern margin that was tectonically active.
As the end of the Cretaceous the climate cooled. The Terminal Cretaceous Event, probably the widely blamed meteorite, was associated with a mass extinction around the world, but apparently except for Australia. The Australian and New Zealand fossil record doesn't display signs of a mass extinction at 65 Ma.
Early in the Palaeocene epoch, between 65 and 58 Ma. At this time the oceans were warm, the abyssal water around Australia was 12-15o C, it is now 2o C. Without much of a temperature gradient between the surface and deeper water, ocean circulation was slow. Between 55 and 50 mya, the spreading in the Tasman Sea and the Corel Sea Basin stopped.
An expanse of oceanic crust separated Australia from Antarctica by the start of the Oligocene, about 36.6 Ma. To the west and east of the rift zone spreading ridges had formed, extending the seafloor, spreading into the Indian Ocean and south of Tasmania, across the Tasman Sea to south of the Campbell Plateau, forming the Pacific-Antarctic Ridge that separates the Antarctic and Pacific Plates.
60 Ma. By this time the expansion of the Tasman Sea had stopped. Australia continued drifting north. Widespread erosion and deposition of sediments occurred across Australia, believed to be associated with one or more cooling events, and probably ice accumulation, in the early Cenozoic. Very sandy channels formed in the east, while in the west wide expanses of swampy alluvial channels formed (G. E. Wilford, R. P. Langford & E. M. Truswell, unpublished data) . The same conditions occurred intermittently until about the Early Oligocene. (Hill, 1994).
50 Ma. It was at about this time that the Transantarctic Mountains were thrust up (Fitzgerald and Gleadow, 1988), leading to the increased accumulation of snow and ice, a feature of Antarctica for much of the Cenozoic.
40 Ma. The movement of the Pacific Plate changed direction at this time to a more WNW direction, eventually colliding with the Australian Plate in New Guinea. From the Mid Oligocene onwards the collision resulted in mountain building and subduction of the Australian Plate beneath Sundaland (Pigram & Symonds, 1991). The spreading rate between Australia and Antarctica had increased about 44 Ma, as the Australian Plate moved to the north, resulting in the formation of a deep marine strait between Tasmania and Antarctica by 38 Ma (Kennett, 1980). This allowed the Circum Antarctic Current to form, which resulted in the increased cooling of Antarctica and the adjacent oceans.
Microcontinental blocks linked the Antarctic Peninsula to the tip of South America up until about this time, since then plate movements and seafloor spreading associated with the opening of the Scotia Sea has separated them. It is not known exactly when the Drake Passage, the expanse of sea between the Antarctic Peninsula and southern South America, opened. Lawver et al. (1985) has suggested that it opened some time between 64 and 34 Ma, but Barker & Burrell (1977) have suggested it was more like 30-25 Ma. In (Hill, 1994). About 42 Ma there was a possible stepping stone island, about 2000 km west of Perth near Broken Ridge, where limestone and chert were exposed It is thought to have lasted only a short time (Tea et al., 1990).
By 30 Ma the last impediment to the movement of water around Antarctica was removed when the connection between the tip of South America and Antarctica and between Tasmania and Antarctica was broken. The formation of the circum-Polar current now developed. This allowed the freezing of the Antarctica continent by shielding it from the warmer water of the adjacent oceans. After 50 Ma Australia began to move more rapidly and by 45 Ma the last connection with Antarctica was broken.
From about 38 - 36 Ma, in the Early Oligocene , a change in the Australian climate occurred, becoming much drier, and duricrusts formed over much of the continent, as the large-scale ice sheets became established in East Antarctica. Evidence for this climatic shift was found in 1988 in the cores of the Ocean Drilling Program (G. E. Wilford, R. P. Langford & E. M. Truswell, unpublished data).
McDougall & Duncan (1988) suggest that from about 30 Ma a series of volcanic islands between the Australian continent and New Caledonia could have influenced plant migration in the Neogene, though the evidence for a land connection is lacking. The only island of this series that has not been submerged is Lord Howe Island, a bit over 6 million years old. The submerged islands forming seamounts in the Tasman Sea (McDougall & Duncan, 1988).
20 Ma. By this time Australia had drifted far enough north for its northernmost parts to move into the zone of influence of the tropical monsoon climate. This led to the formation in well-drained coastal soils of bauxite deposits. A number of rivers that were active in the early Tertiary under this climate regime, but were less active when the rainfall declined resulting in less erosion, which was reduced further by partly duricrusted soil surfaces. There were many lakes at this time. Volcanism occurred sporadically along the northern margin of the Australian Plate as it continued pushing north into Southeast Asia. One result of the northward movement with the accompanying volcanic activity was the formation of islands between Australia and Asian mainland.
10 Ma. By now the increasing aridity was being caused in Australia, especially in the interior, by the greatly expanding ice sheets in Antarctica. This led to reduced erosion and deposition. A similar effect has recently been found to be responsible for the decades long drought across southern Western Australia and Victoria, in this case by the contraction of the weather systems closer to Antarctica that previously brought winter rain to the southern parts of the continent. The rain-bearing clouds no longer reach as far north as they did previously, many completely missing the Australian continent. (Catalyst, ABC TV).
As the Indo-Australian Plate moves northeast the volcanic activity continues on its northern margin in the mountainous island of New Guinea that bears the brunt of the collision with the Pacific Plate, as well as along the subduction zone known as the Pacific "Ring of Fire".
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|