Modern Plate Configuration Gondwana Region Seafloor Spreading
 

Modern plate configuration

The Australian Plate is bounded from the Pacific and Eurasian Plates by convergence, from the Indian Plate by a zone of compressional deformation, and from the Somalian and Antarctic Plates by divergence.

The break up of Gondwana from Permian 250 Ma.
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Gondwana region

The Gondwana super-continent (shown above) comprised South America, Africa, Australia and India. They were grouped around Antarctica as a central core, with a number of smaller continental fragments.


Cretaceous - 130 Ma

By the early part of the Cretaceous period, some 130 million years ago, Gondwana had begun to break up. The first split divided the continent into two sections, a western half, made up of Africa and South America, and an eastern half with Antarctica, India and Australia. The sites of initial separation between the continents are marked by mountainous submarine ridges in the mid-ocean.

 


Cretaceous - 120 Ma

Later in the Cretaceous period, some 120 million years ago, the next of the fragments, India, broke away and began its northward journey through the Indian Ocean.
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Seafloor spreading around Australia

The onset of seafloor spreading west of Australia at ~130 Ma marks the breakup between Greater India and Australia. Roughly at the same time, long-lived subduction east of Australia ceased.

In the late Cretaceous (~61 Ma) the spreading direction between Australian and Antarctica changed from NW-SE to North-South.
 

 


Eocene - 40 Ma

The break between Australia and Antarctica was the last event in the Gondwana breakup. It began about 80 million years ago when a deep rift valley formed along the southern edge of Australia; this widened to form the present Southern Ocean. The formation of this ocean is apparent in the reconstruction at left for the Eocene epoch, some 40 million years ago. Since that time Australia has been moving steadily north at the rate of about 5 cms per year.
 
 

Final separation of Australia from Antarctica

Prior to 43 Ma a triple junction formed north of the Ross Sea, accommodating incipient motion between East and West Antarctica. At the same time spreading between India and Australia ceased and the northward motion of Australia accelerated as it separated from Antarctica.

The Southeast Indian Ridge now completely separated the Australian and Antarctic Plates. At the same time spreading between India and Australia ceased and subduction north of Papua New Guinea was initiated.

Major tectonic events since the early Miocene (~20 Ma) include the breakup of the Indo-Australian plate, and various collisional processes and plate boundary reorganizations north and east of Australia. The increasing effect of rising subducted slab material from the transition zone underneath the Southeast Indian Ridge is postulated as creating the Australian-Antarctic Discordance, with its thin crust, anomalous depth, and fracture zone offsets.

The sector of oceanic crust containing Macquarie Island, between Tasman Sea oceanic crust and continental crust of the Campbell Plateau, was generated from 39 to 10.5 Ma at the Proto-Macquarie Spreading Ridge, that propagated from the Pacific/Antarctic spreading ridge. The Proto-Macquarie Spreading Ridge evolved with time from long ridge segments of NNE-trend to short ridge segments of E-trend in the vicinity of Macquarie Island and generated during the latest episodes of seafloor spreading. The overall trend of the Proto-Macquarie Spreading Ridge was NNE-SSW throughout and roughly coincided with the present day Macquarie Ridge. Plate reconstructions suggest coincident seafloor spreading and strike-slip movements at the Proto-Macquarie Spreading Ridge between 14 and 10.5 Ma. The present day Indo-Australian/Pacific plate margin is coincident with the Macquarie Ridge, an arcuate 2100 km long crustal fracture system connecting the Pacific/Antarctic and Indo-Australian/Antarctic spreading ridges with the Alpine Fault system in New Zealand.
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Last update: December 24, 2002