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The Australian plate is one of the most imposing and older ones on Earth and is in contact with 17 other plates and micro-plates. On its southeast boundary, the plate has been subsiding under the Pacific plate along the Puysegur trench at a rate of 5 ± 0.8 mm/yr [3] since 5-10 Ma [4] towards the northeast direction (Figure 1). The active configuration generates earthquakes of M4-M7.9 on both plates and at their interface [5] (Figure 2). The Australian slab gradually steepens from south to north along the subduction zone until it becomes nearly vertical. The plate boundary abruptly converts to the Alpine Fault (linear dextral transpressive transform fault) which transects the New Zealand since 6.5-10 Ma [3]. Furthermore, the Australian plate subsidizes under the Pacific plate further north, near the Solomon islands, since 10 Ma at a rate of 52 ± 4 mm/yr [4](Figure 3).
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The Southeast Indian ridge (Figure 4) appeared during the separation of the Gondwana continent (34 Ma) and it lies on the southern boundary of the Australian plate, which diverges from the Antarctic plate at a rate of 60-70 mm/yr towards the northeast direction [6] (Figures 5). The mid-ocean ridge is sectioned by transformation faults and two propagating rifts, and at its eastern end lies the Australia-Antarctica Discordance (Figure 5) where the temperature is remarkably low conforming with the observed sea-floor deep levels [ 7]. |
The southwest plate boundary begins at the Rodrigues Triple Junction which marks a common point between the Australian, the Antarctic, and the African (Somalian) plates and exists since 65 Ma [9] (Figure 6). The Australia-African intersection is marked by the Central Indian Ridge (CIR), where the Australian plate is drifting towards the northeast direction at a rate of 40 mm/yr, the direction being conserved for the last 0-4 Myr (Figure 7). An increase of seismic activity is observed at less magmatic sections corresponding to the transform faults intersecting the ocean ridge [8](Figure 8). Moreover, it has been observed that a rigid Indo-Australian plate motion does not correlate with the plate kinematics seen from the CIR. Thus, a viable model would be to separate the Indian and Australian plates [10] by a diffuse boundary [6], which comprises an oceanic spreading ridge and an oceanic convergent boundary [1] (Figure 9). A further sophisticated proposition that accounts for incompatible inter-plate seismic activity and other inconsistencies, would be the formation of the Capricorn plate, which appeared about 11 Ma [10] from the CIR, and has a diffuse eastern plate boundary with Australia [6] (Figure 10).
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On its northern margin, the Australian plate subsidizes under the Sunda plate along the Java Trench (Figure 10) towards the northeast direction at a rate of 71± 2 mm/yr[11] since 80 Ma [12] (Figure 11). The Pacific Ring of Fire along the subduction zone presents a great volcanic and seismic activity (Figures 12,13), the biggest earthquake being of magnitude M9 in 2004 (Figure 14). |
On the northeast side, the Australian plate is in contact with younger micro-plates where the plate boundaries are of varying nature [1] (Figures 15,16,17). |
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Beneath the Cenozoic layer that covers the majority of the Australian continent (Figure 18) lie older blocks that testify for the complex mechanism by which the Australian plate formed. Its origin can be traced from Precambrian (Archean) rocks in the Yilgarn and Pilbara cratons, on the western margin of the land. The assemblage of the west, north, and south sections of the present region happened at the Proterozoic period by the convergence of tectonic provinces, giving rise to Proto-Australia, as it is demonstrated by the building blocks in the middle of Australia. It is not until the Mesozoic (cretaceous), during the separation of the Gondwana super-continent, that the eastern section joined the land, coinciding with Australia's separation from Great India, with its western boundary facing the Indian ocean, and with the uplifting of the eastern highlands (Figure 19). Around 65 Ma, sea floor spreading began which would eventually separate the Australian and Antarctic continents (34 Ma). The great plate converges to the north around 40 Ma, the path being exposed by the plate's passage on a hot plume, leaving the sea mount chains in the Tasman sea [6] (Figure 20).
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The non-seismic Ninety East Ridge (NER) in the Indian sea (Figure 21), originated from a hot spot near the Wharton ridge. NER headed towards the north from the end of Cretaceous until the beginning of Cenozoic. Later, as the Wharton ridge was no longer active, the NER merged the Indian and Australian plates, and ceased activity, which coincided with the birth of the Southeast Indian Ridge [13]. Around the Mesozoic period, the Tasman sea formed from rifting along the Australian east coast, and the Lord Howe Rise and New Zealand were moved to the east [6]. The Tasman Sea comprises the Resolution ridge which is a geologic separation between late Cretaceous to early Eocene blocks at the north and Eocene to Miocene oceanic crust at the south [14] (Figure 22). Around the same period, the Coral Sea and its plateaus formed. They are characterized by a complex bathymetry[15](Figure 23). Australia and New-Guinea are believed to have been joined during the Phanesozoic and were separated by subduction under the little island around the Mezosoic [16]. During the dissolution of the Gondwana super-continent (Jurassic), there was an active rift at the north of the Australian plate, which later became a sea floor spreading, responsible for the origin of the Arafura sea upon the continental shelf area [17] (Figure 24).
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[REFERENCES]
- Modified Google Earth file by professor Yajing Liu, based on an online module developed by Laurel Goodell, Department of Geosciences, Princeton University. http://serc.carleton.edu/sp/library/google_earth/examples/49004.html.
- National Earthquake Information Center, 2004, M7.1 Puysegur Trench Earthquake of 22 November 2004, US Geological Survey, scale 1:1,300,00.
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