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The Australian plate and the micro-plates occupying its eastern and northeastern margins form a rich conglomeration of special features such as hot spots, triple junctions, strange plate rotations, and variations in plate thickness.
The Ninety East Ridge (NER)-Figure 1
The NER is an old volcanic ridge which originated from a hot spot near the Wharton ridge, separating the Indian and Antarctic plates. At the end of the Cretaceous and the beginning of the Cenozoic periods, the NER drifted northwards leaving the trace of its passage on the Indian plate. Later, the Wharton ridge ceased its tectonic activity, the Australian and Indian plates converged through the NER, and the Southeast Indian ridge was activated. The North end of the aseismic ridge is older(77 Myr) and is characterized by the distribution of individual large volcanoes, whereas the south end is younger (43 Myr) and displays elevated continuous chains of volcanoes. Furthermore, the middle of the NER is composed of linear segments and sea mounts. These differences along the NER are explained by the relative distance between the hot spot and the Wharton ridge, where the north end would be farthest and the south end the nearest .
East Australian Hot Spot-Figure 2
The eastern coast of the Australian continent is bordered by eroded volcanoes (Lord Howe Rise, Tasmantid sea mount chain, Norfolk Island) which originated from the East Australian Plume System (EAPS). The plume system formed during the rifting of the Coral sea, around 65Ma . More precisely, the Lord Howe chain (LHC) is the trace left by a stationary plume underneath the moving Australian plate towards the northeast direction. Consequently, the north end of the sea mount chains is youngest (25 Ma). Moreover, the Tasmantid sea mount chain is contemporaneous to the LHC. However, it formed through a hot spot drifting to the south at a rate of 67 mm/yr, and thus, the south end marked by the Gascoyne sea mount is younger (6.4 Ma), and the north end terminated by the Queensland seamount is older (24Ma) . The Australian margin is also characterized by a chain of dormant mafic volcanoes known as the Newer Volcano Group .
The Rodrigues Triple Junction (RTJ)- Figure 3
The RTJ represents the common point shared by the Australian, African and Antarctic plates since 65Ma, and is composed by the Southest and Central Indian ridges, spreading at an intermediate rate, and the Southwest Indian ridge, spreading at a very low rate. It has been observed that the Central Indian ridge is characterized by an unstable segmentation near the triple junction. Thus, it is proposed that the RTJ evolved through two phases during less than 2 Myr : a) the Southeast and the Central Indian ridges join at the triple junction, configuration that becomes unstable due to the asymmetric spreading of the ridges, and the difference in axis orientation or lengthening rates, and finalizes b) by the offset of the two ridges. Furthermore, the Southwest Indian Ridge (SIR) elongates through continuous and discontinuous manners. The former involves the gradual propagation of the SIR and the deformation of the Central Indian ridge, and the latter entails the deformation of a rifting zone as the RTJ drifts northwards. This is followed by the restoration of the Central Indian Ridge, which leads to the stable configuration of the RRR triple junction .
STRANGE PLATE ROTATION
Australian-Pacific Intraoceanic Subduction Zone -Figure 4
The Macquary spreading ridge (before 20 Ma), located south to the New Zealand island, became the Puysegur trench by a transitional process involving the displacement of the Euler rotation pole followed by the internal deformation of the Australian plate . The Australian plate subsidizes under the Pacific plate along the mentioned trench, the dip of the diving crust steepening as the boundary approaches the southern New Zealand island to become the Alpine fault . On the northern section of the island, there is back-arc rifting near the Hikurangi margin. Surprisingly, it is at this location that the old and buoyant pacific crust subsidizes in an oblique orientation underneath the Australian plate . This ultimate configuration is known as an intraoceanic subduction zone and originates from the fast tectonic inversion of ancient spreading ridges at back arc basins due, principally, to the Euler rotation pole's location along or near the interplate boundary. It is known that the subduction boundaries at north and south of New Zealand have been and are extending, until their eventual junction through the absence of the Alpine fault .
PLATES' VARYING THICKNESS
Australian Continent- Figure 5
The crustal thickness of the continent ranges from 24-59 km and presents local discontinuous topography and steep horizontal gradients as well. The crustal thickness does not follow the precept of the magmatic underplating, that is, it is not thicker at older building blocks due to magmatic accretion. Indeed, it is relatively thin at the western Archean cratons (30 km), whereas it is thicker at the younger northern section and Mt. Isa (55 km). The thinnest continental crust lies in Tasmania .
Indian Ocean- Figure 6
The oceanic crustal thickness is 7 km on average except at the Central and Southeast Indian ridges, and the Ninety East ridge (NER). Accordingly, the NER is thicker (20 km) since it originates from a hot and weak lithosphere section, gaining mass due to magma underplating . Moreover, the crust at the oceanic spreading ridges are thicker as well . However, the Southeast Indian ridge undergoes crustal thinning on its eastern side, at the Australian-Antarctic Discordance due to the ultra-slow spreading rate of the ridge .
The oceanic crust gets thicker at the Solomon and New Hebrides islands, since they form fractured arcs , but is even thicker at subduction zones, due to the dipping crust, such as the Puysegur trench , the Java trench , and the Tonga-Kermadec subduction margins. Finally, the arc-arc collision at the Molucca sea leads to the destruction of the oceanic crust located in the middle of the convergent arcs to yield another crust thickness configuration  (Figure 8).
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