TheBanda Arc (main arc, Inner, and Outer) is a dual chain of islands in easternIndonesia that is around 2,300 km long. It is the result of the collision of acontinent and an intra-oceanicisland arc.
The presently active volcanic arc is mounted on stretched continental andoceanic crust whereas the associatedsubduction trench is underlain bycontinental crust, which has subducted deep enough to contaminate the volcanic arc with continental melts.[1] The island of Timor is notable for the lack of volcanic activity. This is due to the island representing the zone of fore-arc and continental collision which prevents volcanic activity from occurring.[2]
The convergence of the Indo-Australianplates andEurasia resulted in the formation of the Sunda and Bandaisland arcs. The transitional zone between the arcs is located south ofFlores Island and is characterized by the change in thetectonic regime along the boundary in the Timor Region.[3][4]
Some academic literature refers to the arcs by location – so that the main arc can be referred to as the 'southern',[5] the 'western'[6] Situated at the centre of three converging and colliding majortectonic plates,Indo-Australia,Eurasia,Pacific, the Banda arc includes youngoceanic crust enclosed by avolcanic inner arc, outer arc islands and a trough parallel to the Australiancontinental margin. It is a complexsubduction setting (where one plate moves under another, sinking into the Earth'smantle), with possibly the largestfold onEarth, extending to a depth of about 650 km (404 mi), in a subducted plate.[7] Although the Australian Continental crust does not subduct, the sub-crustal mantlelithosphere that is attached to the Australian plate continues to subduct underneath the Eurasian plate. This subduction continues to pull the Australian plate in a northward direction today.[2]
TheBanda Arc is a doubleisland arc formed by the collision of theIndo-Australian plate with theEurasian plate that commenced around 15 to 12 million years ago.[2] Principal islands includeTimor,Flores, andSeram.
The island of Timor provides an excellent landscape for understanding the arc-continent collision that is taking place. Although previous interpretations of the island of Timor have placed it as an accretionary wedge, it is in fact a mixture of Banda fore-arc volcanic rocks and the Australian continental margin.[2]
The basic stratigraphy of Timor is layers of young volcanics and sedimentary rocks overlaying metamorphic complexes. The sedimentary and metamorphic layers together form what is called the "Banda Terrane". These layers were thrust on top of the Australianpassive continental margin as the arc and continent collided, so Australian lithologies, like the Gondwana sequence, underlie the island of Timor. Compressive forces continue to duplex the Australian sequences under the island, leading to as much as 2,500 meters of uplift of the Banda Terrane.[8]
Because they are metamorphic, the Banda Terrane sequences could be from a number of sources. Likely sources are 1) rifted metamorphic material from the ancient Great Indonesian Arc, 2) they are part of the original Australian continental basement, or 3) They are metamorphosed exposures of the subducted passive continental margins of Australia.[8]
Petrological studies show that the Banda Terrane was formed as a clay-rich sedimentary unit derived frommafic continental and oceanic arcprotoliths. As such, detrital zircons are part of its composition. There are two groups of zircons found in Banda Terrane rocks: euhedral zircons, and weathered spherical zircons. The age of these zircons, as derived fromU-Pb dating, are 87 Ma, and 113 Ma, respectively. On the other hand, the three major groups of zircon ages found in colliding Australian sediments are 301 Ma, 1882 Ma, and 2400-2700 Ma. The difference in zircon ages indicates that the Banda Terrane is likely not of Australian origin, but rather of Asian origin from the Great Indonesian Arc.[8]
Similar event-age relationships usingLu-Hf dating andfission-track analysis indicate the Banda Terrane likely rifted from the Great Indonesian Arc, collided with the Australian continent 3 Mya, and has been uplifted through duplexing of the underlying Gondwana sequence. Volcanic and sedimentary layers on top of the metamorphic rocks weathered away, leaving the metamorphic cores asklippes now visible on the surface.[8]
However, the results of the collision are much larger than just Timor. The collision has created a 180-degreeisland arc, which is more than 1,000 kilometres (620 miles) long. Geographically, it stretches across easternIndonesia, and is delimited by an active innervolcanic arc. The outer arc contains numerousislands, and its internalgeologic structure contains youngoceanic crust exclusively.[9]
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