Banda Arc



The Banda Arc (main arc, Inner, and Outer) is a dual chain of islands in eastern Indonesia that is around 2,300 km long. It is the result of the collision of a continent and an intra-oceanic island arc.

The presently active volcanic arc is mounted on stretched continental and oceanic crust whereas the associated subduction trench is underlain by continental crust, which has subducted deep enough to contaminate the volcanic arc with continental melts. 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.

The convergence of the Indo-Australian plates and Eurasia resulted in the formation of the Sunda and Banda island arcs. The transitional zone between the arcs is located south of Flores Island and is characterized by the change in the tectonic regime along the boundary in the Timor Region.

Geologic Setting
Some academic literature refers to the arcs by location – so that the main arc can be referred to as the 'southern', the 'western' Situated at the centre of three converging and colliding major tectonic plates, Indo-Australia, Eurasia, Pacific, the Banda arc includes young oceanic crust enclosed by a volcanic inner arc, outer arc islands and a trough parallel to the Australian continental margin. It is a complex subduction setting (where one plate moves under another, sinking into the Earth's mantle), with possibly the largest fold on Earth, extending to a depth of about 650 km, in a subducted plate. Although the Australian Continental crust does not subduct, the sub-crustal mantle lithosphere 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.

Inner and outer arcs
The Banda Arc is a double island arc formed by the collision of the Indo-Australian plate with the Eurasian plate that commenced around 15 to 12 million years ago. Principal islands include Timor, Flores, and Seram. 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
 * The Inner Banda Arc consists of a string of recent and active volcanic islands from Komodo to Kekeh-besar of the Barat Daya Islands, including Flores, Solor, Alor, Wetar, and Damar.
 * The Outer Banda Arc consists of Australian continental margin cover units that were scrapped off the Australian plate and added to the southern edge of the Asian plate. The oldest sedimentary rocks in these islands are from the Early Permian. The Banda arc-continent collision is still active and converging at a rate of 7 cm/a. It stretches from Savu through Rote, Timor, Leti, Babar, Tanimbar, and the Kai Islands, before turning west to Seram, Ambon, and Buru. The outer arc is geologically associated with the Australian continent, though it is a more recent accretion than the neighboring Aru Islands.

The resulting feature is a 180-degree island arc, which is more than 1,000 km long. Geographically, it stretches across eastern Indonesia, and is delimited by an active inner volcanic arc. The outer arc contains numerous islands, and its internal geologic structure contains young oceanic crust exclusively.