10 Movements, collisions and the Banda Arc

Continents and island arcs generally move towards each other because the intervening ocean floor is being subducted, i.e. forced underneath. The old ocean floor, originally forming at a spreading ridge, is being recycled back into the mantle. From about 45 million years ago, Australia moved north, and ocean crust was consumed at north-dipping subduction zones stretching from Sumatra in the west to Halmahera in the east. At about 23 million years ago, the leading part of continental Australia began to collide with the Southeast Asian margin forming the east Sulawesi orogen.

The Banda Arc represents an example of large-scale deformation of the Earth’s crust. The curved Banda arc comprises young oceanic crust enclosed by a volcanic inner arc, outer arc islands and a trough parallel to the Australian continental margin (Spakman & Hall, 2010). The outer non-volcanic arc of islands is formed principally of sedimentary, metamorphic and a few igneous rocks of Permian to Quaternary age. Banda subduction began about 15 million years ago as the Australian plate moved northward at a speed of about 7 cm per year.

On the south side of the Banda Sea, in Timor, collision between the Australian margin and the Banda volcanic arc began in the Pliocene and the site of active deformation is migrating west with time (Hall, 2002). Seram, like much of Timor, consists of relatively deep-water continental margin rocks representing part of the Australian margin formed during the Mesozoic, which were uplifted into the present high mountains of Seram in the Late Miocene.

Whitmore (1981) in the book Wallace’s Line and Plate Tectonics explains (p. 1):

“The boundary between Gondwanaland and Laurasia lies within Celebes or just to its east…  The Inner Banda island arc is Laurasian, the Outer Banda island arc and Moluccas are Gondwanic.”