What Can We Learn from Indonesia That Is Relevant to Other Orogenic Belts ?

The Indonesian region is a huge active orogenic belt with multiple small plates and sutures, but is poorly known compared to many other active and older belts of comparable size. It is quite common to see the region used to provide simple analogues and models, but these may reflect the paucity of data rather than realistic scenarios.

By the late Mesozoic the region was a mosaic assembled from Gondwana blocks separated by ophiolitic sutures. Subduction had ceased in the late Cretaceous after collision of continental fragments and was resumed in the Eocene. Subsequent Cenozoic growth has been staccato. The deep mantle shows there was a different history of subduction north of India compared to that north of Australia. The basement structure influenced the Cenozoic development. Other influences include collisions of India, Australia, Pacific arcs, and subduction.

Rapid movements of small plates has meant that plate boundaries have changed frequently during the Cenozoic. Growth occurred in an episodic way, related primarily to arrival of continental fragments at subduction margins, with subordinate contributions from ophiolite accretion, and arc magmatism. Controls on deformation include a thin weak lithosphere beneath much of the region which is very responsive to changing forces at plate boundaries. Extension and contraction resulted from coupling between the over-riding and downgoing plates, strike-slip faulting and subduction hinge movements. In east Indonesia rollback caused dramatic extension within the collision zone, although ongoing contraction will eliminate most of the evidence for it.

All orogenic belts are different and none is a model for another. Indonesia shows that we need more realistic models of arcs. Geochemistry provides insights but is insufficient to comprehend arc development. In Indonesia we need more data, between the arc and trench, and on land, particularly a better understanding of the deep structure (crust and mantle).