CENOZOIC INDONESIAN TECTONICS

The Indonesian region includes island arcs and collision zones with a record of Mesozoic and Cenozoic subduction of Indian and Pacific lithosphere as well as a history of continental collision at its margins. GPS measurements, seismicity and volcanic activity indicate the complexity and speed of current tectonic movements. The Cenozoic history of the region was no less complex. Subduction beneath Indonesia resumed at 45 Ma, after subduction ceased in the Late Cretaceous, as Australia began to move rapidly northwards. Tectonic influences on the region during the Cenozoic include India–Asia collision, Australia–SE Asia collision, Pacific plate/Philippine Sea plate collisions with the SE Asian margins, and subduction. Indian collision since 50 Ma modified the Asian continent but was not the principal tectonic influence in Indonesia. Although Australian collision with SE Asia has been important, subduction since 45 Ma has been the major driving force. As a result of subduction there is a high heat flow across a region beyond the vicinity of the volcanic arcs. There is probably a thin and generally weak lithosphere beneath much of SE Asia that is responsive to changes in the forces acting at the plate margins. Changes in these forces reflect major plate motions, collision and subduction hinge movements. Subduction led to both extension and contraction, often at the same time. During the Cenozoic, subduction hinge rollback was accompanied by arc volcanism and in most cases by marginal basin formation, as the mantle wedge was replenished by an inflow of hot mantle. In west Indonesia, strike-slip faulting accompanied subduction due to partitioning of oblique convergence. In central Indonesia volcanism ceased for a while in the Neogene as the subduction hinge advanced. In east Indonesia, collision of the Australian margin with the SE Asian margin since 25 Ma combined with subduction hinge rollback to form the present Banda arc. It might be expected that the type of events in the geologically recent history of Indonesia, exemplified by catastrophic earthquakes and volcanic eruptions, would be obvious in the geological record but this is not the case. This may reflect their relatively short-lived nature, the stratigraphic detail needed to identify them, and also the still limited amount of research in this fascinating region.