Abstract

The fore-arc basin of the Sunda Arc off Sumatra is a strongly subsiding trough contained between the rising subduction complex and the elevated continental core. Up to 4 km of Miocene–Recent strata on the eastern flank of the basin have transgressed over an unconformity cut across a Palaeogene continental margin that was uplifted and structurally disrupted during the late Oligocene. The large step-like offsets of the palaeo-shelf edge, and probably some of the structural disruption, can be attributed to right-lateral strike-slip faults that splay across the fore-arc basin from the Sumatra Fault Zone. Offsets range up to 100 km+, producing marginal re-entrants that became sites of turbidite-filled basins trapped behind the growing Neogene accretionary prism. The larger re-entrants may be floored with oceanic crust, but the proximity of exposed subduction complex to the palaeo-margin precludes the existence of a continuous band of oceanic crust beneath the fore-arc basin.

The seaward flank of the fore-arc basin has migrated westwards during Neogene subduction, but not so much as the trench, and in a much more complicated fashion. During the early Miocene, the inner trench slope had a juvenile terrace-like morphology. By late mid-Miocene time, the trench slope break was near sea level and formed a shelf edge high. Thrusting and folding related to subduction probably decreased gradually upslope until late-Pliocene, when large flexures and E-directed reverse faults developed, marking an arcward shift in the rear or eastern boundary of the deformation associated with subduction. The episodic development of arcward-directed structures, involving previously stabilized fore-arc basin strata, suggests periods of increased stress levels along the base of the accretionary prism.

First Page Preview

First page PDF preview
You do not currently have access to this article.