Structural frame work of the fore-arc basin, NW Sumatra

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.