The western Sunda Arc of Indonesia is a continental-margin arc-trench system characterized by oblique subduction along an irregular crustal edge and by transcurrent faulting within the arc terrane. Offscraping and accretion of material along the base of the landward trench slope have led to formation of a prominent outer-arc ridge that has trapped sediment in basins between the volcanic arc and the trench. Sedimentary strata deposited in these fore-arc basins record the Neogene and Quaternary evolution of the arc-trench system and preserve many of the original structural and stratigraphic relationships.

Along the active Sumatra margin, the combination of compression and transcurrent faulting has led to the development of relatively shallow transverse structures that subdivide the fore-arc region into several discrete sedimentary basins. Multichannel seismic reflection profiles reveal the structural style of two prominent transverse highs that separate Nias basin from the deeper

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Meulaboh and Mentawai troughs to the north and the south, respectively.

The Banyak Islands cross-structure is a linear zone of deformation that transects the fore arc obliquely. The continental shelf is narrow and an abrupt change in depth to basement and sediment thickness occurs across a near-vertical fault. The trace of this fault continues onshore in good alignment with a major strand of the right-lateral Sumatra fault system. Offshore, the fault transects the Banyak shelf and appears to intersect the late Pliocene flexure on Nias which is thought to represent the rear edge of subduction-related deformation. Here, it occupies a wider zone of deformation characterized by high-angle reverse faults that probably involve basement. Monoclinal folds and secondary faults have developed in the overlying sedimentary section, and large growth faults with normal separation have developed along the crest of the trench-slope break north of Nias.

Stratigraphic relationships and well data indicate that faulting and folding occurred during Pliocene time. The variation in structural style along strike is indicative of varying crustal structure. The shelf, characterized by rigid crystalline basement with a thin sedimentary cover, exhibits a brittle crustal response, whereas the thick prism of fore-arc basin strata overlies more ductile crust which affords greater crustal mobility.

The wide central Sumatra shelf southeast of Nias does not appear to be related to strike-slip faulting. Seismic profiles show that flat sediments of Miocene and Pliocene age onlap an eroded crustal block of continental character lacking diagnostic structures and obvious trend associations. We believe that this feature existed as a broad, paleobathymetric high that separated the basins early in the history of the fore arc. Buried normal faults, which step down to the trench, offset the basement reflector along the paleoshelf edge. The relatively shallow shelf has persisted as a positive structural element exhibiting a reduced rate of subsidence throughout the Neogene.

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