Abstract

Because most active forearcs are wholly or mainly submarine, knowledge of their geology is derived largely from seismic surveys. However, DSDP has in some cases revealed stratigraphical and structural features unsuspected from seismic data. Forearc islands offer exceptional opportunities for correlating seismic imaging with exposed rocks. The islands of the Sunda forearc have been widely used as models for recent and ancient forearc terrains and most hypotheses have been based on the comparatively well-known geology of Nias, although van Bemmelen (1949) reported considerable stratigraphical and structural differences between the various islands. A very different aspect of Cenozoic history is exposed on Simeulue to the NW of Nias, including an Oligo-Miocene erosional unconformity comparable with sections reported from wells in the forearc basin (Karig et al., 1979). Repeated tectonic inversions of Simeulue may be related in part to transpression and transtension stresses generated by strike-slip motion interacting with sinuosities in the trench and the Sumatran fault system.