Abstract

The Sumatran Forearc is the site of oblique plate convergence between the Indo-Australian and SE Asian-Eurasian Plates. It is generally accepted that the forearc sliver is not behaving as a rigid plate and that the rate of slip increases along the right-lateral Sumatran Fault System from southeast to northwest. Two contrasting hypotheses have been invoked to explain this pattern of decoupling: by using slip vectors to suggest arc-parallel stretching; and a second major right-lateral strike slip zone, parallel to the Sumatran Fault System, to accommodate the oblique subduction. This zone, termed the Mentawai fault zone, lies just to the east of the outer-arc ridge and is indicated as intersecting with other faults on Nias Island. In this paper data are presented from Nias where a thick forearc succession is excellently exposed in three subbasins with half-graben geometries and broadly comparable Palaeogene-Recent histories. Mud diapirism has generated melanges which cut the Oligocene to Recent strata. Outcrop data, LANDSAT, Synthetic Aperture Radar and aerial photographs were used to determine fault offset patterns, and marked extension of the island along north-striking right lateral faults and ESE striking left-lateral faults is revealed. Extension was further accommodated along a set of ENE striking normal faults. In the region where the proposed Mentawai fault zone comes onto Nias there is an important, long-lived, basin-bounding fault. This structure had in excess of 5 km normal throw during the Oligocene-Miocene whilst minor contractional reactivation of the fault occurred during the Pliocene phase of uplift and deformation which affected all of Nias. Data from onshore and offshore the Nias region suggest that all the features visible from seismic sections collected over the Mentawai fault zone to the south of Nias can be explained in terms of inversion of originally extensional structures and mud diapirism. It is suggested that strike-slip motion is of limited importance along the 600 km long Mentawai fault zone. Rather this highly structured zone represents a deformation front whose origin can be explained by Pliocene to Recent subduction-driven inversion at the outer margin of the forearc.