Abstract
Along the Sumatran subduction zone, oblique strain is partitioned between the Sunda megathrust and the strike-slip Sumatran Fault located along the volcanic arc. Additional strike-slip motion may be localized adjacent to the forearc basin system, forming the Mentawai and West Andaman Fault Zones. An alternative interpretation of the West Andaman Fault Zone, as primarily a backthrust, has also been proposed.

We combine profiles from an NSF-funded 2D seismic survey (SUMUT) with previous bathymetry and MCS surveys to characterize the West Andaman Fault Zone adjacent to the Aceh forearc Basin. Pre-stack time migration of SUMUT profiles provides the clearest images to date of this portion of the West Andaman Fault Zone. In these images we interpret inward-dipping fault segments along the West Andaman Fault Zone indicative of a transpressional flower structure. A reflector at the base of the Aceh Basin sediments appears to continue trenchward of the Fault Zone for >15 km and was interpreted previously as a regional-scale Oligocene/early Miocene unconformity. Within the Fault Zone this reflector is disrupted, with some segments raised and some lowered. The geometry of this reflector across the Fault Zone and further seaward is inconsistent with the geometry we would expect if a major backthrust were to daylight at the edge of the basin. Based on this observation and the configuration of faults in a flower structure, we conclude that the West Andaman Fault Zone where imaged is predominantly strike-slip in nature. As such, the West Andaman is likely part of a system of faults including the Sumatra Fault that accommodate the significant shear component of strain in this oblique subduction zone. Additionally, the location of the West Andaman Fault within the forearc may be controlled by the rheologic contrast between the marginal plateau and the forearc basin.