Deformation of the northern Sumatra accretionary prism from high-resolution seismic reflection profiles and ROV observations

Following the 2004 Great Sumatran–Andaman Mw ≈ 9.2 earthquake, high-resolution seismic reflection, multibeam bathymetric and remotely operated vehicle data were acquired to investigate the tectonic framework of the rupture zone and search for evidence of seafloor and near-surface displacement. Three distinct regions off northern Sumatra were investigated; 1) a portion of the Sunda Trench, 2) the adjacent frontal deformation zone, and 3) the seaward flank of the Aceh (forearc) Basin. A thick (> 1.5 km) sediment section within the Sunda Trench shows evidence of shallow normal faulting, possibly representing early stages of assimilation into the accretionary wedge. The frontal deformation zone consists of ridges of predominantly landward-verging thrust folds. Seaward-verging backthrust faults at or near the base of the steep slope commonly reach the seafloor. We do not observe a single, laterally extensive structural offset at the deformation front that might be interpreted as contributing to the 2004 tsunami. Rather, a series of small-offset (tens of metres) faults were noted across this broad zone of the frontal accretionary wedge. The western boundary of the Aceh (forearc) Basin is the West Andaman strike-slip fault, juxtaposing the accretionary complexʹs forearc high with basin fill sediments. Neither the seismic nor ROV data show evidence of recent seafloor displacement along the fault trace. Basin infill demonstrates consistent along-strike patterns of tilting and seaward subsidence during sedimentation, while modern fill is flat-lying and coherent across the entire basin. Intercalated chaotic layers interpreted as mass transport deposits may record a history of seismicity, but recent examples of such deposits were not observed on the modern seafloor, either seaward of the deformation front or in the Aceh Basin. Lack of any evidence of faulting, offset or disruption of sediments within Aceh Basin suggests that there was little impact of the 2004 earthquake in this area. Distributed faults throughout the frontal deformation zone, combined with observations of landward-verging folds at the deformation front, folding within piggy-back basin sediments, and lack of evidence of disruption along the West Andaman Fault zone and within the forearc basin all support strain partitioning across the margin. A proposed strong wedge interior may act as a backstop during major thrust events, constraining deformation to the frontal deformation zone and the slope apron. Tsunami generation in response to the 2004 event did not result from surficial displacements along a single fault or narrow fault zone at the toe of the deformation front, but was more likely a result of vertical displacement across the entire outer forearc.