Tectonic evolution of the South Sumatra–Java forearc system from deep seismic reflection data

The South Sumatra–Java subduction zone marks the central part of the Sunda Subduction system, where the Indo-Australian plate subducts beneath the Eurasian plate. This subduction zone consists of a distinct accretionary wedge complex, a forearc high that is subaerially exposed at some places, and a deep forearc basin. In this thesis, I have analyzed newly acquired high-resolution deep seismic reflection data that provide detailed seismic images of structures and depositional sequences within the forearc system. These data have helped us to shed light up on the evolution of the basin, origin of major structures within the forearc basin–forearc high area, the development of the forearc high islands, and segmentation of seismic ruptures along this margin. Deformation in the South Sumatra forearc basin is associated with compressional structures along the margin. Folds-thrusts developed in the early Paleogene shelf and formed a NW-SE trending structural high. Subsequently, a N–S fault separated depocenters of the basins, marked by offset of the paleo-shelf edge, followed by deposition of Lower Oligocene–Lower Miocene sediments in broad NNE–SSW to NE–SW half grabens. In the Early Miocene, folds-thrusts formed in the lower slope, suggesting compressional deformation farther seaward. Later on, uplift of the continental margin reached its peak, as indicated by the erosional surface farther seaward. Accretionary wedge system has developed in the Early–Middle Miocene by the formation of landward-vergent backthrust and seaward-vergent thrusts seaward of the forearc basin. The Late Miocene–Pliocene compression triggered further uplift of the accretionary wedge and folds-thrusts in forearc basin sediments. The Mentawai Fault Zone (MFZ) lies at the center of the forearc basin in the southeast and in the front of the forearc high islands in the northwest. The MFZ forms a backthrust along the boundary between the accretionary wedge and continental backstop. On the sea floor the MFZ exhibits arcuate ridges convex toward northeast, formed by landward-vergent folds-thrusts. These compressional structures developed in-sequence deformation, which represents the deformation in the front of the inner part of accretionary wedge. Compressional structures along the MFZ, which are also disturbed by diapirs and mud volcanoes, represent motion along a doubly vergent accretionary wedge margin. Uplift and deformation in the subduction front appear to be associated with the subducting Investigator Fracture Zone (IFZ) and other bathymetric highs on the oceanic plate. Tilting of the forearc basement and its overlying sediments is also recognized above the prolongation of subducting highs, suggesting the role of the subducting bathymetric high in the emergence of the forearc high islands. Furthermore, localized circular uplift in the shelf–slope area is also observed above a deep subducted seamount observed beneath the forearc mantle. The forearc basement appears to have been translated by a N–S deep-seated fault in the center of the South Sumatra forearc basin, which coincides with segmentation of seismic rupture. Segmentation of shallow thrust earthquake ruptures during 2000–2008 in the South Sumatra margin appears to have been associated with barriers due to subducting IFZ in the northwest and structures in the upper plate in the southeast. Compressional phase is recognized during the latest Paleogene by the development of folds-thrusts along the slope of the Java forearc. Subsequently, landward-vergent backthrust and seaward-vergent imbricated thrusts formed the accretionary wedge in the seaward margin of the forearc basin during Early–Middle Miocene. Continuous contraction of the subduction zone since Late Miocene induced landward-vergent folds-thrusts in the front of the accretionary wedge, here referred to as the Offshore South Java Fault Zone, which represent the growth of the inner part accretionary wedge toward north. Subducting bathymetric highs have induced further localized uplift of the forearc high and landward deflection of the subduction deformation front.