Structure and morphotectonics of the accretionary prism along the Eastern Sunda-Western Banda Arc

Abstract

Seismic reflection profiles across the accretionary prism along the Eastern Sunda-Western Banda Arc reveal variation in structure that relates to the incipient collision with Australia. The morphology of the arc-trench system changes from ridged, south of Bali, Lombok and Sumbawa, to sloped south of Sumba. East of Sumba, the accretionary wedge is backthrust over the forearc basin, incorporating forearc sediments and basement. Frontal accretion in the Java Trench is characterized by imbricate thrusting of a “thin” pelagic sediment cover at the toe of the accretionary wedge. The morphology near the toe of the wedge appears to be controlled by faults in the subducting oceanic plate that strike parallel to the deformation front and may have originated in the outer-trench swell. The prism incorporates oceanic basement by trenchward verging thrusts, which cut across the accretionary prism and extend into the subducting slab. A comparison between the volume of the accretionary prism and the amount of sediment delivered to the Java Trench in the past 30 Ma shows that probably little of the sediment has been subducted. The decrease in width of the prism from Bali to Sumbawa corresponds to an eastward younging trend of the arc-trench system from late Oligocene to early Miocene. South of Sumba the width of the accretionary prism increases considerably, due to the accretion of thick continental margin carbonates which deform by thrust-bounded folds. Buoyancy of the partially subducted marginal Scott plateau increases basal shear stresses, adding to the growth of a large accretionary wedge. Further east, the subduction of thick continental crust results in even higher basal shear stresses that are distributed throughout the accretionary wedge. They cause the progressive development of backthrusts and internal deformation, leading to shortening and thickening of the wedge.