Structure, evolution and tectonic activity of the eastern Sunda forearc, Indonesia, from marine seismic investigations

Forearc structures of the eastern Sunda Arc are studied by new multichannel reflection seismic profiling. We image a high along-strike variability of the subducting oceanic plate, the interface between subducting and overriding plate, the accretionary wedge, the outer arc high and forearc basins. We highlight ongoing tectonic activity of the entire outer arc high: active out-of-sequence thrust faults connecting the plate interface with the seafloor, slope basins showing tilted sedimentary sequences on the outer arc high, vertical displacement of young seafloor sediments, and tilted sedimentary sequences in the Lombok forearc basin. While frontal accretion plays a minor role, the growth of the outer arc high is mainly attributed to oceanic sediments and crustal fragments, which are attached to the base of the upper plate and recycled within the forearc. We image ongoing large-scale duplex formation of the oceanic crust. The incoming oceanic crust is dissected by normal faulting into 5–10 km wide blocks within a 50–70 km wide belt seaward of the deep sea trench. These blocks determine the geometry and evolution of duplexes attached to the base of the overriding plate landward of the trench. Long-lasting and ongoing subsidence of the Lombok Basin is documented by distinct seismic sequences. In the Lombok Basin we image mud diapirs, fed from deeply buried sediments which may have been mobilized by rising fluids. We propose a wrench fault system in the eastern Lombok forearc basin that decouples the subduction regime of the Sunda Arc from the continent–island arc collision regime of the western Banda Arc. The observed tectonic activity of the entire forearc system reflects a high earthquake and tsunami hazard, similar to the western part of the Sunda Arc.

Research highlights
► The eastern Sunda forearc is imaged by new multichannel seismic reflection profiling.
► Duplex formation at crustal scale is found at the upper/lower plate interface.
► Splay faults and tilted piggy-back basins show tsunami hazard.
► Forearc growth is due to oceanic underplating and recycling within the outer arc high.
► Mud diapirs in the forearc basin are mobilized by rising fluids.