Crustal structure of the central Sunda margin at the onset of oblique subduction

The convergent margin of the central Sunda Arc in Indonesia was the target of a reflection and refraction seismic survey conducted in 1998 and 1999. A long two seismic lines across the subduction complex off southern Sumatra a nd off Sunda Strait, coincident multichannel and wide-angle data were colle cted, complemented by two refraction strike-lines in the forearc basin off Sumatra. The combined analysis of the acquired data allows us to present a detailed model of the subduction zone where initiation of strain partitioni ng occurs due to the onset of oblique subduction. The dip of the subducted plate is well defined along both dip-lines and a lateral increase from 5 de grees to 7 degrees from beneath the outer high off Sumatra to Sunda Strait is supported by complementary gravity modelling. The downgoing slab is trac ed to a depth of more than 30 km. On both reflection dip-lines, a clearly d eveloped backstop structure underlying a trench slope break defines the lan dward termination of the active accretionary prism and separates it from th e outer high. Active subduction accretion is supported by laterally increas ing velocities between the deformation front and the active backstop struct ure. Seismic velocities of the outer high are moderate along both lines (<5 .8 km s(-1) at 20 km depth), suggesting a sedimentary composition. Reduced reflectivity beneath a rugged top basement traced along the outer high of b oth dip-lines supports a high degree of deformation and material compaction . Several kilometres of sediment has accumulated in the forearc domain, alt hough a distinct morphological basin is only recognized off southern Sumatr a and is not developed off Sunda Strait. The bathymetric elevation of the J ava shelf that is encountered in the southern Sunda Strait corresponds to i ncreased velocities of a basement high there and is connected to extensiona l structures of the Sunda Strait transtensional basin. Differences observed in the morphology of the forearc domain are also reflected in the lower cr ustal structure. Off southern Sumatra, the velocity-depth model clearly ind icates a continental-type crust underlying the forearc basin, whereas lower velocities are found beneath the Sunda Strait forearc domain. Off Sumatra, some 3-D constraint on the upper plate structure is gained from the refrac tion strike-lines, which in addition is supported by synthetic data modelli ng.