From normal to oblique subduction: Tectonic relationships between Java and Sumatra

The convergent motion of the Indian-Australian and the Eurasian Plates results in subduction at the Sunda Arc. Obliquity of subduction beneath Sumatra induces large strike-slip faults in Sumatra and its margin, whereas the subduction is almost perpendicular to the trench southwest of Java. The nature of the transition between these two subduction regimes is of major interest. New data collected with the Indonesian R.V. Baruna Jaya III, show that the Cimandiri Fault Zone of west Java continues out to sea. Sinistral activity seen on land, can be the conjugate of dextral strike-slip faulting along a NW-SE prolongation of the Sumatra strike-slip fault in the forearc domain. A structural transition is occurring south of the Pelabuhan Ratu Gulf and may therefore correspond to a change in the subduction regime. To the west, oblique subduction induces partitioning of the motion into convergent motion and northwestward strike-slip motion. To the east, opposite Java, subduction is normal and typical forearc basin develops. In the transition area, the curvature of the margin induces a northwestward increase of the obliquity of subduction and consequently of the lateral component of the partitioned motion. North-westward, displacement of the forearc domain results in internal extensional deformation and ablation of accreted sediments south of the Sunda Strait, explaining the concave shape of the deformation front.