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 Are. Obliquity of subduction benea th Sumatra induces large strike-slip faults in Sumatra and its margin, whereas the subduction is almost perpendicular to the trench southwes t of Java. The nature of the transition between these two subduction r egimes is of major interest. New data collected with the Indonesian R. V. Baruna Jaya III, show that the Cimandiri Fault Zone of west Java co ntinues out to sea. Sinistral activity seen on land, can be the conjug ate of dextral strike-slip faulting along a NW-SE prolongation of the Sumatra strike-slip fault in the forearc domain. A structural transiti on is occurring south of the Pelabuhan Ratu Gulf and may therefore cor respond to a change in the subduction regime. To the west, oblique sub duction induces partitioning of the motion into convergent motion and northwestward strike-slip motion. To the east, opposite Java, subducti on is normal and a typical forearc basin develops. In the transition a rea, the curvature of the margin induces a northwestward increase of t he obliquity of subduction and consequently of the lateral component o f the partitioned motion. North-westward, displacement of the forearc domain results in internal extensional deformation and ablation of acc reted sediments south of the Sunda Strait, explaining the concave shap e of the deformation front.