Structural framework of the Sunda Shelf and vicinity

Abstract

The Sunda Shelf is one of the most extensive coherent shelves in the world. A geophysical survey was conducted over the southern Sunda Shelf (Java Sea). Water depth, sediment thickness, and the gravity and magnetic fields were continuously measured. Expendable radiosonobuoys were used for seismic refraction measurements. These geophysical data supplemented by earlier studies over the northern Sunda Shelf and geological data from land areas provide a comprehensive picture of the structural framework of the entire Sunda Shelf. In addition, structural studies over some of the deep-sea floors surrounding the Sunda Shelf are combined with those over the Sunda Shelf to develop an evolutionary scheme of the Sunda Shelf and adjacent deep seas. Seismic reflection profiles show that the Sunda Shelf consists of three major units: the northern Sunda Shelf basinal area, the Singapore Platform, and the Java Sea basinal area. In the northern Sunda Shelf are two large sedimentary basins, the Brunei and Gulf of Thailand basins, which are separated by the Natuna Ridge. In the Java Sea are several other basins separated by uplifts. The basins in the eastern Java Sea are narrow and long and seem to result from compressional forces, while those in the western Java Sea are more circular and seem to result from tensional forces. Radiosonobuoys revealed small basement features and resolved many strata having different velocities. Faults are abundant throughout the Sunda Shelf and clearly control the distribution and shapes of the basins. The faults strike north-south in the western Java Sea and northeast-southwest in the eastern Java Sea. A major discontinuity trending north-south (termed here the Natuna Rift in the northern Sunda Shelf and the Billiton Depression in the central Sunda Shelf) cuts the structures of the entire Sunda Shelf. The discontinuity continues south across Central Java to the deep ocean floor. Analysis of magnetic anomalies shows that the area can be divided into several distinct magnetic provinces that do not always follow the major structural units mapped by the seismic reflection data. These magnetic provinces coincide with corresponding provinces of lithic units. The gravity field over the central and southern Sunda Shelf averages around +30 mgal. Local gravity anomalies having relative amplitude of lO-25 mgal are superimposed on the regional background level. Although the local gravity anomalies were helpful in resolving the upper crustal structures, the cause for the relatively high regional gravity is unknown. The structural elements on the Sunda Shelf are interpreted as the result of past interaction between the Indian Ocean-Australian, Pacific, and Asian plates. The evolution of the Sunda Shelf during the Mesozoic resulted from horizontal differential movement in a north-south direction as both the Indian Ocean and Pacific plates were moving to the north. In Eocene time two major events affected the evolution of the Sunda Shelf: the direction of movement of the Pacific plate changed from north to west-northwest producing northeast- southwest trending structural elements in the eastern Java Sea, and a spreading ridge that previously existed in the deep sea south of the shelf (Wharton Basin) was subducted along the Java Trench.