Structural framework of the Sunda Shelf and vicinity /

by Ben-Araham, Zvi

The Sunda Shelf is one of the mO.'t extensive coherent shelves in t'~e world. A geophysical iìurvey was conducted over the sou':.:hern Sunda Shelf (Java Sea). Water depth, sediment thickness, and the gravity and magnetic fields were continuously measured. Expendable radiosonobuoys were used fo:= seismic refraction measurements ~ These geophysica 1 data supplemented by earl:.er studies over the northern Sunda Shelf and geological data from land areas provide a con?rehensive picture of the structural framework of the entire Sunda Shelf. In addition, structural studies over some oft-he 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 sea s . 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 ãre more circular and seem to result from tensiolia: forces. Radiosonobuoys' revea led sma 11 basement features and resolved many strata having different velocities. .... . .. " \ '- Faults 3re abundant throughout eie Sunda Shelf and clearly cont~ol the distribution and shapes of the basins. The faults s-irike north-south in the T.iestern Java Sea and northeast-southwest in the eastern Ja\,a Sea. A major discontinuity J.:r:ending north-south (te::i-=d here the Natuna Rift in the r.orthern Sunda Shelf and the Billiton Depression in the centi~'il Sunda Shelf) cuts the 3tructures of the entire Sunda Shelf. The discontinuity conti iues south across Central Java to the deep ocean floor. Analysi 3 of magnetic anomalies ~lOWS that the area can be divided into severa 1 distinct magnf~tic provinces that do not always follow the major structural units mapped by the seismic refl-:ction data. These magne-cic provinces coincide with corresp)nding provinces of lithii: units. The gravity field over ~Je central and southern S~nda Shelf averages around +30 mjal. Local gravity anomalies having relative amplitude of lO-25 mgal are superimpo::3ed on the regional background l,:vel. Although the local gravity anomalies were helpful in r,=solving the upper crustal structures, the cause for the rela-t.ively high regional gravity is unknown. The striictura 1 elements on the Sunda Shelf are interpreted as th= result of past interaction between the Indian Ocean-Austra lian, Pacific, and Asian plates. The evolution of the Sunda Shelf during the Mesozoic resulted from horizontal differen-t.ial 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. 3 Thesis Supervisor: Dr. K. O. Emery Title: Senior Scientist