Age distributions of monazites in the Late Cretaceous to Late Eocene turbidite from northwestern Borneo and its tectonic setting

摘要

Late Cretaceous to Late Eocene turbidite is widely distributed in the northwestern part of Borneo. It is known as the Rajang Group. Sand samples were collected from the four rivers that flow through this stratum. More than 4000 monazite grains were analyzed by electron microprobe analyzer to obtain their ages. The monazite age distributions of the four rivers show three main peaks at 200-300 Ma, 400-500 Ma and 1850-1900 Ma, and a weak cluster at 700-1100 Ma. Such age distributions show that the detrital grains were not supplied from the Indochina Peninsula, but from the South China Craton. There is a huge unconformity, called the Sarawak Orogeny, which formed after the deposition of the Rajang Group. The Sarawak Orogeny is considered to be formed at Latest Eocene, close to the time of the opening of the South China Sea that began as early as the Early Oligocene. Hence, it is probable that the northwestern Borneo, which is situated close to South China, has been moved to its current position during the opening of the South China Sea. There is no clear tectonic event after the opening of the South China Sea between northwestern Borneo and the central part of the Indonesia Archipelago including southern Borneo. Assuming that Borneo and at least the central Indonesia Archipelago were moved at the time of the opening of the South China Sea, the reduced connection between the Indian and Pacific oceans during the Miocene and also the Wallace line for fauna and flora may be explained more simply by the present model than by the other reconstruction models.