The tectonostratigraphic evolution of SE Asia

A new model for the Tertiary tectonic and stratigraphic evolution of SE Asia is presented integrating stratigraphic data from many of the major sedimentary basins in the area. The model can be divided into four phases. (1) Stage I (50–43.5 Ma) during which the India-Eurasia collision was initiated and proceeded contemporaneously with oceanic subduction beneath southern Eurasia. The continental collision caused a slow-down in the oceanic spreading rates in the Indian Ocean reducing the convergence velocity along the Sunda Arc subduction system and resulting in a phase of extension in the adjacent fore-arc and back arc areas. The isolated rift basins in the fore-arc and the East Java area were filled with transgressive then open marine sediments since these basins were on the low-lying edge of the Sunda craton whereas the mid-Eocene sea was not able to penetrate in to the more cratonic backarc rift basins of Sumatra or NW Java which were filled with fluvio-lacustrine sequences. (2) Stage II (43.5–32 Ma) was triggered by the termination of oceanic subduction beneath the India-Eurasia collision zone. This locked up the spreading system in the Indian Ocean and caused a major plate reorganization effective in the Indian, Southern and Pacific oceans. The plate reorganization in the Indian Ocean slowed convergence rates yet again along the Sunda Arc producing a second phase of rifting and rift basin deposits. The plate reorganization in the Pacific Ocean resulted in the onset of extension in the South China Sea and the deposition of fluvio-lacustrine sequences in the isolated rift basins whereas in the East Kalimantan area a failed rift system in the Makassar Straits resulted in isolated rift basins filled with deltaic and marine sequences which were overlain by more extensive post-rift marine shales. The first major collision of the Luconia Shoals block with a subduction system along the NW Borneo margin resulted in the deposition Balingian delta sequence. (3) Stage III (32–21 Ma) is contemporaneous with the first phase of seafloor spreading in the South China Sea during which the entire continental area south of a line bisecting and joining the South China Sea and the Gulf of Thailand rotated clockwise about a pole near the head of the Gulf of Thailand. This block rotation formed the Malay Basin within which a large fluvio-lacustine sequence was deposited. The rotation also produced a phase of increased plate convergence rates and inversion along the Sunda Arc ending the rifting in these basins. The subsequent sedimentary fill in these basins records a marine transgression which is interpreted to be partially eustatic in origin and partly a result of the subsidence associated with the onset of post-rift thermal sag phases. Elsewhere in East Kalimantan extensive marine and carbonate sequences were deposited whilst in the South China sea the incursion of marine environments led to the deposition of inboard transgressive deltaic and marine deposits and marine and carbonate deposits offshore in more distal settings. (4) Stage IV (21–0 Ma) was initiated by the cessation of the first phase of seafloor spreading in the South China Sea caused by the collsion of the Baram block with the NW Borneo subduction system. This event is interpreted to be contemporaneous with the onset of continental shortening in Tibet via block rotation and lateral extrusion along strike-slip faults. Major collisions, an inevitable result of the particular configuration established by the plate-reorganization at 43.5 Ma, occurred throughout this stage in the NW Borneo, Sulawesi and Timor areas and together with the rotation of Sumatra, and extrusion-related wrench faulting in the Malay-West Natuna and Bac Bo/Yinggehai Basin areas resulted in a extensive structural invertsion and highly variable basin fill sequences. A eustatic high in the middle Miocene marks a period of maximum marine transgression onto the Sunda craton and the deposition of extensive marine shale deposits. This was followed by dominantly regressive sequences puctuated by major eustatic sea-level falls at about 5 and 10 Ma. Particular structural styles and eustatic models are discussed in relation to the proposed model and the distribution of hydrocarbons in the Tertiary basins of southeast Asia is presented. This distribution is explained in terms of the different types of source rocks present in the different basins.