Orogenic events in Southern South China Sea and their relationship with the subduction of the Proto South China Sea

The original location of the Proto South China Sea and the processes related to its subduction and termination have remained under debates, and no consensus has been reached up to date. It is not only closely related to the expansion of the South China Sea, but also significant to the study of the geodynamics of the region. After a thorough review of previous literatures, this paper is devoted to the orogenic processes and their bearing on the subduction of the Proto South China Sea. The results show that the tectonic activities in the Southern South China Sea could be divided into two phases. The first phase of the activities happened from Early Cretaceous to Late Cretaceous , while the oceanic crust of the Paleao-Pacific subducted below the island of Borneo. The subduction zone was located along the current Lupal line, which caused the Zengmu-Nansha block to move towards the southwestern Borneo. It was closed in Late Cretaceous by a collision orogeny. Since Borneo itself is composed of many blocks and suffered multiple collisions during the period of Eocene, there were the deformation and reorganization of blocks after collision. Eventually, the final phase of the deformation (i.e. the Sarawak Movement) was completed in Late Eocene (37 Ma). The second phase of movement was from Late Eocene (35 Ma) to Middle Miocene (15.5 Ma). The Proto South China Sea from east of the West Baram Line to Cagayan of Philippines, was subducting eastward from the West Baram Line towards the island of Borneo. It swooped and then spread to Sabah and the area to the south of Palawan and stopped at the Mindoro Island, the Philippines. The resulting drag force was driving the expansion of the South China Sea. Similar to the effect of the subduction of the Paleo-Pacific plate, the subduction of the Proto South China Sea triggered the closure of the sea between the Borneo Island and the Nansha block in Middle Miocene (15.5 Ma), causing the collision between the Nansha block and the Borneo island in Sabah (i.e. the Saba orogen) and the collision between the Palawan micro-continent and the Philippine island arc (i.e. the Palawan collision). The unconformities are widely distributed in this region as the product of this subduction process in the Southern South China Sea. They can also be observed on the Palawan Island. Nowadays, the Nansha Trough and the Palawan Trough are no longer regarded as the foreland of the subduction zone. The former is the tectonic response to the gravity-driven deformation of the Sabah Neogene sedimentary wedge, while the latter is the collision forefront generated by the southern South China Sea. Strong subsidence occurs under the action of late accretion wedges. The real subduction zones are located respectively in the southeast of the Nansha Trough and in the southeast of the Palawan Trough. There is evidence supporting the hypothesis that subduction process of Proto South China Sea ceased about 10 Ma ago in the Mindanao of the Philippines, thus closed the entire Paleo-South China Sea.