The Relationship Of Tectonic Evolution Between The Proto-South China Sea And The South China Sea Basin

The South China Sea is located at a juncture among the Eurasia, the Indian-Australia, and the Pacific plates, and therefore influenced by the Tethys Tectonic Domain on the west and the Pacific Tectonic Domain on the east at the same time, which leads to its development under a complex tectonic setting. In addition, the effect of its resources and environment has also been widely concerned at home and abroad. In these years, researches on lithospheric deep structure, superficial structural deformation, magmatism, sedimentary basin evolution, sea floor spreading era, and paleomagnetism of the South China Sea region have made some progress, which provides important constraints for understanding the tectonic evolution of the South China Sea region during the Meso-Cenozoic. The Proto-South China Sea was an important tectonic unit during the formation process of the South China Sea. However, previous tectonic evolution models of the South China Sea rarely consider the impact of the Proto-South China Sea, especially its subduction and closure. The location and characteristics of the Proto-South China Sea, and the records, time, way, and mechanism of its subduction process, which are essential for plates reconstruction of the southeast Asia, are also rarely studied.In order to address the above issues, this paper discussed the subduction process of the Proto-South China Sea, combined with sedimentary formation, magmatism, relics of oceanic crust in the active continental margin of the Proto-South China Sea and structural analysis of northwestern Kalimantan. Furthermore, through comparing the subduction process of the Proto-South China Sea with the onset, spreading and subdution of the South China Sea, we explore the relationship between tectonic evolution of the Proto-South China Sea and the South China Sea, thereby revealing the dynamics of Cenozoic evolution of the Southeast Asia. We gain some insight through the study:1. The Proto-South China Sea was a late Mesozoic continental marginal sea which developed in the transition part of the Tethys Ocean and Paleo-Pacific Ocean. Throuth analysis of ophiolite melange, magmatite and sedimentary formation of the northern Kalimantan, we recognize an Jurassic-Cretaceous ocean basin, the Proto-South China Sea, on the north of Kalimantan, and it was subducted beneath the Kalimantan Block since the Late Cretaceous.2. In a regional context, we interpret the closure of the Proto-South China Sea occurred in a ’scissor-like’ fashion, with the deformation younging progressively from southwest to northeast. Thus, the Proto-South China Sea in Sarawak had been completely closed in late Eocene when the Luconia Block collided with West Borneo along Lupar Suture, resulting in the formation of a regional unconformity on Rajang Group in Sarawak, central Kalimantan and southern Sabah. The sedimentary environment changed from deep to shallow sea in Sarawak region. While deep sea turbidite developed in Sabah area through to the Miocene, indicating a remnant of Proto-South China Sea still existed in this area. In early Miocene, the collision of the Nanshan Block on the east of the West Baram Line and the Kalimantan Block led to the finally closure of the Proto-South China Sea, and the formation of a regional unconformity surface on Crocker Group.3. The South China Sea formed between 32 and 16 Ma, opening from NE to SW gradually.23Ma is an important time, when the rifting of the northern continental margin basins of the South China Sea terminated and the Southwestern Subbasin began to open due to the southward jump of the mid ocean ridge, thereby forming the unconformity T6o in the basins of the southern South China Sea.4. The South China Sea formed under dextral transtensional stress field in the East Asia continental margin during 32-16 Ma, and the drag effect of the southward subduction of the Proto-South China Sea promoted the disintegration and southward drifting of the Nansha-Palawan micro-block from the South China Block. In early Paleogene, the closure of the Proto-South China Sea resulted in the collision of the Nanshan Block and the Kalimantan Block, while the South China Sea was still spreading. In early Miocene (16 Ma), the continuous northward movement of the Austrilian Plate led to the transition of the stress field from dexatral transtension into sinistral transcompression in the East Asia margin, finally ending the spreading of the South China Sea.