Kinematic Evolution of The Rajang Group In the Sibu Zone, Sarawak, Borneo in the framework of the evolution of the South China Sea

Abstract

The island of Borneo has an interesting tectonic history. The formation of the Island is still not completely understood due to its complex evolution, but certain parts have been studied intensively for their economic value such as the offshore regions of Sarawak. Regions such as the onshore area of Sarawak have received less attention in research studies. One area that is still discussed is the accretionary prism of the Rajang group. This group consists mostly of turbidite sediments and has experienced multiple major deformation phases. Understanding the evolution of this prism can help with further understanding of the evolution of the island and its significance in the frame work of the South China sea. The aim of this study is first understood the mechanisms responsible for the deformation of the wedge such as oblique subduction and the transition from thin to thick-skinned faulting. Then we can determine the structural setting of the region and place the findings in a larger scale context to identify the kinematic evolution of the region. This has been done via field observations and further analysis of the data to determine the different deformation phases. The fieldwork and subsequent analysis identified four deformation phases, among which three are compressional and one extensional. These phases can be correlated to large scale geological events that have impacted the region. The results have been compared to existing models to create an updated kinematic evolution of the Rajang group. The first deformation phase is linked to the subduction of oceanic crust at the Lupar line. This subduction caused compression in the accretionary prism resulting in a clear foliation and isoclinal folding. During this period of deformation, multiple major lineaments similar to the Lupar line formed due to the southward movement of the South China plate and the counterclockwise rotation of Borneo. In the later stages of this compression, the Rajang accretionary prism formed its imbricate thrust system. The subduction stopped around 45 Ma when the Luconia block collided with the accretionary complex starting the Sarawak Orogeny. This collision caused major uplift in the region due to a combination of transpresisional movement and new thick-skinned faulting. This transpressional movement resulted in the creation of a second foliation and open folding throughout most of the Rajang group. At 32 Ma, the opening of the South China Sea started and initiated the third deformation phase. This phase is characterized by strike slip faulting throughout the prism. It reactivated the inherited major lineaments and created a new phase of strike slip deformation. The final extensional deformation recorded in this study is caused by the extensional collapse after the rapid uplift and is observed by normal faulting in the studied region.