Cretaceous - Miocene sedimentary rocks of northern Borneo preserve records of subduction of the Paleo-Pacific and Proto-South China Sea, providing important evidence for reconstructing the tectonic evolution of Southeast Asia since the Mesozoic. However, the genesis and tectonic setting of these sediments remain controversial. In this study, new Sr isotope, combined with Nd isotope data were used to determine the provenance contribution of the Cretaceous – Late Eocene Lubok Antu mélange and the Rajang Group. Detrital zircon ages and sedimentary geochemistry data of the Cretaceous - Miocene strata are also used to better understand the tectonic evolution of Borneo. Results show that more than 60% of the sediments came from a magmatic belt during the Late Cretaceous to Early Paleocene, and more than 50% from the Malay Peninsula during the Paleocene to the Late Eocene. The proportion of different detrital zircon ages and sedimentary geochemical characteristics in Borneo changed from west to east during the Cretaceous to the Miocene, which may be related to drainage changes caused by the gradual closure of an ocean basin. Subduction ceased in central Borneo during the Early Paleocene, slightly later than Late Cretaceous cessation in western Borneo. The collapse of magmatic belt lead river drainages from the Malay Peninsula to flow into Borneo. Whereas subduction continued in Eastern Borneo until the Miocene. Opening of the South China Sea cut off the drainage from the Malay Peninsula, and the inner rocks in Borneo once again became the main source of sediments.

The continental margin south to central Vietnam is notable for its high elevation plateaus many of which are covered by late Cenozoic basalt flows. It forms the westernmost margin of a wide continental rift of the South China Sea (East Vietnam Sea), and uplift has been considered a result of either rifting or younger intraplate basalt magmatism. To investigate margin development apatite thermochronometry was applied to a dense array of samples collected from across and along the margin of south to central Vietnam. Results, including thermal history models, identified a distinct regional episode of fast cooling between c. 37 and 30 Ma after which cooling rates remained low. The fast cooling coincides with a period of fast extension across the South China Sea (East Vietnam Sea) region that preceded continental break-up recorded by Oligocene grabens onshore. A thermal model is used test different processes that might influence the inferred cooling including a distinct pulse of exhumation; a decrease in exhumation followed by an associated transient decrease in geothermal gradients and, underplating coincident with rifting. Thermal relaxation following Mesozoic arc magmatism is ruled out as geotherms returned to background rates within 20 Myrs of emplacement, well before the onset of fast cooling. Models support fast cooling attributed to accelerated erosion during early stages of rifting. Some additional heating from either underplating, and/or hot mantle upwellings is also possible. No evidence was found to support regional uplift associated with the intraplate magmatism, enhanced monsoon-driven erosion or seafloor spreading dynamics