Linking the Tectonic evolution of Hainan Island and Borneo since the Paleozoic: Implication for Interaction Between the Tethyan and Paleo‐Pacific Tectonic Domains in Southeast Asia

The interaction between the Tethyan and Paleo-Pacific subduction systems in Southeast Asia since the Paleozoic is still poorly understood. Hainan Island and Borneo, located near the junction of the Tethyan and Paleo-Pacific domains, provide the critical regions for understanding the dynamic processes and interaction between the two tectonic domains. Using new and published zircon U-Pb-Hf isotopes, and the detrital zircon Eu/Eu* data, we evaluate crustal evolution and the relative crustal-thickness variations in both regions. The data sets indicate broadly synchronous intervals of reduced apparent crustal thickness inferred from Eu/Eu* at 440–410 Ma, 370–350 Ma, 270–220 Ma and 160–140 Ma, together with largely negative εHf(t) values through the Paleozoic-Triassic, which characterized by the mixed mantle-derived material and older crustal components, probably associated with the Paleo-Pacific subduction. Jurassic–Cretaceous zircons from Borneo shift to mainly positive εHf(t), supporting the mantle-derived material addition and a Tethyan–Paleo-Pacific double-subduction system beneath Sundaland. However, the Hainan retains mostly negative εHf(t), consistent with continued Paleo-Pacific subduction and older crustal components. These results clarify how the Tethyan and Paleo-Pacific systems interacted in Southeast Asia.