Interaction Between the Tethyan and Paleo‐Pacific Tectonic Domains in Southeast Asia: Late Triassic Initiation of an Inward‐Dipping Double Subduction System

Interaction between Tethys and the Paleo‐Pacific subduction zones in Southeast Asia during the Mesozoic remains poorly understood. Using new and published zircon U‐Pb and Hf data sets from Borneo (Paleo‐Pacific domain) and Sumatra (Tethyan domain), we propose that isotopically juvenile magmatism was active on both sides of Sundaland due to the initiation of inward‐dipping double subduction during the latest Triassic when Indochina collided with Sibumasu, as evidenced by a pronounced positive shift in zircon εHf(t) values from both Cenozoic sedimentary successions and Mesozoic magmatic rocks in Sumatra and Borneo. From the latest Triassic to Cretaceous, the contrasting positive εHf(t) values ranges between Borneo and Sumatra, with Borneo showing a broad range and Sumatra a narrower variability, imply that the inward‐dipping double subduction system evolved asymmetrically due to differences in slab dip angles between the subducting Meso‐Tethys and Paleo‐Pacific oceanic lithosphere. After 80 Ma, this asymmetric double subduction system was disrupted, marked by the complete cessation of arc magmatism in Borneo while isotopically juvenile magmatism continued on Sumatra. Our findings emphasize that, when compared to the contemporary single‐sided subduction system of the western Meso‐Tethyan domain and the northern Paleo‐Pacific domain, SE Asia developed more juvenile crust due to large‐scale upper plate extension driven by inward‐dipping double subduction.