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Abstract The Mesozoic subduction history of the Paleo‐Pacific plate below the East Asian margin remains contentious, in part because the southern part is poorly understood. To address this, we conducted a sediment provenance study to constrain Mesozoic subduction history below West Sarawak, Borneo. A combination of detrital zircon U‐Pb geochronology, heavy minerals, trace element, and bulk rock Nd isotope data were used to identify the tectonic events. The overall maturity of mineral assemblages, dominantly felsic sources, abundant Precambrian‐aged zircons, and low εNd(0) values (average −13.07) seen in Late Triassic sedimentary rocks suggest a period of inactive subduction near Borneo. Slab shallowing subduction occurred between 200 and 170 Ma based on subdued magmatism and tectonic compression across West Sarawak. From c. 170 to 70 Ma there was widespread magmatism and we interpret the Paleo‐Pacific slab steepened. Collectively, we show the Paleo‐Pacific plate subduction had variable slab dip histories in Borneo. 

Abstract 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. 

Abstract Most mélanges in exhumed subduction‐accretion complexes are polygenetic, recording significant information about the nature of geological processes during their formation. Here, we apply micro‐chemical analysis and illite K‐Ar dating to constrain the deformation mechanism and timing of the pervasively sheared scaly matrix in the accretionary complex rocks presently known as “Kenting Mélange” in the Hengchun Peninsula (South Taiwan). Our results reveal that parts of the matrix were formed in Cretaceous (96.7 ± 8.6 Ma and 108 ± 18.4 Ma) due to pressure solution. These new, older matrix ages suggest that the Kenting Mélange, which was considered as Cenozoic and interpreted to have been associated with the subduction of the South China Sea, preserves different primary chaotic units (e.g., mélange and/or olistostrome). Our findings imply the Kenting Mélange is actually polygenetic and allow part of Kenting Mélange that we named the “proto‐Kenting Mélange” to be interpreted as a remnant of a primary mélange, which was mixed and/or juxtaposed in the Cenozoic Kenting Mélange. The block‐in‐matrix fabric with a pervasively sheared scaly muddy matrix, along with the preservation of slightly older oceanic crust blocks, suggests that the proto‐Kenting Mélange is most likely an ocean plate stratigraphy mélange. This unit initially formed near a Paleo‐Pacific subduction margin during the latest Early Cretaceous. Our results reveal a nearly 3000‐km‐long physical archive of latest Early Cretaceous subduction–accretion processes, which took place adjacent to the continental margin of East Asia during the consumption of Paleo–Pacific ocean floor during the latest Mesozoic.