SE Asia comprises a heterogeneous assemblage of fragments derived from Cathaysia (Eurasia) inthe north and Gondwana in the south, separated by suture zones representing closed former ocean basins.The western part of the region comprises Sundaland, which was formed by Late Permian-Triassicamalgamation of continental and arc fragments now found in Indochina, the Thai Penisula, PeninsularMalaysia, and Sumatra. On Borneo, the Kuching Zone formed the eastern margin of Sundaland since theTriassic. To the SE of the Kuching Zone, the Gondwana-derived continental fragments of SW Borneo and EastKalimantan accreted in the Cretaceous. South China-derived fragments accreted to north of the KuchingZone in the Miocene. Deciphering this complex geodynamic history of SE Asia requires restoration of itsdeformation history, but quantitative constraints are often sparse. Paleomagnetism may provide suchconstraints. Previous paleomagnetic studies demonstrated that Sundaland and fragments in Borneounderwent vertical axis rotations since the Cretaceous. We provide new paleomagnetic data fromEocene-Miocene sedimentary rocks in the Kutai Basin, east Borneo, and critically reevaluate the publisheddatabase, omitting sites that do not pass widely used, up-to-date reliability criteria. We use the resultingdatabase to develop an updated kinematic restoration. We test the regional or local nature of paleomagneticrotations againstfits between the restored orientation of the Sunda Trench and seismic tomography imagesof the associated slabs. Paleomagnetic data and mantle tomography of the Sunda slab indicate thatSundaland did not experience significant vertical axis rotations since the Late Jurassic. Paleomagnetic datashow that Borneo underwent a ~35° counterclockwise rotation constrained to the Late Eocene and anadditional ~10° counterclockwise rotation since the Early Miocene. How this rotation was accommodatedrelative to Sundaland is enigmatic but likely involved distributed extension in the West Java Sea betweenBorneo and Sumatra. This Late Eocene-Early Oligocene rotation is contemporaneous with and may havebeen driven by a marked change in motion of Australia relative to Eurasia, from eastward to northward, whichalso has led to the initiation of subduction along the eastern Sunda trench and the proto-South China Sea tothe south and north of Borneo, respectively.
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