Shihu Li

Clockwise rotations recorded in redbeds from the Jinggu Basin of northwestern Indochina

Paleomagnetic data have been used to infer clockwise rotations and significant southward motion of the Indochina block during Cenozoic extrusion from the India-Asia collision zone. Because the Cenozoic of the Indochina block has been sparsely sampled to date and is key to determining the timing of this tectonic motion, we performed an extensive paleomagnetic study on Paleocene to Oligocene redbeds and middle Miocene sandy silts from the Jinggu Basin (23.5°N, 100.7°E), in northern Indochina. Paleomagnetic results from the redbeds pass fold tests, but they show exclusively normal polarity. There is controversy, however, on the age assignment to the lower part of the succession. If the age of the redbeds is indeed Paleogene, this indicates a prefolding remagnetization associated with Oligocene deformation of the Indochina block. If the age of the redbeds would be Late Cretaceous, their magnetization may be primary, and deposition could have taken place during the Cretaceous normal superchron. However, the abundance of secondary hematite in the redbeds in combination with the biostratigraphy, which indicates a Paleogene age for at least the upper two formations of the redbeds in the Jinggu Basin, implies a pervasive remagnetization. The middle Miocene sediments pass both the fold test and the reversals test and contain magnetite as well as hematite as carriers, suggesting a primary magnetization. Our large data set from the redbeds (>2000 paleomagnetic directions) demonstrates an ∼30°−35° clockwise rotation of the Jinggu Basin with respect to Eurasia, for both scenarios, i.e., when compared to a ca. 100 Ma pole (if the redbeds carry a primary natural remanent magnetization) or to a ca. 30 Ma pole (if the sequence is remagnetized). The middle Miocene results, however, indicate that the Jinggu Basin experienced no significant (2° ± 5.6°) rotation with respect to Eurasia. Since no major deformation has occurred within northern Indochina during Late Cretaceous to Eocene times, our results reflect a major clockwise rotation of the Indochina block during its Oligocene to early Miocene extrusion from the India-Asia collision zone.

Cenozoic Rotation History of Borneo and Sundaland, SE Asia Revealed by Paleomagnetism, Seismic Tomography, and Kinematic Reconstruction

Abstract SE Asia comprises a heterogeneous assemblage of fragments derived from Cathaysia (Eurasia) in the 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‐Triassic amalgamation of continental and arc fragments now found in Indochina, the Thai Penisula, Peninsular Malaysia, and Sumatra. On Borneo, the Kuching Zone formed the eastern margin of Sundaland since the Triassic. To the SE of the Kuching Zone, the Gondwana‐derived continental fragments of SW Borneo and East Kalimantan accreted in the Cretaceous. South China‐derived fragments accreted to north of the Kuching Zone in the Miocene. Deciphering this complex geodynamic history of SE Asia requires restoration of its deformation history, but quantitative constraints are often sparse. Paleomagnetism may provide such constraints. Previous paleomagnetic studies demonstrated that Sundaland and fragments in Borneo underwent vertical axis rotations since the Cretaceous. We provide new paleomagnetic data from Eocene‐Miocene sedimentary rocks in the Kutai Basin, east Borneo, and critically reevaluate the published database, omitting sites that do not pass widely used, up‐to‐date reliability criteria. We use the resulting database to develop an updated kinematic restoration. We test the regional or local nature of paleomagnetic rotations against fits between the restored orientation of the Sunda Trench and seismic tomography images of the associated slabs. Paleomagnetic data and mantle tomography of the Sunda slab indicate that Sundaland did not experience significant vertical axis rotations since the Late Jurassic. Paleomagnetic data show that Borneo underwent a ~35° counterclockwise rotation constrained to the Late Eocene and an additional ~10° counterclockwise rotation since the Early Miocene. How this rotation was accommodated relative to Sundaland is enigmatic but likely involved distributed extension in the West Java Sea between Borneo and Sumatra. This Late Eocene‐Early Oligocene rotation is contemporaneous with and may have been driven by a marked change in motion of Australia relative to Eurasia, from eastward to northward, which also has led to the initiation of subduction along the eastern Sunda trench and the proto‐South China Sea to the south and north of Borneo, respectively.