TRIASSIC PALAEOMAGNETIC CONSTRAINTS ON PALAEOGEOGRAPHIC SIBUMASU AND INDOCHINA IN RECONSTRUCTIONS OF THE PANGAEA SUPERCONTINENT

Palaeogeographic reconstructions of East Asia in Pangaea are notably in dispute among various models proposed by different authors, and the lack of reliable and quantitative palaeomagnetic constraints, in particular, from the Southeast Asian terranes, seem to be the main reason leading to the controversy over palaeopositions of Southeast Asia in Pangaea. Here we report new palaeomagnetic results from Permian to Triassic rocks from western Yunnan, Myanmar, and Thailand with an aim to locate the Sibumasu and Indochina blocks in the formation of Pangaea, and then to discuss when and where the majority of the East Asian blocks and/or terranes collided each other as well as the relationship between East Asia and the main of Pangaea. Characteristic Remanent Magnetisations (ChRMs) isolated from Permian sediments, middle Triassic limestones from the Simao terrane, and middle Triassic limestones from the Baoshan terrane are interpreted as Cretaceous remagnetisations based upon their directional similarity to that of Cretaceous and negative fold test results or slight deterioration in directional grouping after tilt correction. Whereas the Triassic mafic volcanics from the Jinggu arc and a few middle Triassic limestones from the Simao terrane, Late Triassic basalts from the Baoshan terrane, and some Triassic limestones from the Shan State terrane in Myanmar all yield stable ChRMs through stepwise AF and thermal demagnetisation on volcanic rocks or hybrid thermal and AF demagnetisations on limestones. These ChRMs are interpreted as primary magnetisations in the light of detailed rock magnetic experiments and field stability tests and indicate that both the Sibumasu and Indochina blocks were located around ~10°N in the Late Triassic so that the two blocks may have collided to be a unit by the Late Triassic and the Eastern Palaeo-Tethys Ocean may have closed no later than the Late Triassic, which is compatible with recent palaeontological and geochemical constraints.