Preliminary paleomagnetic study on Late Paleozoic to Early Mesozoic rocks in Indochina and its paleogeographic implications

The Indochina block is one of the main blocks in East Asia and its paleogeographic positions in formation of the Pangea supercontinent are still in debate because of the lack of reliable Late Paleozoic to Early Mesozoic paleomagnetic constraints.Here we present some preliminary Late Paleozoic to Early Mesozoic paleomagnetic results from the Simao block, the northern part of the Indochina block in West Yunnan of China. Following detailed rock magnetic and paleomagnetic experiments on a total of 205 drilled samples in 20 sites, characteristic remanent magnetizations (ChRMs) were isolated from most samples following thermal or integrated thermal and alternating field (AF) demagnetization. Results from 40 samples of the Permian sediments yield a mean paleomagnetic direction of Dg=27.6°, Ig=41.5°, k=82.1, α95=2.7° before and Ds=45.2°, Is=35.8°, k=66.4 ,α95=3.0° after tilt correction. Following magnetic fabric analysis and thin sections analysis with microscope, strong tectonic deformation and medium degree metamorphism were identified in these samples. Results from 65 samples of Middle Triassic limestones yield a mean direction of Dg=46.9°, Ig=38.4°, k=31.8, α95=2.5° before and Ds=52.2°, Is=46.5°, k=35.6°, α95=2.4° after tilt correction. General recrystallization was founded in these samples following thin section analysis with microscope. These two mean directions, with slight change in data grouping after tilt correction, appear to have a secondary origin.However, mean direction of 24 samples obtained from the other Middle Triassic limestones profile is of Dg=45.6°, Ig=70.0°, k=76.4, α95=3.1° before and Ds=82.5°, Is=22.4°, k=68.7°, α95=3.3° after tilt correction, corresponding to a paleopole at 182.7°E,11.3°N with A95=3.3°. We interpret this direction as primary remanence in the light of its significant difference with Cretaceous poles, no significant metamorphism founded on hand-samples and thin sections. Besides, mean direction of 40 samples of mafic volcanics drilled in Jinggu arc is of Dg=171.2°, Ig=7.4°, k=7.6, α95=29.6° before and Ds=163.2°, Is=16.2°, k=19.6°, α95=17.7° after tilt correction, these ChRMs are of dual polarity and pass fold and reversal tests indicating a pre-folding origin, which implies that the Jinggu arc may be apart from the Simao block and nearly 80° clockwise rotated relative to the Simao block in the Late Triassic. Preliminary paleomagnetic results reported here indicate that the Simao block, which is thought to be the northern part of the Indochina block, may have located at ~10°N in the Late Triassic.