Large southward motion and clockwise rotation of Indochina throughout the Mesozoic: Paleomagnetic and detrital zircon U-Pb geochronological constraints

We report a combined paleomagnetic and U-Pb geochronologic study of sedimentary rocks from the Huai Hin Lat and Nam Phong formations of Mesozoic age in NE Thailand in order to provide independent constraints on the tectonic movement of the Indochina Block during convergence of the major blocks now comprising East Asia. The maximum allowable depositional age of the two formations is estimated to be 227 Ma and 215 Ma, respectively, from detrital zircon U-Pb geochronologic analysis which also indicates a sediment source transition in the Khorat Plateau Basin during the Middle-Late Jurassic. A formation mean paleomagnetic direction of Dg/Ig=21.4°/38.1° (kg=19.5, α95=9.6°) before and Ds/Is=43.0°/48.0° (ks=47.4, α95=6.1°, N=13) after tilt correction is derived from samples with different lithologies, bedding attitudes, magnetic carriers and polarities and yields a positive fold test. Hence, the magnetization is likely primary. The revised Mesozoic APWP of the Indochina Block yields paleolatitudes (for a reference site of 22°N, 102°E) of 33.4 ± 7.2°N during the Norian Late Triassic, 25.9 ± 5.9°N during the Late Triassic to Early Jurassic, 23.9 ± 8°N during the Late Jurassic to Early Cretaceous, 27.5 ± 3.2°N during the Early Cretaceous and 24.5 ± 4.9°N by the Late Cretaceous; corresponding declinations are 45.2 ± 8.6°, 38.0 ± 6.6°, 36.3 ± 8.8°, 29.6 ± 3.6° and 24.9 ± 5.4° respectively. These data indicate a significantly southward displacement accompanied by clockwise rotation during the Mesozoic. A reconstruction of the Indochina Block within the now well-studied merging process of South China and North China indicates that the Indochina Block was located at a higher latitude than the South China Block during the Norian stage of Late Triassic times whilst no significant relative poleward displacement apparently occurred during the Early Jurassic to Early Cretaceous interval. Our study supports a post-Cretaceous tectonic extrusion model with a southeastward displacement of Indochina with respect to the South China Block estimated to be 1000 ± 850 km since the Late Cretaceous.