Abstract
The terminal collision of India with Asia brought about significant tectonic deformation into the Asian Continent. In order to depict these deformational features in the southeastern margin of the Indochina Peninsula, Late Cretaceous red beds of the Dak Rium Formation were sampled from Da Lat area (11.7°N, 108.3°E), the Kontum massif, Vietnam. Thermal demagnetization procedure revealed the presence of characteristic remanent magnetization (with unblocking temperatures around 680°C) in most samples. Primary nature of this magnetization is ascertained through positive fold test. A tilt-corrected mean direction calculated for the Dak Rium Formation is Dec/Inc=11.4°/35.4° (ks=331.7 α95=1.7°, N=21), with a corresponding paleopole at 76.5° N, 161.2° E (A95=1.7°). Compared with the European APWP, a negative inclination flattening (−15.2°±4.9°) is observed, indicating southward displacement of the Kontum massif by 9.2°±4.9° since Late Cretaceous. Since, comparable amount of southward displacement has been reported from 16 localities of the Indochina and Shan–Thai blocks, it seems a reasonable assumption to predict that the Indochina Peninsula underwent a southward displacement of 5.1°±2.4° (570km±270km) as a composite unit (including the Shan–Thai Block, Indochina Block and Kontum massif). On the contrary, 27.0°±9.8° counter-clockwise rotation of the Kontum massif with respect to the Indochina Block indicate a significant amount of internal deformation along the tip of south moving Indochina Peninsula. This rotation of the Kontum massif is attributed to 32–17Ma left lateral motion along the East Vietnam Boundary Fault, which makes up the 1000km long seaward continuation of the Ailao Shan–Red River shear zone. Tectonic deformation of the Indochina Peninsula was, thus, partly accommodated by local block rotation (e. g., counterclockwise rotation of the Kontum massif) and to some extent by internal tectonic deformation of the Shan–Thai Block.
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► The Indochina Peninsula experienced a southward displacement by 5.1°±2.4° between 32 Ma and 17 Ma. ► During this displacement the Kontum massif rotated CCW by 27.0°±9.8° within the Indochina block. ► Left lateral motion along the East Vietnam Boundary Fault brought about this CCW rotation.