Cretaceous palaeomagnetism of Indochina and surrounding regions: Cenozoic tectonic implications

Abstract

Abstract Results of a detailed palaeomagnetic study of Cretaceous-age volcanic, intrusive and sedimentary rock formations from southern Vietnam (24 sites, 163 core samples) are presented. The palaeomagnetic and supplementary rock magnetic studies indicate that magnetite and titanomagnetite are the predominant magnetic carriers in the volcanic and intrusive rock samples, whereas hematite is the principal carrier in the red-beds. The mean palaeomagnetic direction of twenty-one sites from southern Vietnam yields D = 14.5°, I = 33.3°, α 95 = 6.3°, k s / k g = 1.04, which corresponds with a VGP at λ = 74.2°N, φ = 171.1°E, A 95 = 5.9°. Comparison of the pole with the Eurasia mean Cretaceous palaeopole shows that relative to Eurasia southern Vietnam has experienced a southward displacement of 6.5° ± 5.1°, but with insignificant rotation since the Cretaceous. Previously reported Cretaceous palaeomagnetic data, combined with new palaeomagnetic data from this study and analysis of regional structural trends, indicate that Sundaland can be divided into several fault-bounded tectonic domains (Shan-Thai, Indochina, offshore Sundaland), each with a different rotation and/or translation history. Such differential motion might explain, for example, Oligocene transtension and basin formation in the Gulf of Thailand and central onshore Thailand (between the Shan-Thai and Indochina blocks). Our data combined with previously acquired palaeomagnetic data across Southeast Asia, also suggest that, during the Cenozoic, Indochina and parts of Sundaland underwent complex internal deformation and did not behave as a rigid block.