New paleomagnetic constraints on the extrusion of Indochina: Late Cretaceous results from the Song Da terrane, northern Vietnam

Samples were collected for paleomagnetic investigations from the Upper Cretaceous Yen Chau Formation in the Song Da terrane (21.7°N, 103.9°E) bounded by the Ailao Shan–Red River fault system and Song Ma fault, in an attempt to examine extrusion tectonics of East Asia. Primary nature of the high-temperature (600–690 °C) component of magnetization from 13 sites is supported by a positive fold test. The tectonic corrected data provides the characteristic Late Cretaceous paleomagnetic direction for the Song Da terrane (D=6.4°, I=32.0° with α95=8.5°, N=13 sites), corresponding to a paleopole lying at 82.9°N, 220.7°E (A95=6.9°). Comparison with coeval paleomagnetic poles for the neighboring tectonic blocks indicates no latitudinal translation of the Son Da terrane with respect to the South China Block, whereas the Shan-Thai Block and the southern part of the Indochina Block revealed a southward displacement of 10.5±9.5° in latitude with respect to the Song Da terrane. We conclude that the southeastern segment of the Ailao Shan–Red River fault system to the east of the Dien Bien Phu fault is not a demarcation of the extruded Indochina Peninsula. The Indochina Block extruded probably along some faults between the Song Da terrane and the Khorat Plateau.