Tertiary paleomagnetic data from northwestern Yunnan, China: further evidence for large clockwise rotation of the Indochina block and its tectonic implications

Eocene red sandstones were collected at 15 sites for paleomagnetic study from two areas (Lanping and Yunlong) of the western part of Yunnan province, China, in the northern part of Indochina. The high-temperature magnetization component with an unblocking temperature of around 680°C is isolated after stepwise thermal demagnetization. Characteristic directions from nine accepted sites reveal a positive fold test at the 99% confidence limit, indicating that the high-temperature magnetization component is possibly of Eocene age. A tilt-corrected mean direction from nine accepted sites is D=266.1°, I=−39.8° with α95=11.2°, corresponding to a paleopole at 14.5°N, 169.7°E with A95=10.9°. A comparison with previously reported Tertiary paleomagnetic data indicates that the Lanping basin of the northern part of the Indochina block rotated clockwise through 91° (±15°) with respect to the Yangtze craton. This large magnitude clockwise rotation is probably caused by the superimposition of regional and local rotations. Our previous middle Cretaceous paleomagnetic results from the Yunlong area indicate that about 40° of clockwise rotation out of 90° is explained by tectonic rotation due to a relatively rigid body rotation of the Simao Terrane during indentation of India. The remaining component of rotation is probably ascribed to further Eocene/post-Eocene rotational motion of the tectonically independent small blocks within the Lanping basin associated with fault movement.