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 fr om two areas (Lanping and Yunlong) of the western part of Yunnan province, China, in the northern part of Indochina. The high-temperature magnetizatio n component with an unblocking temperature of around 680 degreesC is isolat ed after stepwise thermal demagnetization. Characteristic directions from n ine accepted sites reveal a positive fold test at the 99% confidence limit, indicating that the high-temperature magnetization component is possibly o f Eocene age. A tilt-corrected mean direction from nine accepted sites is D =266.1 degrees, I=-39.8 degrees with alpha (95) = 11.2 degrees, correspondi ng to a paleopole at 14.5 degreesN, 169.7 degreesE with A(95) = 10.9 degree s A comparison with previously reported Tertiary paleomagnetic data indicat es that the Lanping basin of the northern part of the Indochina block rotat ed clockwise through 91 degrees (+/-15 degrees) 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 Cretac eous paleomagnetic results from the Yunlong area indicate that about 40 deg rees of clockwise rotation out of 90 degrees is explained by tectonic rotat ion due to a relatively rigid body rotation of the Simao Terrane during ind entation of India. The remaining component of rotation is probably ascribed to further Eocene/post-Eocene rotational motion of the tectonically indepe ndent small blocks within the Lanping basin associated with movement. (C) 2 001 Elsevier Science B.V. All rights reserved.