Palaeomagnetic evidence for southward displacement of the Chuan Dian fragment of the Yangtze Block

Cretaceous red sandstones of the Feitianshan Formation and the Xiaoba Formation were sampled at 33 sites from the Dadeli and Mishi synclines of Xichang (27.9degreesN, 102.3degreesE). The study area is a part of the Chuan Dian fragment bounded by the Xianshuihe-Xiaojiang and the Red River fault systems, which in turn constitute the southwestern part of the Yangtze Block. Almost all the samples give a characteristic palaeomagnetic direction with unblocking temperatures up to 680degreesC. The primary nature of magnetization is ascertained by a positive fold test with a 99 per cent confidence level for the Dadeli Syncline. The tilt corrected overall mean direction of the 33 sites is D=3.7degrees, I=41.5degrees (alpha(95)=3.4degrees), with a corresponding palaeopole at 85.2degreesN, 241.7degreesE( A(95)=3.5degrees). This pole occupies the near-sided position with respect to the estimate of the Cretaceous pole of the Sichuan Basin, indicating that the Xichang area experienced a significant southward displacement. Combined with earlier reported palaeomagnetic data from the Chuan Dian fragment, a significant southward displacement of 6.7+/-3.5 in latitude is estimated for the whole fragment with respect to the Sichuan Basin since the Late Cretaceous. Extrusion dynamics in the Asian continent due to its collision with India brought about the southward displacement of the Chuan Dian fragment. Declination data indicate that the southern part of the Chuan Dian fragment was subjected to clockwise sense rotation of up to 45degrees. This significantly large tectonic rotation probably occurred during extrusion of this fragment from the north.