PALEOMAGNETIC STUDY OF CRETACEOUS ROCKS FROM THE YANGTZE BLOCK, CENTRAL YUNNAN, CHINA - IMPLICATIONS FOR THE INDIA-ASIA COLLISION

More than 200 samples for this study were collected at 30 sites from the Cretaceous Matoushan and Puchanghe Formations around the city of Chuxiong (25-degrees-N,101.5-degrees-E). Twenty-four sites have characteristic directions with a high temperature component above 350-degrees-C. Paleomagnetic directions from 21 sites of the Cretaceous formations reveal clockwise deflection in declination (D = 44.6-degrees, I = 41.3-degrees, and alpha95 = 10.7-degrees). The reliability of the direction is ascertained through a positive fold test, and the presence of normal and reversed polarities. Comparison with paleomagnetic data from other areas indicates that the Chuxiong area was rotated clockwise through 26-degrees + 17-degrees with respect to the eastern part of the Yangtze block, whereas it was subjected to more than 44-degrees of clockwise rotation with respect to the neighboring blocks of Lhasa (56-degrees +/- 21-degrees) and western Sichuan (44-degrees +/- 16-degrees) since Cretaceous time. The clockwise rotation of Chuxiong is a reflection of rotational motion of the large region including the southeastern Qiangtang Terrane and the Jinsha suture-Red River fault. Paleomagnetic data and tectonic features suggest that the rotation occurred prior to the folding in the central Yunnan province, probably during the Paleogene. This large rotation is a spectacular example of the deformation in the southeastern part of the Asian continent due to the collision of India.