NEW CRETACEOUS PALEOMAGNETIC POLES FROM THE TARIM BASIN, NORTHWESTERN CHINA

Most paleomagnetic studies of the Tarim have been focused on the Paleozoic with little data published for Mesozoic rocks. However, the ancient configuration of this part of Asia has been affected by the Indian-Eurasian collision. During the second French-Chinese Karakorum expedition, from Kashkar to Kunjerab, in the Autumn of 1990, we sampled about 500 cores from 53 sites of Upper and lower Cretaceous red sandstones from three areas near Uytak. Yingjisha and Wuqia (38.5-39.5-degrees-N, 75-76.5-degrees-E). Isothermal remanent magnetization acquisition indicates the presence of minerals with high magnetic coercivity. Stepwise thermal demagnetization confirms that hematite is the dominant carrier and allowed the separation of low temperature components (LTC, < 300-degrees-C) from high temperature components (HTC, > 600-degrees-C) in most samples. The mean direction of LTCs is aligned with that of the present Earth field. Normal and reversed polarities have been found for the HTCs of each age group. HTCs show positive fold tests for at least one sub-study where the attitudes of bedding were sufficiently different to allow the test to be performed. More important, the for sub-studies from Yingjisha and Wuqia give fully compatible results. Results from the Uytak area show a wide dispersion in declinations. as could be expected from the rather severe deformation and proXiMitY to major thrust and Strike-slip faults. Data obtained in the Kuche area from northern Tarim by Li and co-workers and in southern Junggar by Chen and co-workers are statistically identical to the new data from Yingjisha and Wuqia. Data from Paicheng in northern Tarim appear to have been subject to local rotation. Excluding those sites which appear from paleomagnetic study to have been significantly rotated, we calculated a mean Cretaceous pole for the Tarim basin at 69.1-degrees-N-degrees, 223.8-degrees-E (A95 = 3.5-degrees, N = 5). This result indicates that the total post-Cretaceous rotation of Tarim relative to Junggar and to Eurasia has been negligible (2.6 +/- 0.1-degrees and 0.4 +/- 4.6-degrees, respectively). Continental shortening and rotation between Tarim and Junggar, within the Tien Shan, are not resolved by paleomagnetism although the values obtained (400 +/- 600 km, 3 +/- 6-degrees) are compatible with tectonic models of the range. A major discrepancy, warranting further geological investigation, is that between the Tarim and North China block poles, which would imply some 1400 +/- 1060 km of north-south relative displacement.