A PALEOMAGNETIC STUDY OF MESOZOIC SEDIMENTS FROM THE JUNGGAR AND TURFAN BASINS, NORTHWESTERN CHINA

Mesozoic sedimentary rocks have been collected from the Karamay area (northwestern Junggar block) and in the Turfan basin (northeastern Tarim block) for a paleomagnetic study with the aim of testing the rigid nature of these blocks and to better constrain their paleogeographic relationships within central Asia prior to the collision with India. The results from the Karamay area are disappointing, Lower Cretaceous grey sandstones having apparently been recently remagnetized. This remagnetization is carried by magnetite, and yields highly clustered normal polarity directions. Both a medium (MTC) and a high (HTC) unblocking temperature component are isolated from flat-lying or weakly tilted Jurassic red sandstones. The MTC could be carried by maghemite and the HTC is carried by hematite. The two components are statistically identical, and seem to correspond to an ancient magnetization. However, there is no fold test and comparison with previous data from Junggar and the reference APWP of Siberia does not allow us to decide between a Middle Jurassic primary magnetization and an Upper Jurassic or Cretaceous remagnetization. Stronger constraints are obtained from Upper Jurassic to lower Eocene fine-grained red sandstones from anticlines that have been thrust southwards, in the Turfan area of South Tien Shan. After the elimination of a recent overprint that is probably related to weathering, a high coercivity, high unblocking temperature component carried essentially by hematite is isolated. All directions from the Jurassic to the lower Eocene, which yield positive fold and reversal tests, are statistically indistinguishable. This is compatible with a primary origin for these magnetizations, similar to what has previously been found in Junggar, Tarim and western Tibet. The corresponding Turfan pole is not statistically distinct from that of Tarim for the same period, in accordance with the notion of a large, quasi-rigid Tarim block remaining attached to Eurasia in a rather constant position and at a constant latitude throughout the second half of the Mesozoic. This new datum can therefore be included to derive a new mean pole for the Tarim that is valid for the period from the Middle Jurassic to the end of the Cretaceous: 68.4 degrees N, 224.5 degrees E (A(95) = 3.1 degrees).