A CRETACEOUS POLE FROM SOUTH CHINA, AND THE MESOZOIC HAIRPIN TURN OF THE EURASIAN APPARENT POLAR WANDER PATH

To contribute to the apparent polar wander path (APWP) of the South China Block and Eurasia in general, we collected paleomagnetic samples from Mesozoic red beds around the city of Ya'an (30-degrees-N, 103-degrees-E) in the western tip of the Sichuan Basin. In this paper we present the results from 373 oriented cores taken from one section representing 3 km of sedimentary rocks. The section is dated with continental ostracods and with a magnetostratigraphic correlation between a densely sampled 272-m sequence and the polarity time scale, giving an upper Jurassic to Upper Cretaceous or Lower Tertiary age. The remanent direction is remarkably stable throughout the section (D = 2.0-degrees, I = 34.2-degrees, k = 63.1, alpha-95 = 3.6-degrees, N = 26/28 sites). While this fact might suggest that the section has been remagnetized, paleomagnetic and rock magnetic tests indicate that the remanence is primary. The pole position (78.6-degrees-N, 273.4-degrees-E, dp = 2.4-degrees, dm = 4.1-degrees) corresponds to a rather low paleolatitude (lambda = 18.8-degrees +/- 2.4-degrees) but is consistent with other Cretaceous poles from China. If one accepts the Eurasian APWP of Irving and Irving (1982), this result would imply that more than 1000 km of shortening took place between South China and Eurasia, following the acquisition of the remanence. However, there is no geological evidence for this large shortening. We propose that the remanence was acquired within the time corresponding to the tip of the hairpin turn (approximately 150-50 Ma) in the revised APWP of Besse and Courtillot (this issue). The local geology suggests that the syncline from which the samples were taken has been rotated by 15-degrees +/- 5-degrees counterclockwise, which is reflected in a similar discrepancy between the measured paleodeclination and that predicted by the Besse and Courtillot (this issue) Eurasian APWP. After correcting for this rotation, the pole position is 70.9-degrees-N, 225.2-degrees-E (dp = 2.4-degrees, dm = 6.5-degrees). We conclude that Eurasia was fully assembled by the end of the Jurassic and that the Mesozoic Eurasian hairpin turn is a real feature.