PALEOMAGNETISM OF CRETACEOUS TO LOWER TERTIARY ROCKS FROM SOUTHWESTERN SICHUAN - A REVISIT

A re-study has been carried out on the Cretaceous to lower Tertiary rocks from southwestern Sichuan province on the South China Block. The characteristic remanent magnetization (ChRM) resolved from the Paleocene-Eocene Leidashu Formation is of dual polarity and very shallow inclination. The anomalously low paleolatitude (6.9 +/- 6.3-degrees-N) indicated by the inclination is not supported by geological evidence from this region and is probably due to inclination error related to depositional and/or post-depositional processes. A positive fold test, as well as the prevalence of normal polarity, suggests that the ChRM of the Mid-Upper Cretaceous Xiaoba Formation might be acquired during the Cretaceous Long Normal Superchron (CLNS). The tilt-corrected mean direction of the Xiaoba Formation (D/I = 8.1-degrees/38.8-degrees, alpha-95 = 6.6-degrees, N = 18 sites) is not significantly different from a preliminary result (D/I = 357.6-degrees/31.6-degrees, alpha-95 = 8.6-degrees, N = 12 samples) we reported earlier for the collective Xiaoba and Leidashu formations. The resultant paleopole (81.5-degrees-N/220.9-degrees-E, A95 = 7.1-degrees) is coincident with other Cretaceous poles for the Chinese blocks. Comparison with the Eurasian apparent polar wander path (APWP) indicates neither significant rotation nor latitudinal translation of the study area relative to Eurasia. The newly obtained mean direction for the Xiaoba Formation is believed to be more reliable than our earlier result since it is based on a larger sample size and is apparently free from effects of the shallow inclination observed in the Leidashu Formation, and should supercede our previous preliminary result. The paleopole derived from seven sites of the Lower Cretaceous Feitianshan Formation (69.0-degrees-N/204.6-degrees-E, A95 = 4.3-degrees) deviates from the track from the Late Jurassic to Late Cretaceous poles; the deviation might be reduced by further sampling.