Abstract
Shallow palaeomagnetic inclinations have been frequently reported from the red beds of central and southeast Asia. To trace the origin of this phenomenon, inclination variation in southeast Asia is examined on lithological to locality scale. Lithological aspect of this variation has been studied using the Early Cretaceous Bailong and Cangxi formations of the Bazhong area (32.1 degrees N, 106.7 degrees E), the northern Sichuan Basin. Samples from 36 sites, consisting 18 pairs of successive sandstones and mudstones layers, were collected for this purpose from a synclinal structure. Stepwise thermal demagnetization of most samples revealed the presence of stable characteristic remanent magnetization, which is generally unblocked by 680 degrees C. Positive fold and reversal tests suggest a primary origin for this component, yielding the Early Cretaceous palaeomagnetic direction of declination/inclination = 20.9 degrees/26.5 degrees (ks = 37.2, alpha(95) = 4.4 degrees, N = 30). 10 pairs of sandstone and mudstone layers show almost identical inclinations (I-sandstones = 23.3 degrees +/- 3.7 degrees and I-mudstones = 24.7 degrees +/- 2.4 degrees), but 27 degrees shallower than that expected from the Eurasian apparent polar wander paths (APWPs), indicating that no lithological variation in inclination has occurred. Location-wise variation in inclination shallowing is examined through palaeomagnetic data from Late Jurassic to Eocene red beds distributed around southeast Asia. Based on these investigations, no inclination shallowing is observed in the eastern part of the South China Block (SCB), whereas large degree of shallowing is observed in the Sichuan and Xining-Ninhe basins. Variation in inclination shallowing from one sedimentary basin to another could probably be caused by changes in the depositional environment. The eastern part of the SCB, where no inclination shallowing have been observed, is characterized by Basin and Range type tectono-geological setting. In contrast, the foreland basins are bounded to the north by east-west striking high altitude orogenic belt. Swift development of these basins as a result of flexural subsidence is the most likely reason for inclination shallowing.