Inclination variation in the Late Jurassic to Eocene red beds from southeast Asia: lithological to locality scale approach

Details Inclination variation in the Late Jurassic to Eocene red beds from southeast Asia: lithological to locality scale approach Inclination variation in the Late Jurassic to Eocene red beds from southeast Asia: lithological to locality scale approach

Aug 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011geoji.186..471s&link_type=abstract

Geophysical Journal International, Volume 186, Issue 2, pp. 471-491.

Mathematics

Logic

Palaeomagnetism Applied To Tectonics, Palaeomagnetism Applied To Geologic Processes, Rock And Mineral Magnetism

Scientific paper

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°N, 106.7°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 °C. Positive fold and reversal tests suggest a primary origin for this component, yielding the Early Cretaceous palaeomagnetic direction of declination/inclination = 20.9°/26.5° (ks= 37.2, α95= 4.4°, N= 30). 10 pairs of sandstone and mudstone layers show almost identical inclinations (Isandstones= 23.3°± 3.7° and Imudstones= 24.7°± 2.4°), but 27° 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.

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