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

Abstract

S U M M A R YShallow 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, α 95 = 4.4 • , N = 30). 10 pairs of sandstone and mudstone layers show almost identical inclinations (I sandstones = 23.3 • ± 3.7 • and I mudstones = 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.

Oroclinal origin of the Simao Arc in the Shan-Thai Block inferred from the Cretaceous palaeomagnetic data

“…These are compatible to a difference between the observed and expected Cretaceous inclinations (7.4 • ± 2.6 • ) reported from the Shan-Thai Block (K1 and K2 in Table 2), which has been declared primary as a result of the positive fold test. This compatibility between the present and previously reported data sets (Sato et al 2011) confirm a widespread presence of steep inclinations in the region, which tectonically suggest a southward displacement of the Shan-Thai Block (Leloup et al 1995).…”

Paleomagnetism of the Late Cretaceous Red Beds From the Far Western Lhasa Terrane: Inclination Discrepancy and Tectonic Implications

“…Even though the site mean direction of the Jingzhushan Formation passes McElhinny (1964) fold test, the mean inclination calculated from 31 paleomagnetic sites of the northern limb of folds ( I s = 19.0°) is significantly lower than that of 23 paleomagnetic sites of the southern limb of folds ( I s = 51.8°) (Table 1). This large discrepancy of the mean inclination of the two limbs of folds may be attributed to several factors: (1) the depositional and/or compaction of sedimentary rocks may cause the inclination shallowing (Ding et al, 2015; Kodama, 2012; Tan et al, 2010; Tauxe & Kent, 2004); (2) the original strata were not horizontal but inclined when the sedimentary rocks got magnetization, so the inclination obtained from the two limbs of folds was quite different (Sato et al, 2011; Tong et al, 2019); (3) the chemical remanent magnetization (CRM) was acquired after deposition of the red beds, which can obscure the primary component (Jiang et al, 2015; Kodama & Dekkers, 2004); (4) the remanence directions obtained from the two limbs of folds may have been influenced by the distortional strains (Cogné et al, 1991; Kanamatsu & Herrero‐Bervera, 2006; Kligfield et al, 1983). The above four possibilities are discussed separately.…”

The Interaction of the Eastward Extrusion of the Songpan‐Ganzi Terrane and the Crustal Rotational Movement of the Sichuan Basin Since the Late Paleogene: Evidence From Cretaceous and Paleogene Paleomagnetic Data Sets of the Sichuan Basin

“…Because the paleomagnetic data sets used to construct the apparent polar wander path (APWP) of East Asia (Cogné et al, ) contain Cretaceous and Paleogene paleomagnetic data from the Sichuan Basin, the Cretaceous pole of the eastern edge of SCB is considered more appropriate for paleomagnetic studies of southwestern China (Tong, Sun, et al, ; Wang et al, ). In addition, the Cenozoic rotational deformation of the edge of the Sichuan Basin is much smaller than that of the southeastern edge of the Tibetan Plateau (Enkin, Chen, et al, , Enkin, Courtillot, et al, ; Sato et al, ; Tong, Sun, et al, ; Wang et al, ; Zhao et al, ). Thus, it is necessary to compare the amount of rotation calculated using the different APWPs of Europe (Besse & Courtillot, ; Torsvik et al, ) and the eastern edge of SCB (Tong, Sun, et al, ), respectively, in order to confirm whether the rotational characteristics relative to the different reference poles would change.…”

“…However, the Cenozoic crustal kinematic characteristics of the Sichuan Basin are still been strongly debated. During the past three decades, numerous paleomagnetic data obtained from the Cretaceous and Cenozoic red‐beds on the edge of the Sichuan Basin, such as the Bazhong and Tongjiang‐Wanyuan area on the northern edge (Sato et al, ; Wang et al, ; Zhao et al, ), the Yaan and Xinjin areas on the southwestern and western edges, respectively (Otofuji et al, ; Enkin, Chen, et al, , Enkin, Courtillot, et al, ; Zhao et al, ), and the Yibin area on the southern edge (Tong, Sun, et al, (Figure a), have revealed contradictory kinematic characteristics of clockwise or counterclockwise rotational deformation. Wang et al () suggested that the activity of the Xianshuihe strike‐slip fault drove the Sichuan Basin to experience integral counterclockwise rotational movement since ~13 Ma, which controlled the right‐lateral strike‐slip movement of the northeastern segment of the LMSFB.…”