The stationary Cretaceous paleomagnetic pole of Sichuan (South China Block)

Abstract

Upper Jurassic to lower Tertiary sites have been sampled from three localities, separated by up to 150 km, in the Sichuan Basin (∼30°N, 103°E) of the South China Block. Each locality has at least one stability test, but none alone gives a fully reliable result. When taken together, however, identical directions are found with a global average D=12.7°, I=34.2°, k=42.9°, α95=3.3° (N=44 sites) and corresponding pole 73.9°N, 234.4°E, dp=2.2°, dm=3.8°, paleolatitude λ=18.8°. There is no significant apparent polar wander (APW) with age, which is consistent with an acquisition of magnetization during the period of the stable tip of the Eurasian APW loop (∼130–70 Ma), as defined with poles from other continents transferred onto Eurasia. This age range is compatible with the available paleontological and magnetostratigraphic age constraints but is shorter than that suggested by the assigned formation ages. The paleolatitude of Sichuan is significantly lower (8.0°±4.0° at 90 Ma) than that predicted by the reference APW path, either because of northward shortening during the Tertiary or because of errors in the APW path. Cretaceous poles from other Chinese blocks are consistent with the Sichuan pole, but there is evidence that these blocks have suffered local rotations up to 10° during the Tertiary, probably as a result of the India‐Eurasia collision.

The Lower Cretaceous reference pole for North China, and its tectonic implications

“…Moreover, this would raise insuperable problems in Table 2. for the south-west of the Sichuan basin. The Upper Cretaceous pole reported by Enkin et al (1991) is located on the far side of the high-quality (Q > 4) mean pole (Table 2), which implies either the occurrence of northward intracontinental shortening (about 800 km) during the Tertiary in the Qinling fold belt or an error in the age of the rocks as suggested by the original authors (Enkin et al 1991). However, magnetic inclination shallowing due to the coarse grain size and compaction effects cannot be ruled out (e.g.…”

New Late Cretaceous and Paleogene paleomagnetic results from south China and their geodynamic implications

“…One group of poles was obtained from the coastal provinces of Fujian, Guangdong and Zhejiang, located between the Zhenhe‐Dapu and Changle‐Nanou faults; a second group of poles was obtained from the western part of Sichuan province, situated to the east of Longmenshan fault zone, or from the Chuan‐Dian fragment located between the Red River faults zones and the Xianshuihe‐Xiaojiang fault zones (Figure 1). Some poles have clearly suffered from local rotations about vertical axes, as suggested by the original authors [ Enkin et al , 1991; Gilder et al , 1993, 1999; Otofuji et al , 1998]. For example, the coastal provinces of Fujian and Guangdong are rotated clockwise with respect to the stable SCB (Figure 11a) [ Gilder et al , 1993].…”

“…These Chinese blocks are assumed to have been fully assembled with Eurasia and to have suffered no relative latitudinal movement by the Early Cretaceous [ Zheng et al , 1991; Huang and Opdyke , 1992; Gilder and Courtillot , 1997; Yang and Besse , 2001]. However, many local or regional vertical axis rotations have occurred in the SCB since the Cretaceous [ Enkin et al , 1991; Gilder et al , 1993; Huang and Opdyke , 1992; Otofuji et al , 1998]. Because of either the collision beginning at 55 Ma between India and Asia in the west [ Dewey et al , 1989] or the subduction of the circum‐Pacific plate beneath the Asian continental margin in the east since the Early Cretaceous, the SCB may have suffered from significant intercontinental deformation and block rotations.…”

“…Previous paleomagnetic studies of the Late Cretaceous in the SCB mainly concentrated on the continental margin of the SCB (the western Sichuan and coastal provinces) [ Enkin et al , 1991; Gilder et al , 1993; Chan , 1991; Huang and Opdyke , 1992] (Figure 1). These paleomagnetic poles from the continental margin show generally a distribution along a small circle centered in the sampling areas, indicating either the local deformations due to the influence of strike‐slip faults zones such as the Xianshuihe‐Xiaojiang fault system, the Red River fault system, or the Changle‐Nanou fault zone.…”

“…The early Tertiary poles reported by Zhao et al [1994] and Gilder et al [1993] were derived from 35 samples (five sites) and 17 samples (only one site), respectively, at a close area from Guangxi province (Figure 1). Previous studies also indicated that the Cretaceous paleolatitude of western Sichuan was significantly lower than that predicted by the poles of “stable” Eurasia [ Enkin et al , 1991; Huang and Opdyke , 1992], the discrepancy was resulted either from the northward shortening in the Qinling fold belt during the Tertiary or from errors in the reference apparent polar wander path (APWP) of Eurasia [ Enkin et al , 1991]. …”