Palaeomagnetic evidence for southward displacement of the Chuan Dian fragment of the Yangtze Block

Abstract

S U M M A R YCretaceous red sandstones of the Feitianshan Formation and the Xiaoba Formation were sampled at 33 sites from the Dadeli and Mishi synclines of Xichang (27.9 • N, 102.3 • E). The study area is a part of the Chuan Dian fragment bounded by the Xianshuihe-Xiaojiang and the Red River fault systems, which in turn constitute the southwestern part of the Yangtze Block. Almost all the samples give a characteristic palaeomagnetic direction with unblocking temperatures up to 680 • C. The primary nature of magnetization is ascertained by a positive fold test with a 99 per cent confidence level for the Dadeli Syncline. The tilt corrected overall mean direction of the 33 sites is D = 3.7 • , I = 41.5 • (α 95 = 3.4 • ), with a corresponding palaeopole at 85.2 • N, 241.7 • E(A 95 = 3.5 • ). This pole occupies the near-sided position with respect to the estimate of the Cretaceous pole of the Sichuan Basin, indicating that the Xichang area experienced a significant southward displacement. Combined with earlier reported palaeomagnetic data from the Chuan Dian fragment, a significant southward displacement of 6.7 • ± 3.5 • in latitude is estimated for the whole fragment with respect to the Sichuan Basin since the Late Cretaceous. Extrusion dynamics in the Asian continent due to its collision with India brought about the southward displacement of the Chuan Dian fragment. Declination data indicate that the southern part of the Chuan Dian fragment was subjected to clockwise sense rotation of up to 45 • . This significantly large tectonic rotation probably occurred during extrusion of this fragment from the north.

The longest theropod trackway from East Asia, and a diverse sauropod-, theropod-, and ornithopod-track assemblage from the Lower Cretaceous Jiaguan Formation, southwest China

“…For calculation of hip heights and speed estimates, the methods of Alexander (1976), Thulborn (1990), and Gonz alez Riga (2011) were adopted. The different values estimated from these different methods were juxtaposed but are left undiscussed.…”

“…The relative stride length (SL/h) may be used to determine whether the animal is walking (SL/h 2.0), trotting (2 < SL/h < 2.9), or running (SL/h ! 2.9) (Alexander, 1976;Thulborn, 1990). The SL/h ratios of the Hanxi ornithopod trackways HXO1eO2 range from 2.06 to 2.20 (Table 1) and accordingly suggest a trotting gait.…”

“…Afterwards, part of the early exposures of that trackway were covered by mining debris rubble. This is discussed in a review paper by Vila et al (2008). 11) The Upper Jurassic Purgatoire River site (Colorado, USA) exhibits sauropod (Brontopodus) trackways with lengths of about 50e60 m (Lockley, 1986;Lockley et al, 1997).…”

“…Based on the stratigraphy and geologic ages, the Feitianshan Formation (BerriasianeBarremian, Tamai et al, 2004) is older than the Jiaguan Formation (BarremianeAlbian, Chen, 2009), as such, the dinosaur trackways in the Panxi region are younger than those from the Sichuan Basin. The dinosaur fauna of the Jiaguan and Feitianshan formations are similar (Table 3).…”

A diversified vertebrate ichnite fauna from the Feitianshan Formation (Lower Cretaceous) of southwestern Sichuan, China

“…The Feitianshan Formation appears equivalent to the Tianmashan Formation or Chengqiangyan Formation from the Sichuan Basin based on stratigraphic correlation and sedimentary facies analysis (CGCMS, 1982). When Tamai et al (2004) discussed the paleomagnetics of the Sichuan and Yunnan fragments of the Yangtze Block, they proposed a Berriasian to Barremian age for the Feitianshan Formation.…”