Passive crustal clockwise rotational deformation of the Sichuan Basin since the Miocene and its relationship with the tectonic evolution of the fault systems on the eastern edge of the Tibetan Plateau
We conducted a paleomagnetic study of the Upper and Lower Cretaceous red-beds in the Yibin area, in the southern part of the Sichuan Basin. High-temperature magnetic components were isolated between 580 °C and 695 °C. An ambiguous fold test resulted in alternative interpretations of the Cretaceous primary magnetization or Oligocene-Miocene remagnetization. Nevertheless, the two different interpretations of the paleomagnetic data consistently show that the Yibin area on the southern edge of the Sichuan Basin experienced clockwise rotation, relative to the eastern edge of the South China Block and Europe. A comparison with the Cretaceous and Paleogene paleomagnetic data from the northern and western part of the Sichuan Basin indicates that, except for the southeastern corner of the Sichuan Basin, the crust of the Sichuan Basin generally experienced significant clockwise rotational deformation since the early Miocene. The northeastward escape of the Songpan-Ganzi Terrane and the counterclockwise rotation of the Yangtze Fold Belt applied a right-lateral shear stress to the region surrounding the Sichuan Basin since the Miocene, which induced the passive crustal clockwise rotational deformation of the Sichuan Basin. Subsequently, the non-synchronous crustal rotational deformation of the Sichuan Basin and its surrounding blocks controlled the tectonic evolution of their boundary tectonic belts: for example, the right-lateral strike-slip movement of the Longmenshan fault belt and the southeastward extension of the Xianshuihe strike-slip fault since the Miocene. INTRODUCTION The Paleogene collision of India and Eurasia, and their subsequent continuous convergence, not only induced latitudinal shortening and vertical thickening of the Asian crust, but also led to lateral crustal extrusion around Tibet since the Oligocene. GPS measurements, structural deformation, and paleomagnetic studies on the southeastern edge of the Tibetan Plateau have shown that the Indochina Block experienced southward clockwise rotational escape along the Red-River strike-slip fault since ca. 32 Ma (Wang et  al., 1998; Sato et  al., 2001, 2007; Shen et al., 2005; Gan, et al., 2007; Tong et al., 2013; Li et al., 2017). Although the Chuandian Fragment, on the northern side of the Indochina Block, also experienced clockwise rotational movement along the Xianshuihe-Xiaojiang strike-slip fault, it did not undergo significant southward extrusion (Otofuji et al., 1998; Yang et al., 2001; Yoshioka et al., 2003; Tong et al., 2015; Wang et al., 2016). On the eastern side of the Tibetan Plateau, the Sichuan Basin, as the western part of the Yangtze Block, is sandwiched between the  SongpanGanzi Terrane and the Yangtze Fold Belt. It is surrounded by fourfold-fault belts: the Longmenshan fault belt on the western edge, the Zhangjiajie fault belt on the eastern edge, and the Daba Shan arcuate fold-thrust belt and the Xianshuihe strike-slip fault on the northern and southwestern edges, respectively (Burchfiel et  al., 1995; Burchfiel and Wang, 2003; Yuan et al., 2000; Xie et al., 2006; Robert et al., 2010; Zhang et  al., 2010; Wang et  al., 2013; Wang et al., 2014; Tian et al., 2016; Fig. 1A). The Songpan-Ganzi Terrane, in the central Tibetan Plateau, experienced eastward extrusion, and was resisted by the Sichuan Basin along the Longmenshan fault belt (Shen et al., 2005; Gan et al., 2007). During the Cenozoic, the convergence between the South China Block and the Pacific Plate induced an E-W–trending stress, which overlapped with the stress applied by the eastward extrusion of the Songpan-Ganzi Terrane, in the region of the Sichuan Basin. Against this stress background, the Cenozoic crustal motion and tectonic deformation of the Sichuan Basin coordinates the transformation of the Pacific and Tethys structural domains in the South China Block. To reveal the Cenozoic crustal kinematic characteristics of the Sichuan Basin, during the last three decades paleomagnetic studies have been conducted on the Cretaceous and Paleogene sediments of the Yaan-Feixianguan area in the southwestern corner of the Sichuan Basin (Zhuang et  al., 1988; Otofuji et  al., 1990; Enkin et al., 1991; Zhao et al., 2017), and in the TongjiangBazhong area on the northwestern edge of the Sichuan Basin (Sato et  al., 2011; Wang et al., 2013; Zhao et al., 2017). However, these studies have reached contradictory conclusions. The Cretaceous paleomagnetic data obtained from the northwestern edge of the Sichuan Basin show that it experienced clockwise rotation relative to Eurasia (Sato et  al., 2011; Wang et al., 2013; Zhao et al., 2017). In contrast, most of the Cretaceous and Paleogene paleo magnetic data from the southwestern corner of the Sichuan Basin indicate counterclockwise rotation since the late Cretaceous (Zhuang et  al., 1988; Otofuji et  al., 1990; Enkin et  al., 1991; Zhao et al., 2017). Moreover, some paleomagnetic data from the same strata, in the adjacent area, indicate clockwise rotation (Zhuang et al., 1988; Enkin et al., 1991). Zhuang et al. (1988) suggested that the Yaan and Feixianguan area, in the southwestern corner of the Sichuan Basin, first experienced clockwise rotation during the Cretaceous and Paleocene, and subseGSA Bulletin; January/February 2019; v. 131; no. 1/2; p. 175–190; https:// doi .org /10 .1130 /B31965 .1; 7 figures; 3 tables ; published online 15 August 2018. tyb816@ 163 .com For permission to copy, contact editing@geosociety.org © 2018 Geological Society of America Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/131/1-2/175/4604368/175.pdf by China Geological Library, Yong Cao on 06 March 2019