An integrated north–south paleo-Dadu-Anning River: New insights from bulk major and trace element analyses of the Xigeda Formation

The Xianshuihe-Anninghe fault extends SE–S and constitutes the southeastern margin of the Tibetan Plateau. However, the Dadu River which is associated with the fault does not flow following the path, but makes a 90° turn within a distance of 1 km at Shimian, heading east, and joins the Yangtze River, finally flowing into the East China Sea. Adjacent to the abrupt turn, a low and wide pass near the Daqiao reservoir at Mianning separates the N–S course of the Dadu River from the headwater of the Anning River which then flows south into the Yunnan Province along the Anninghe fault. Therefore, many previous studies assumed southward flow of the paleo-Dadu River from the Shimian to the Anning River. However, evidences for the capture of the integrated N–S paleo-Dadu-Anning River, its timing, and causes are still insufficient. This study explored the paleo-drainage pattern of the Dadu and Anning Rivers based on bulk mineral and geochemical analyses of the large quantities of fluvial/lacustrine sediments along the trunk of the Dadu and Anning Rivers. Similar with sands in the modern Dadu River, the Xigeda sediments also exhibit a granitoid affinity with the bulk major mineral compositions of quartz (>50%), anorthite (about 10%), orthoclase (about 5%), muscovite (about 5%), and clinochlore (about 4%). Correspondingly, bulk major elements show high SiO₂, with all samples >60%, and some of them >70%, low TiO₂ (⩽0.75%), P₂O₅ (⩽0.55%), FeO* (⩽5%), and relatively high CaO (1.02%–8.51%), Na₂O (1.60%–2.52%), and K₂O (2.17%–2.71%), with a uniform REE patterns. Therefore, synthesizing all these results indicate that these lacustrine sediments have similar material sources, which are mainly derived from its course in the Songpan-Ganzi flysch block, implying that the paleo-Dadu originally flowed southward into the Anning River and provided materials to the Xigeda ancient lake. The rearrangement of the paleo-Dadu River appears to be closely related to the locally focused uplift driven by strong activities of the Xianshuihe-Xiaojiang fault system.

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