Paleogeographic Patterns of E-C Transition Period in the Upper Yangtze and the Geodynamic Mechanism

Key words: 
• Palaeogeographic pattern /
• Geodynamics /
• E-C transition period /
• Rodinia breakup /
• the Upper Yangtze region

Abstract: During the Ediacaran-Cambrian (E-C) transition period, the margin of Upper Yangtze block distinctly presented a platform-shelf pattern, and the continental shelf was developed in Guangyuan-Mianyang-Moxi,Chengkou, Ebian, Laochangping in Pengshui and Bijie.The shelf was nearly North-South distribution with trumpet-like, opening to the edge of the open sea. Among them, the Guangyuan-Mianyang-Moxiis the largest, of which length is about 200 km, the widest place is greater than 100 km and the narrowest place 30 km,area of about 54 000 km². Since the SinianDoushantuo Formation to Cambrian Qiongzhusi Formation, from the initial appearance to the final demise, the continental shelf has experienced about 110 Ma. Carbonate sediments was developed on platform, dark phosphorus-bearing chert, siliceous dolomite, clipping with phosphate rock band (Well Zi 4), were developed on deep shelf facies of Maidiping Formation, Qiongzhusian Formation and Canglangpu Fomration, and carbonate granules was developed on the outer ring shelf. The intensively distributed magmatic rocks along the line of Longmenshan to Micangshanare about 800-760 Ma and Hf isotope values indicate the trigeminal rift develops in the western and northern margins of the Upper Yangtze with the breakup of Rodinia. The northeastern rift valley along boundary of the Songpan Ganzi block and the Yangtze block, and the crack depth has reached the upper mantle (intracontinental rift), the oceanic rift valley toward the northwest may be extend into the ancient Qilian sea along Danba,Xiaojin-Jinchuan. Along the Mianyang-Guangyuan, AnkangPingli,Leshan-Neijiang-Nvji well into a line, the continental rift which reach into the interior of the Upper Yangtze platform were developed. These continental rifts controlled the differentiation platform-shelf pattern in the western and northern margin of the Upper Yangtze during the E-C transition period. From the breakup of Rodinia continental and the sedimentary pattern of the Upper Yangtze plate margin, the differentiation of the platform-shelf pattern on the continental margin is the paleogeographic effect of the supercontinent breakup.