Thrust structures and uplift periods of the Nyainrong metamorphic complex and its neighboring areas

Regional thrust systems consisting of thrust faults, tectonic slices and sheets, nappes, structural windows and accompanied folds were discovered in the Nyainrong metamorphic complex and its neighboring areas on the basis of field studies and geological mapping. Tectonic slices and thrust sheets surrounding the Nyainrong metamorphic complex were thrust southwest-westward over the Upper Cretaceous red beds and the Jurassic-Lower Cretaceous marine sedimentary rocks. Regional intensive thrust led to detach-ments among the ophiolite and Jurasssic mélanges, the Carboniferous-Permian clastic-carbonate, the Paleozoic low-grade metamor-phic rocks and the high-grade metamorphic basement, accompanied by ductile shear zones and oblique thrust faults in the Nyainrong metamorphic complex. Dextral-slip fault trending in northwest-southeast direction along the Tumen-Tuojiu-Zase zone trans-formed to regional thrust systems trending in east-west direction in southern Tanggula Range. Regional thrust and gravitational slide resulted in 86~120km southwest-westward movement of the Jurassic mélange and the Paleozoic covering layers, forming the Dong Co tectonic sheets of ophiolite mélanges along the Naqu-Baerda-North Baingoin in the west-southwest of the Nyainrong meta-morphic complex. Tectonic schist, slikensides and linear minerals trending in east-west direction were formed along the detachment faults beneath the Dong Co thrust slices of ophiolite mélanges. Fission-track dating indicates that thrust uplift mainly occurred at the late stage of the Early Cretaceous-the early stage of the Late Cretaceous (111±5~91±5Ma), late stage of the Late Cretaceous (89±6~69±5Ma) and Late Paleocene-Early Eocene (55±4~44±2Ma), and the corresponding rates of thrust uplift range from 0.10mm/a to 0.69mm/a in the Nyainrong metamorphic complex and its neighboring areas. Thrust uplift also occurred in Late Eocene and Oligo-cene in some areas. The process and mechanism of regional thrust in the Nyainrong metamorphic complex and its neighboring areas are discussed on the basis of the data available.