Structural evidence for the Permo-Triassic tectonic evolution of the Yidun Arc, eastern Tibetan Plateau

Deformation across the Yidun Arc occurred as a two-phase process; (i) Early Triassic deformation of Paleozoic sequences resulting from closure of the Jinsha Jiang Suture, and (ii) Late Triassic deformation of Middle to Late Triassic rocks as a result of final basin closure. Deformation in the western Yidun Arc is dominantly west-vergent. A north-south striking west-directed thrust, the Jinsha Thrust, forms the boundary between the western Yidun Arc and the eastern Qiangtang Block. The Jinsha Thrust incorporates within it ophiolitic material of the Jinsha Jiang Suture, and the structural geometry across the Jinsha Thrust suggests that Early Triassic subduction was east-dipping along the Jinsha Jiang Suture. Deformation within Middle to Late Triassic sequences of the eastern Yidun Arc, eastern Qiangtang Block and western Songpan Garzê Fold Belt is thin-skinned, being restricted largely to a single generation of upright folds and associated thrusts. Late Triassic westwards subduction underneath the combined western Yidun Arc-Qiangtang Block resulted in arc volcanism in the eastern Yidun Arc. Coeval arc volcanics in the northern margin of the Qiangtang Block suggest that the suture along the northern margin of the Qiangtang Block may be correlated with the Garzê-Litang Suture rather than the Jinsha Jiang Suture.