Diverse Structural Styles of the Northeastern Tethyan Himalaya in Southern Tibet Reveal the Early Collisional Tectonics of India and Asia

The poor understanding of the structural history of the Late Triassic flysch in the northeastern Tethyan Himalaya has caused many disputes regarding India-Asia collisional tectonics. Here, we conducted an integrated study including tectonostratigraphic analysis, structural analysis, and zircon U‒Pb and muscovite ⁴⁰Ar/³⁹Ar dating to unravel the structural evolution of the Late Triassic flysch. Field geological mapping revealed that the flysch can be subdivided into three structural units, including the Gyaca mélange, Qiongjie-Dengmu shear zone and Langjiexue fold-thrust belt. The three units all contain voluminous siliciclastic rocks deposited in an abyssal submarine fan environment at the Indian passive continental margin. The Gyaca mélange shows a two-stage deformation process that generated a collisional mélange resulting from the subduction of the Indian passive continental margin. ⁴⁰Ar/³⁹Ar dating of top-to-the-south shear bands and block-in-matrix structure indicate that the India-Asia collision happened no later than the Selandian (ca. 60 Ma). The divergent imbricated thrusting and folding of the Gyaca mélange and Langjiexue fold-thrust belt formed a positive flower structure. The zircon U‒Pb and ⁴⁰Ar/³⁹Ar dating of syntectonic dikes and sericite flakes within the Gyaca mélange yield ages of ca. 56–55 Ma for the genesis of the divergent structures. Moreover, the Qiongjie-Dengmu dextral shear zone yields a sericite ⁴⁰Ar/³⁹Ar age of ca. 35 Ma and thus indicates a transient strike-slip stress regime. Generally, the early India-Asia collision tectonics in the northeastern Tethyan Himalaya showed episodic evolution with changing structural styles from mélange formation to imbricate fold-thrust belt development and finally strike-slip shear generation.