Structural Geology and Kinematic Patterns of the Yaoshan Complex, Southeastern Tibet: Implications for Tectonic Evolution of the Ailao Shan-Red River (ASRR) Shear Zone

Structural geology, strain and kinematics of shearing of the ASRR shear zone are essential in elucidating the nature of shearing along the shear zone and its role in the tectonic evolution of eastern Tibet. This study reports new progress on our understanding the structural geology and tectonic significance of the Yao Shan metamorphic complex (YSMC), the NW extension of the Day Nui Con Voi complex along the ASRR shear zone. Structural analysis reveals that the YSMC constitutes a large linear antiform cored by highly sheared Proterozoic high grade rocks. Progressive shearing of the rocks generated early pure and late simple shearing-dominated fabrics at decreasing temperatures. L-fabrics and A-type folds at various scales are characteristic structural elements in the complex. A linear antiform is best outlined by the outer shell Cambrian-Triassic marbles, metabasalt and phyllitic rocks. In addition, these rocks contain shear fabrics consistent with those in the high grade core. The AMS data of 186 specimens from 20 sites along a section across the central YSMC shows generally high anisotropy of magnetic susceptibility implying high shear strain. Most samples possess flattening strain AMS fabrics, in spite of fabrics types of the tectonites. Magnetic foliations due to oriented growth of biotite and magnetite have intersection angles of less than 15° with mylonite foliation (S fabrics), indicating high shear strain. Meanwhile, the intersection suggests that both dextral shearing and sinistral shearing occur across the complex, which is compatible with outcrop and microscopic observations. It is suggested that the Yao Shan-Day Nui Con Voi complex is a linear a-type anticlinal fold formed by crustal scale syn-shearing folding and doming. The present study demonstrates that the Yao Shan-Day Nui Con Voi anticlinal structure was formed as a result of middle and lower crustal flow along the ASRR shear zone in southeastern Tibet during Indian-Eurasian continental collision.