Structural feature and its significance of the northernmost segment of the Tertiary Biluoxueshan-Chongshan shear zone, east of the Eastern Himalayan Syntaxis

Here we describe ductile, ductile-brittle and brittle deformation styles in the northern segment of the Tertiary Biluoxueshan-Chongshan shear zone lying to the east of the Eastern Himalayan Syntaxis. In the northernmost part of the zone in the vicinity of the Eastern Himalayan Syntaxis, it consists of mylonitic gneiss, granite, and schist. Based on field relations and mineral assemblages, the rocks are classified into gneiss belt in the west limb, including banded gneiss, augen mylonite and migmatite gneiss, and schist belt in the east limb. Except for the massive granite pluton, the other three tectonites are affected by polystage deformation (D1–D4). Fold deformation of the first stage D1 is isoclinal to tight pattern with nearly N-S fold axes and steeply axial planar cleavage S1, which resulted in the local crustal thickening under a contractive setting. D2 overprinted D1 and is characterized by tight folds with steep axes and N-S fold axial planar, which are also characterized by large-scale ductile strike-slip shear foliation S2, parallel to the nearly N-S trending axial planes of D1 and D2. The structural pattern of D2 represents a transpression along the zone. D3 occurred during the late stage of the transpression or post-transpression, producing the NW-SE and NE-SW trending strike-slip faults of the third stage D3. Following the D3 deformation, the zone was exhumed to shallow crustal level where the various tectonites underwent a brittle transtensional deformation D4, combined with one N-S trending strike-slip component and one normal faulting component. Structures and previous geochronologies presented in the paper suggest that the study area is correlated with those in the adjacent tectonic zones, Ailaoshan-Red River shear zone and Gaoligong shear zone in the western Yunnan. It underwent intensive polyphase deformation, namely, crustal thickening, transpression, and transtension, responding to syn-collision and post-collision of India-Eurasia from 65 Ma to current period east of the Eastern Himalayan Syntaxis.