The Biluoxueshan transpressive deformation zone monitored by synkinematic plutons, around the Eastern Himalayan Syntaxis

The Biluoxueshan deformation zone is an Early Eocene dextral transpressive zone located around the Eastern Himalayan Syntaxis, and it consists mainly of granite, mylonitic granite, mylonitic gneiss, and schist. Structural evidence indicates that the granitic plutons were emplaced into the zone as subvertical sheeted bodies, then progressively deformed and partly mylonitized. The zone is characterized by a vertical to subvertical foliation and a subhorizontal stretching lineation. Kinematic criteria indicate a dextral sense of shear. A solid-state tectonic fabric is preserved in the mylonitic and magmatic rocks. The submagmatic and weak solid-state fabrics in these granitic plutons all are concordant, and they record horizontal shortening, subvertical extension, and a significant component of dextral simple shear. The plutons are interpreted in terms of syntranspressional magmatism. The SIMS U–Pb ages of zircons in the granite plutons are 57–46 Ma, and the ages of the mylonitic granites range from 34 to 22 Ma. We interpret the timing of the transpressive deformation to be between 57 and 22 Ma, and it may have started in the Late Paleocene and/or Early Eocene, corresponding to a period of crustal thickening due to India–Asia convergence. Movements dominated by dextral ductile shear occurred during the Early Oligocene to Early Miocene, accompanied by the continuous intrusion of granites. The total displacement along the zone is greater than 100 km. 40Ar/39Ar analysis of both the mylonites and the undeformed granites provides additional limits on the timing of vertical exhumation deformation, which is most consistent with a Middle Miocene cooling event along the transpressional zone (19–16 Ma). The Biluoxueshan shear zone, therefore, displays a complex geological history of deformation since ca. 57 Ma, and it has played a significant role in the overall intracontinental deformation, rather than merely being a strike-slip fault for the southeastwards differential extrusion of the Indochina block.

► The Biluoxueshan deformation zone is an Early Eocene transpressive zone around the Eastern Himalayan Syntaxis.
► Structural evidences indicate the syntranspressional magmatism occurred between 57 and 22 Ma along the zone.
► Vertical exhumation of the metamorphic complex occurred at Middle Miocene.