Geometry, kinematics, and regional significance of the Chong Shan shear zone, Eastern Himalayan Syntaxis, Yunnan, China

The geology east of the Eastern Himalayan Syntaxis is poorly known, although it figures prominently in many models for the Cenozoic tectonics of the India-Eurasia collision and subsequent intracontinental deformation. The little known Chong Shan shear zone a similar to 250-km-long and similar to 10-km-wide metamorphic belt composed mainly of mylonitic augen gneisses and migmatites, forms a major shear zone developed during early Cenozoic extrusion of the Indochina crustal (lithospheric?) fragment. Foliation within the shear zone is moderately to steeply west dipping, and stretching lineations are subhorizontal, consistent with dominantly strike-slip transport. Kinematic indicators including rotated porphyroclasts, S-C fabrics, and asymmetric folds provide evidence for both dextral and sinistral movements. Our preliminary geochronological studies indicate that the Chong Shan shear zone has been active since at least ca. 34 Ma, and perhaps as early as 41 Ma. Strike-slip shearing continued at least until ca. 29 Ma, perhaps as late as ca. 24 Ma, and terminated by ca. 17 Ma. The Chong Shan shear zone, therefore, is not a belt of Precambrian metamorphic rocks as previously interpreted, but a Cenozoic shear zone of great significance, which was contemporaneous with movement on the left-lateral Ailao Shan shear zone and the right-lateral Gaoligong Shan shear zone two shear zones that bound the Indochina fragment on the east and west, respectively. Our data from the Chong Shan shear zone along with the data presented elsewhere from the Gaoligong Shan shear zone indicate that while the region between the Gaoligong Shan shear zone and Ailao Shan shear zone extruded to the southeast, it did not extrude as a single rigid block, but rather it was dismembered into at least two major fragments, the Baoshan to the west and the Lanping-Simao to the east separated by the Chong Shan shear zone. Our study of the Chong Shan shear zone suggests it and the other major early Cenozoic shear zones formed part of a broad major shear zone that passes into eastern Tibet between the Qiang-tang and Lhasa tectonic units.