GEOMETRY, KINEMATICS AND REGIONAL SIGNIFICANCE OF THE CHONG SHAN SHEAR ZONE, YUNNAN, CHINA

The presence of the Chong Shan Shear Zone (CSSZ), a very long (300 km)and wide (10-20 km) metamorphic belt of high-grade rocks, is often neglected in models of Eurasian deformation. The CSSZ, which coincides with a topographic high and the Palaeo-Tethys suture in eastern Eurasia, is composed of a zone of mylonitic augen gneisses and migmatites. Metasediments occur only locally. Foliation is moderately to steeply west-dipping. Stretching lineations defined by the alignment of elongate mineral grains are sub-horizontal, consistent with dominantly strike-slip transport. Kinematic indicators such as rotated porphroclasts, S-C fabrics, and asymmetric folds are common at microscopic and outcrop scale and provide evidence for both dextral and sinistral movements. Although sinistral strike-slip transport is widely recognized in the structural development of the region, the role of dextral shearing has not been explored. The lower Palaeozoic sedimentary section of the Baoshan tectonic unit can be followed into the high-grade metamorphic core of the CSSZ with increasing metamorphic grade from west to east. The dominant easterly vergence of folds and increasing metamorphic grade from west to east suggest a northeasterly vergent thrusting over Jurassic-Cretaceous sediments in a transpressional setting. The protolith for the orthogneissic section may be the northern continuation of the Lincang granite of Permo-Triassic age. The nature of eastern boundary of the CSSZ is still unclear, but it is very long and very sharp fault boundary, and separates the CSSZ rocks from the Lanping-Simao-Qamdo folded redbeds, making this one of the most prominent and continuous contacts around the Eastern Syntaxis. New U-Pb monazite ages of 26.9-27.3 Ma along with our previously published muscovite Ar/Ar ages of 13.43-18.73 Ma, from weakly foliated leucogranites within the strongly mylonitic gneisses suggest that CSSZ is not a belt of Precambrian metamorphic rocks, but a Tertiary shear zone of great significance, which accommodated the post India-Eurasia collision and indentation along with the left-lateral Ailao Shan and the right-lateral Gaoligong Shan shear zones.