Palaeozoic tectonics of the south-western Chinese Tianshan: new insights from a structural study of the high-pressure/low-temperature metamorphic belt

The south-western Chinese Tianshan orogenic belt is famous for its omphacite-bearing blueschists and associated eclogite-facies metavolcanic rocks. Although numerous petrologic, geochemical and geochronological studies are available, structural data and interpretations are still rare. This paper provides new structural data, including bulk geometry of structures and kinematic analyses, based on field and laboratory studies along the Akyazhi, Keburt and Muzaert Rivers. The study area is divided into three tectonic units, namely (1) a Southern Unit composed of weakly metamorphosed sedimentary rocks of Silurian age; (2) a Central HP/LT Unit composed of blueschist-eclogite-facies metamorphic rocks derived from basalts, pelites and volcaniclastic rocks; (3) a Northern Unit, which consists of a Carboniferous magmatic arc developed upon an amphibolite-facies metamorphic continental basement. Our structural analysis documents a polyphase deformation. The main event (D₁) is reflected by Devonian to Carboniferous top-to-the northwest ductile shearing, coeval with HP/LT metamorphism. This is followed by north-directed thrusting (D₂) of the Southern Unit over the Central HP/LT Unit, coeval with retrogression of the high-pressure rocks. A top-to-the-S (SE) deformation (D₃) overprints the earliest events and is observed in the Northern and Central Units. Lastly, Permian dextral ductile-brittle wrenching (D₄) overprints the older flat-lying fabrics. D₄ is conspicuous along the Nalati Fault that separates the Northern Unit from the Central HP/LT Unit. The absolute timing of these deformation events is discussed in the light of available radiometric dating. The structural, metamorphic and geochronological data are integrated into a geodynamic model of the south-western Chinese Tianshan that emphasizes south-directed subduction of microcontinents located between Tarim and Junggar.