Late Permian post-orogenic tectonics of Tianshan Belt proved by structural, geochronological, geochemical and paleomagnetic evidences

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Holding Date 21 September 2008

The amalgamation of the Central Asian Orogenic Belt (CAOB), or Altaids, has been a matter of debate since the last two decades. The Tianshan Belt, located in the southern margin of the CAOB or Altaids, provides a good place to study the Paleozoic tectonic evolution of the Central Asia. We performed a synthetical study of structure, geochronology, geochemistry and paleomagnetism on Paleozoic rocks exposed in the western Chinese Tianshan Belt. Geometric and structural investigations suggest two large-scale ductile shear zones develop sub-parallelly along the ridges of the mountains, and are dominated by LP-HT metamorphic and highly deformed rocks that are characterized by steep foliation including sub-horizontal lineation, kinematic observation reveals dextral sense of movement. Ar-Ar datings of secondary mica in mylonites indicate that the tecto-thermal event occurred during 285~245 Ma. Granitic plutons cropping out within or at the margins of large-scale dextral strike-slip shear zones partly display ductile foliation and elongated shape and are parallel to the orientation of the contiguous shear zones. Zircon U-Pb dating suggests that the synkinematic granite were formed during 280~265Ma, their geochemistry shows an evolution from high-K calc-alkaline towards alkaline series indicating the coexistence of diverse magmatic sources and emplacement controlled by the shear zone. The characteristic remanent magnetizations were isolated from the Carboniferous and Permian rocks of Yili block, comparison of the Late Carboniferous (C2) and Late Permian (P2) paleomagnetic poles of the Yili Block with available coeval poles of Junggar and Tarim, indicate that no significant covergence but important counterclockwise rotations of the Yili¨CJunggar blocks with respect to Tarim took place during C2 to P2. Above data consistently suggest that the post-orogenic tectonics of the Tianshan Belt occurred in Late Permian, and is represented by large-scale ductile shear associated synkinematic plutonism, and was induced by post-collisional rotation.

This research is supported by the National Basic Research Program of China (973 Programs) (Nos. 2007CB411301 and 2001CB409804).

tags: Structure Geochronology geochemistry Paleomagnetism Post-orogeny Permian TianshanBelt Tectonic & Seismotectonic