Geodynamic significance of preserved Carboniferous subduction complex in Atbashi range (South Tianshan, Kyrgyzstan) and inferences for crustal-scale structure of north Tarim-Tibet orogenic system

Abstract

A subduction complex of carboniferous age is preserved in the South Tianshan belt of Kyrgyzstan. It is made of a LP-LT accretionnary prism comprising an obducted ophiolite thrusted by a HP complex made of a sedimentary channel including eclogites boudins and a continental unit. Its structure and metamorphic history are investigated to reconstruct the geodynamic evolution of the northern Tarim in the Upper Palaeozoic. This study gives insights into the crustal-scale structure of this mountain belt, currently intensely reactivated by the India-Asia collision. Eclogites boudins have a N-MORB type composition similar to the unmetamorphosed obducted ophiolite sequence. Evidence for eclogite facies in both acidic and mafic lithologiesand geological structure are in agreement with a deep subduction channel mainly comprised of metasediments. Eclogites boudins are investigated for a preliminary PT study. Prograde stage (I) begins in blue-schist/eclogite facies transition at 520 ± 30°C - 17 ± 1 kbar. Condit ions of peak metamorphism (II) in eclogite facies range from 550 ± 30°C - 18.5 ± 1 kbar to 540- 595°C - 21 kbar. Retrograde stage (III) is also in the eclogite facies conditions at 515 ± 30°C - 16.7 ± 1 kbar. The geological structure and metamorphic conditions of this suture zone implies the subduction of a narrow oceanic basin in a south-dipping subduction zone, while another north-dipping subduction was active below Middle Tianshan. Final stacking of Middle and South Tianshan occurred at 320-310 Ma. These antithetic subduction zones are still reflected in the main structures of Tianshan. Reactivation of the south­ dipping structures since 30 Ma is ascribed to explain the current localisation of uplift and deformation.