Paleozoic geodynamics evolution of Tianshan orogenic belt (NW China) : Welding of Tarim and Junggar contiental blocks.
Résumé
The E-W trending Tianshan range links Tarim and Junggar blocks. Its geodynamic development is clarified in the eastern Tianshan; new correlations with the western Tianshan and the continental nature of Junggar plate are documented. The eastern Tianshan resulted from a double stage Paleozoic northward accretion. A Middle Devonian suture zone runs in South Tianshan, north of Tarim passive margin, including Silurian flyschs, nappes of ophiolitic mélanges, dated as Silurian to Early-Midle Devonian, and ophiolitic bodies with a sole of amphibolites-granulites. The kinematic criteria yielded by metasedimentary greenschists indicate a northward thrusting prior to the Carboniferous intrusions. The Central Tianshan unit is composed of Ordovician island-arc volcanic rocks and a Silurian flych deposited on the Proterozoic substratum and Cambrian carbonates, and intruded by Carboniferous granites. Kinematic criteria indicate north-verging thrusting and folding, sealed by the unconformity of Lower Carboniferous conglomerates and limestones. North Tianshan is an Upper Devonian-Carboniferous continent-based island arc made of calc-alkaline basaltic and andesitic volcanics and volcaniclastics. It was affected during Late Carboniferous by northward thrusting and folding of thin-skinned tectonics style, sealed by the Middle Permian unconformity. The geodynamic evolution implies two stages. Firstly, after a southward oceanic subduction beneath the Ordovician Central Tianshan arc, the closure of South Tianshan back-arc basin led to the accretion of Tarim plate to the arc. Secondly, the continent-based North Tianshan volcanic arc built by southward subduction of the Junggar paleo-ocean collided during Late Carboniferous time with Junggar. The whole area was affected by two stages of post-collision strike-slip shearing. In the western Tianshan, new data suggest the following conclusions. The so-called "Yili block", between Yining and Narat, is actually the western extension of the North Tianshan Carboniferous island arc, diplaced by a huge late strike-slip motion, and not an independant plate nor the extension of Central Tianshan Ordovician-Silurian island arc, which is in fact located to the south of it. Its basement and the Junggar basin one, exposed in Bole area, are continental with a sedimentary sequence covering the Proterozoic. The South Tianshan is wider there, showing nappes of ophiolitic mélanges thrusted northward over the Lower Paleozoic Tarim-type sedimentary pile.