Paleozoic structural and geodynamic evolution of eastern Tianshan (NW China): welding of the Tarim and Junggar plates

to cite the paper EPISODES Volume: 30 Issue: 3 Pages: 162-186 Published: September, 2007International audienceChinese East Tianshan is a key area for understanding the Paleozoic accretion of the southern Central Asian Orogenic Belt. A first accretion-collision stage, before the Visean, developed the Eo-Tianshan range, which exhibits north-verging structures. The geodynamic evolution included: i) Ordovician-Early Devonian southward subduction of a Central Tianshan ocean beneath a Central Tianshan arc; ii) Devonian oceanic closure and collision between Central Tianshan arc and Yili-North Tianshan block, along the Central Tianshan Suture Zone; iii) Late Devonian-earliest Carboniferous closure of a South Tianshan back-arc basin, and subsequent Central Tianshan-Tarim active margin collision along the South Tianshan Suture Zone. A second stage involved: i) Late Devonian-Carboniferous southward subduction of North Tianshan ocean beneath the Eo-Tianshan active margin (Yili-North Tianshan arc); ii) Late Carboniferous-Early Permian North Tianshan-Junggar collision. The Harlike range, unit of Mongolian Fold Belt, collided with Junggar at Mid- Carboniferous, ending a north-dipping subduction. The last CAOB oceanic suture is likely the North Tianshan Suture Zone, between Yili-North Tianshan and Junggar. During the Permian, all the already welded units suffered from a major wrenching, dextral in Tianshan, sinistral in Mongolian Fold Belt, due to opposite motion of Siberia and Tarim

Primary Carboniferous and Permian paleomagnetic results from the Yili Block (NW China) and their implications on the geodynamic evolution of Chinese Tianshan Belt

International audienceIn order to better understand the tectonic role of the Yili Block on the Paleozoic evolution of the Chinese Tianshan Belt, we performed a primary paleomagnetic study on Carboniferous and Permian rocks from different areas in the Yili Block, NW of China. More than 320 sedimentary and volcanic samples were collected from 39 sites. Except for the Ordovician samples showing a weak and unstable magnetic remanence, the majority of this collection presents characteristic remanent magnetization carried by magnetite and hematite. In the study area, though positive fold test has been observed on the Early Carboniferous rocks, a general remagnetization of these rocks has been identified and attributed to the Late Carboniferous magmatism Moreover, all Early and Late Carboniferous samples from the interior of the Yili Block yield stable and coherent magnetic directions with exhaustively reverse magnetic polarity. The Late Carboniferous (C2) is considered as the magnetic remanence age since these rocks are covered or intruded by synchronous magmatic rocks of the Yili arc, which lasted until to ~310 Ma. The C2 paleomagnetic pole is therefore calculated at 68.6°N, 290.6°E with !95=6.4° and n=15. The Late Carboniferous rocks located close to a deformation zone present a consistentmagnetic inclination but significant different declination with respect to other areas and are suspected to have probably experienced a local rotation. Although no fold test can be performed due to the monoclinal bedding, stable magnetic components are isolated from Late Permian (P2) red beds in the interior of the Yili Block with also a solo reverse magnetic polarity, the P2 paleomagnetic pole of the Yili Block has been, therefore, calculated from the characteristic remanent magnetization: 79.7°N, 172.0°E with !95=11.3° and n=5. Keeping important uncertainties in mind, comparisons of the C2 and P2 paleomagnetic poles of the Yili Block with available coeval poles of Junggar, Tarim and Siberia indicate (1) no significant relative motion between the Yili and Junggar blocks since the Late Carboniferous, (2) no significant or weak latitudinal relative motion occurred since the Late Carboniferous among these blocks, but (3) the 46.2°±15.1° and the 31.6°±15.1° counterclockwise rotations of the Yili-Junggar blocks with respect to Tarim and Siberia took place during C2 to P2. These rotations are accommodated by the Permian dextral strike-slip faults along the northern and southern sides of Tianshan Belt and sinistral strike-slip faulting along the Erqishi Fault of Altay Belt, resulting in about 1000 km and 600 km lateral displacements in the Tianshan and Altay belts, respectively

Accrétions contientales en Asie centro-orientale : évolution géodynamique et structurale du Tianshan et du Junggar oriental (nord-ouest Chine) au Paléozoïque.

M. Jacques CHARVET Professeur Université d'Orléans - PrésidentM. Georges MASCLE Professeur Université de Grenoble - RapporteurM. Patrick MONIE Chargé de Recherche CNRS Montpellier - RapporteurM. François LEFEUVRE Directeur de Recherche CNRS Orléans - ExaminateurM. Liangshu SHU Professeur Université de Nanjing (Chine) - ExaminateurM. Damien DELVAUX Chercheur Musée d'Afrique Centrale Tervuren (Belgique)In eastern central Asia (north Xinjiang), the Paleozoic orogen of Tianshan separates the two blocks of Tarim and Junggar. It is divided into three units: the south Tianshan is composed of gneissic and Silurian ophiolitic nappes; in the central Tianshan, an Ordovician volcanic arc and Silurian flyschs overlay a Proterozoic basement; the north Tianshan is characterised by a Devonian to Carboniferous calc-alkaline volcanism.The structural study of Tianshan and of the eastern border of Junggar provides new information on the chronology and kinematic of deformations which were responsible for the Palaeozoic structuration of this area. Except a pre-Sinian D1 deformation recognised in the Proterozoic substratum of Tianshan, three stages are distinguished. The first one, D2, is north-verging and coeval with the emplacement of the central Tianshan ophiolites of Mishigou and of the south Tianshan ophiolitic nappes of Kumux. The Middle to Late Devonian age of D2 is constrained by the Early Devonian age of ophiolites and the unconformity of Early Carboniferous conglomerates. A Middle to Late Carboniferous deformation D3 is divided into two stages: first, a southward shearing along the boundary between Junggar and Siberian blocks; second, a northward one which occurred in all the north Tianshan unit as folds and thrusts. D3 was coeval with the accretion of the Junggar block with the Tianshan. Finally, a double stage strike-slip deformation D4 dated at 290 and 245 Ma occurred all around the Junggar basin. It was dextral in whole Tianshan and roughly sinistral in Chinese Altay area.In conclusion, the new structural and geochemical data are synthesised in a geodynamic model showing the Cambrian to Permian history of Tianshan and Junggar, marked by the successive accretion of the involved continental blocks and arcs.En Asie centro-orientale (Xinjiang nord), l'orogène paléozoïque du Tianshan sépare les blocs du Tarim et du Junggar. Trois unités y sont distinguées : le Tianshan sud, constitué de nappes gneissiques et ophiolitiques siluriennes ; le Tianshan central, caractérisé par un arc volcanique ordovicien et des flyschs siluriens sur un socle protérozoïque ; le Tianshan nord, représenté par un arc volcanique calco-alcalin dévono-carbonifère.L'étude structurale du Tianshan centro-occidental et de la bordure orientale du Junggar a permis de préciser la chronologie et la cinématique des déformations responsables de la structuration paléozoïque controversée de cette région. A part une phase de déformation anté-sinienne D1 reconnue dans le socle protérozoïque des Tianshan sud et central, trois déformations ont été distinguées. La première, D2, à vergence nord, est considérée comme contemporaine de la mise en place des ophiolites de Mishigou au nord du Tianshan central et des ophiolites de Kumux dans le Tianshan sud. L'âge de D2, compris entre le Dévonien moyen et le Dévonien supérieur, est contraint par les âges (Dévonien inférieur) des mélanges et par la discordance du Carbonifère inférieur. Une déformation D3, d'âge Carbonifère moyen-supérieur, est divisée en deux stades : le premier, à vergence sud, est observée le long de la limite entre le Junggar et le bloc Sibérie ; le second, à vergence nord est responsable du développement de plis et de chevauchements dans l'unité du Tianshan nord. Elle serait contemporaine de l'accrétion du bloc du Junggar avec l'arc du Tianshan nord. Enfin, une phase décrochante D4 a affecté tout le pourtour du bassin du Junggar entre 290 et 245 Ma. Dextre dans tout le Tianshan, elle est globalement senestre dans l'Altay chinois et accommode les rotations relatives des blocs du Junggar, du Tarim et de l'Eurasie. En conclusion, les nouvelles données structurales et géochimiques exposées dans ce travail sont synthétisées dans un modèle géodynamique retraçant l'histoire de cette région de l'Asie entre le Cambrien et le Permien, marquée par l'accrétion successive des blocs continentaux et des arcs impliqués

Paleozoic Accretion-Collision Events and Kinematics of Ductile Deformation in the Eastern Part of the Southern-Central Tianshan Belt, China

International audienceThe Tianshan range could have been built by both late Early Paleozoic accretion and Late Paleozoic collision events. The late Early Paleozoic Aqqikkudug-Weiya suture is marked by Ordovician ophiolitic mélange and a Silurian flysch sequence, high-pressure metamorphic relics, and mylonitized rocks. The Central Tianshan belt could principally be an Ordovician volcanic arc; whereas the South Tianshan belt, a back-arc basin. Macro- and microstructures, along with unconformities, provide some kinematic and chronological constraints on 2-phase ductile deformation. The earlier ductile deformation occurring at ca. 400 Ma was marked by north-verging ductile shearing, yielding granulite-bearing ophiolitic mélange blocks and garnet-pyroxene-facies ductile deformation, and the later deformation, a dextral strike-slip tectonic process, occurred during the Late Carboniferous(Early Permian. Early Carboniferous molasses were deposited unconformably on pre-Carboniferous metamorphic and ductilely sheared rocks, implying the end of the early orogeny. The large-scale ductile strike-slip along the Aqqikkudug-Weiya zone was possibly caused by the second tectonic event, the Hercynian collision between the northern Tarim block and the southern Siberian block. Late Paleozoic granitic magmatism and superimposed structures overprinted this Early Paleozoic deformation belt. Results of geometric and kinematic studies suggest that the primary framework of the Southern-Central Tianshan belt, at least the eastern part of the Tianshan belt, was built by these two phases of accretion events

Paleozoic geodynamics evolution of Tianshan orogenic belt (NW China) : Welding of Tarim and Junggar contiental blocks.

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

Reconnaissance et valorisation des engagements étudiants via les OpenBadges

International audienceLaw No. 2017-86 of 27 January 2017 on equality and citizenship requires higher education institutions to provide arrangements within the organisation and the conduct of studies for students who have responsibilities in associations, trade unions, or who are undertaking civic service or military volunteer work.In parallel, the aim is to be able to recognise the skills acquired in these “extracurricular” activities, notably in the March 2022 circular.By virtue of its various statutes and values, the Institut Polytechnique UniLaSalle (ULS) wishes to be involved in an ambitious policy that recognises the engagement of the School's students