Plate tectonics of the Altaids

Abstract: The Altaids consist in a huge accretionary-type belt extending from Siberia through Mon-golia, northern China, Kyrgyzstan and Kazakhstan. They were formed from the Vendian through the Jurassic by the accretion of numerous displaced and exotic terranes (e.g. island arc, ribbon microcontinent, seamount, basaltic plateau, back-arc basin). The number, nature and origin of the terranes differ according to the palaeotectonic models of the different authors. Thanks to a geo- dynamic study (i.e. definition of tectonic settings and elaboration of geodynamic scenarios) and plate tectonics modelling, this work aims to present an alternative model explaining the Palaeozoic palaeotectonic evolution of the Altaids. Based on a large set of compiled geological data related to palaeogeography and geodyna¬mic (e.g. sedimentology, stratigraphy, palaeobiogeography, palaeomagnetism, magmatism, me- tamorphism, tectonic...), a partly new classification of the terranes and sutures implicated in the formation of the Altaids is proposed. In the aim to elaborate plate tectonics reconstructions, it is necessary to fragment the present arrangement of continents into consistent geological units. To avoid confusion with existing terminology (e.g. tectonic units, tectono-stratigraphic units, micro- continents, terranes, blocks...), the new concept of "Geodynamic Units (GDU)" was introduced. A terrane may be formed by a set of GDUs. It consists of a continental and/or oceanic fragment which has its own kinematic and geodynamic evolution for a given period. With the same ap-proach, the life span and type of the disappeared oceans is inferred thanks to the study of the mate-rial contained in suture zones. The interpretation of the tectonic settings within the GDUs comple-ted by the restoration of oceans leads to the elaboration of geodynamic scenarios. Since the Wilson cycle was presented in 1967, numerous works demonstrated that the continental growth is more complex and results from diverse geodynamic scenarios. The identification of these scenarios and their exploitation enable to elaborate plate tectonics models. The models are self-constraining (i.e. space and time constraints) and contest or confirm in turn the geodynamic scenarios which were initially proposed. The Altaids can be divided into three domains: (1) the Peri-Siberian, (2) the Kazakhstan, and (3) the Tarim-North China domains. The Peri-Siberian Domain consists of displaced (i.e. Sayan Terrane Tuva-Mongolian, Lake-Khamsara Terrane) and exotic terranes (i.e. Altai-Mongolian and Khangai-Argunsky Terrane) accreted to Siberia from the Vendian through the Ordovician. Fol-lowing the accretion of these terranes, the newly formed Siberia active margin remained active un-til its part collision with the Kazakhstan Superterrane in the Carboniferous. The eastern part of the active margin (i.e. East Mongolia) continued to act until the Permian when the North-China Tarim Superterrane collided with it. The geodynamic evolution of the eastern part of the Peri-Siberian Domain (i.e. Eastern Mongolia and Siberia) is complicated by the opening of the Mongol-Okhotsk Ocean in the Silurian. The Kazakhstan Domain is composed of several continental terranes of East Gondwana origin amalgamated together during the Ordovician-Silurian time. After these different orogenic events, the Kazakhstan Superterrane evolved as a single superterrane until its collision with a Tarim-North China related-terrane (i.e. Tianshan-Hanshan Terrane) and Siberian Continent during the Devonian. This new organisation of the continents imply a continued active margin from Siberia, to North China through the Kazakhstan Superterrane and the closure of the Junggar- Balkash Ocean which implied the oroclinal bending of the Kazakhstan Superterrane during the entire Carboniferous. The formation history of the Tarim-North China Domain is less complex. The Cambrian northern passive margin became active in the Ordovician. In the Silurian, the South Tianshan back-arc Ocean was open and led to the formation of the Tianshan-Hanshan Terrane which collided with the Kazakhstan Superterrane during the Devonian. The collision between Siberia and the eastern part of the Tarim-North China continents (i.e. Inner Mongolia), implied by the closure of the Solonker Ocean, took place in the Permian. Since this time, the major part of the Altaids was formed, the Mongol-Okhotsk Ocean only was still open and closed during the Jurassic. Resume: La chaine des Altaides est une importante chaine d'accretion qui s'etend en Siberie, Mon-golie, Chine du Nord, Kirghizstan et Kazakhstan. Elle s'est formee durant la periode du Vendian au Jurassique par l'accretion de nombreux terranes deplaces ou exotiques (par exemple arc ocea-nique, microcontinent, guyot, plateau basaltique, basin d'arriere-arc...). Le nombre, la nature ou encore l'origine different selon les modeles paleo-tectoniques proposes par les differents auteurs. Grâce a une etude geodynamique (c'est-a-dire definition des environnements tectoniques et ela-boration de scenarios geodynamiques) et a la modelisation de la tectonique des plaques, ce travail propose un modele alternatif expliquant l'evolution paleo-tectonique des Altaides. Base sur une large compilation de donnees geologiques pertinentes en termes de paleo-geographie et de geodynamique (par exemple sedimentologie, stratigraphie, paleo-biogeographie, paleomagnetisme, magmatisme, metamorphisme, tectonique...), une nouvelle classification des terranes et des sutures impliques dans la formation des Altaides est proposee. Dans le but d'elabo¬rer des reconstructions de plaques tectoniques, il est necessaire de fragmenter l'arrangement actuel des continents en unites tectoniques coherentes. Afin d'eviter les confusions avec la terminolo¬gie existante (par exemple unite tectonique, unite tectono-stratigraphique, microcontinent, block, terrane...), le nouveau concept d' "Unite Geodynamique (UGD)" a ete introduit. Un terrane est forme d'une ou plusieurs UGD et represente un fragment oceanique ou continental defini pas sa propre cinetique et evolution geodynamique pour une periode donnee. Parallelement, la duree de vie et le type des oceans disparus (c'est-a-dire principal ou secondaire) est deduite grâce a l'etude du materiel contenu dans les zones de sutures. L'interpretation des environnements tectoniques des UGD associes a la restauration des oceans mene a l'elaboration de scenarios geodynamiques. Depuis que le Cycle de Wilson a ete presente en 1967, de nombreux travaux ont demontre que la croissance continentale peut resulter de divers scenarios geodynamiques. L'identification et l'ex-ploitation de ces scenarios permet finalement l'elaboration de modeles de tectonique des plaques. Les modeles sont auto-contraignants (c'est-a-dire contraintes spatiales et temporelles) et peuvent soit contester ou confirmer les scenarios geodynamiques initialement proposes. Les Altaides peuvent etre divisees en trois domaines : (1) le Domaine Peri-Siberien, (2) le Domaine Kazakh, et (3) le Domaine Tarim-Nord Chinois. Le Domaine Peri-Siberien est compose de terranes deplaces (c'est-a-dire Terrane du Sayan, Tuva-Mongol et Lake-Khamsara) et exotiques (c'est-a-dire Terrane Altai-Mongol et Khangai-Argunsky) qui ont ete accretes au craton Siberien durant la periode du Vendien a l'Ordovicien. Suite a l'accretion de ces terranes, la marge sud-est de la Siberie nouvellement formee reste active jusqu'a sa collision partielle avec le Superterrane Ka-zakh au Carbonifere. La partie est de la marge active (c'est-a-dire Mongolie de l'est) continue son activite jusqu'au Permien lors de sa collision avec le Superterrane Tarim-Nord Chinois. L'evolu¬tion geodynamique de la partie est du Domaine Siberien est compliquee par l'ouverture Silurienne de l'Ocean Mongol-Okhotsk qui disparaitra seulement au Jurassique. Le Domaine Kazakh est compose de plusieurs terranes d'origine est-Gondwanienne accretes les uns avec les autres avant ou pendant le Silurien inferieur et leurs evolution successive sous la forme d'un seul superterrane. Le Superterrane Kazakh collisione avec un terrane Tarim-Nord Chinois (c'est-a-dire Terrane du Tianshan-Hanshan) durant le Devonien et le continent Siberien au Devonien superieur. Ce nouvel agencement des plaques induit une marge active continue le long des continents Siberien, Kazakh et Nord Chinois et la fermeture de l'Ocean Junggar-Balkash qui provoque le plissement oroclinal du Superterrane Kazakh durant le Carbonifere. L'histoire de la formation du Domaine Tarim-Nord Chinois est moins complexe. La marge passive nord Cambrienne devient active a l'Ordovicien et l'ouverture Silurienne du bassin d'arriere-arc du Tianshan sud mene a la formation du terrane du Tianshan-Hanshan. La collision Devonienne entre ce dernier et le Superterrane Kazakh provoque la fermerture de l'Ocean Tianshan sud. Finalement, la collision entre la Siberie et la partie est du continent Tarim-Nord Chinois (c'est-a-dire Mongolie Interieure) prend place durant le Permien suite a la fermeture de l'Ocean Solonker. La majeure partie des Altaides est alors formee, seul l'Ocean Mongol-Okhotsk est encore ouvert. Ce dernier se fermera seulement au Jurassique.