Transcontinental shear zones in northern Asia
|Category||Tectonic & Seismotectonic|
|Location||International Geological Congress,oslo 2008|
|Holding Date||27 September 2008|
During the Permian (270-250 Ma), the evolution of the Altaids was controlled by the sinistral Irtysh-Gornostaev and dextral Scytho-Turanian shear zones. The Irtysh-Gornostaev shear zone can be traced as the NW-trending narrow zone stretching from Chinese Altai to the Polar Ural where it cuts the structures of the Uralides. The displacement along the Irtysh-Gornostaev shear zone was estimated as 2000 km (aengör and Natal’in, 1996). The Scytho-Turanian shear zone is of the E-W trend and stretches from Beishan to the southern margin of the Russian craton. Its structure and kinematic history are poorly known because of younger sediments covering most of the zone length. Recent paleomagnetic studies (Van der Voo et al. 2006) have established 40-90° counterclockwise rotation of Permian declinations in the northern wall of this zone.
In the east, the Irtysh-Gornostaev and Scytho-Turanian shear zones join each other in eastern Mongolia. Kinematic features of their eastern E-W-striking continuation was inferred as sinistral from the kinematic history of the Altaids and Manchurides (aengör and Natal’in, 1996) however direct structural observations supported this inference appeared later and only in a few locations.
Recent studies in Inner Mongolia and Southern Mongolia have resulted in discoveries of numerous usually wide shear zones with steep dipping foliation and subhorizontal stretching lineations. One of them stretches along the northern boundary of the Sonidzuoqi (Baolidao) unit, which is the Ordovician-Permian magmatic arc identical to the Uliastai active continental margin of the South Gobi microcontinent. The Sonidzuoqi unit may represent a strike-slip sliver of the Uliastai arc that traps a part of the Uliastai forearc region, the Lugingol-Hegenshan unit. If this inference is correct the sinistral displacement of the Sonidzuoqi unit should be more than 600 km. The early Permian Lugingol flysch also reveals bedding parallel shearing associated with asymmetric steep plunging folds, NE-striking cleavage with steep plunging cleavage/bedding intersections, and wide zones of quartz veins showing en-echelon arrangements. All of them indicate sinistral sense of shear. The same structural style has been observed along the Solonker suture in Mongolia. The mélange along this suture includes blocks both oceanic and magmatic arc origin embedded in matrix with steep foliation. The creation of this mélange can be easier explained by arc-slicing faulting than by pure subduction.
In northern China and Southern Mongolia, the transcontinental shearing could be younger in age (250-20 Ma) as geological data and rare Ar/Ar dating show. Yet younger shearing one may see at the Altaid-Manchuride-Nipponide junction where it was responsible for the eastward escape of the Turan, Malokhingan, and Jiamusi units in front of the Jurassic-Early Cretaceous Great Hingan magmatic arc.