Origins of some microcontinental blocks distributed in eastern Asia, such as the South Kitakami-Kurosegawa, Khanka and Songnen, and paleogeographic relationships among them during the Paleozoic to Mesozoic are discussed from the viewpoints of their paleobiogeography and tectonic history. The South Kitakami-Kurosegawa block originated in the subduction zone along the northern margin of the Gondwanaland, situating near the South China and Australia cratons, during the Early Paleozoic. It separated from the Australia at Late Devonian time with the South China and had located near the South China until the Jurassic. The Khanka and Songnen blocks have Precambrian basements and had located near the Siberia craton during the Paleozoic, excepting the latest Period. They separated from the Siberia and began to drift southward at the earliest Permian, and collided to the northern margin of the North China craton at the latest Permian. The South Kitakami and Khanka had belonged to the same biogeographic realm during the Middle Permian to Mesozoic. This close biogeographic relationship is considered to be resulted by the rearrangement of the continents and change of ocean circulation system around the eastern Asia, and do not mean the close geographic situation. The South Kitakami-Kurosegawa and Khanka had been located in the different geographic domain before the Cretaceous: The South Kitakami-Kurosegawa is near the South China, whereas the Khanka along the northern margin of the North China.

View full abstract

Structural development of the Motai-Nagasaka area of the South Kitakami Belt, NE Japan is discussed in light of precise mapping and microscopic observation of deformation zones. The following succession of events is recognized in the study area, which is occupied, in apparent ascending order, by the high-P/T type Motai Metamorphic Rocks, 500-440 Ma basement igneous rocks, and Paleozoic strata that cover the basement igneous rocks, 1) the formation of the Boundary Shear Zone along the top of the Motai Metamorphic Rocks and their exhumation through a sinistral or top-to-the-south shearing, 2) initiation of sinistral shearing along the Tsuwamono-zawa and Sarusawa shear zones, 3) asymmetric minor s-folding and the formation of major synclinorium as the result of the drag and/or transpression along the above two sinistral shear zones, 4) formation of slate zones in the western limbs of above minor and major synclines, and 5) cooling of the 120-105 Ma granite in the Kitakami Mountains. A restoration model is finally presented of the Early Cretaceous sinistral displacement in the study area, which suggests that the study area was shortened about 65% in N-S direction and thickened about 150% in E-W direction through Early Cretaceous sinistral shearing.

View full abstract

An E-to ENE-trending group of shear zones, the Inner Mongolian Shear Zone, is reported from the Central Asian-Mongolian mobile belt along the northern margin of the North China Block (NCB) in the mideastern part of Inner Mongolian Autonomous Region, China. The Inner Mongolian Shear Zone is subdivided into the Bayan Obo-Xar Moron River, Ondor Sum-Xilingol and Erenhot-Hegenshan shear zones from south to north, and all of them runs along the boundaries of previously distinguished tectonic zones. The Inner Mongolian Shear Zone, having sinistral strike-slip displacement component, contains sheared rocks originated from Late Paleozoic "Variscan" granites, but gives no evident deformation to Late Jurassic sedimentary rocks and Cretaceous "Yanshanian" granites. Considering that the "Variscan" granite occurs in both the Central Asian-Mongolian mobile belt and the NCB, we suggest that the Inner Mongolian Shear Zone rearranged the tectonic zones of the mobile belt in the latest stage or after its amalgamation with the NCB.

View full abstract