Plate Tectonics of Eastern Eurasia in the Light of Fault Systems

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Intra-plate stress field and crustal deformation are considered to be definitely controlled by the condition of plate boundaries and the heterogeneity of lithosphere. Therefore, the method to estimate the boundary conditions inversely from the intra-plate stress field and the crustal deformation is probably an effective approach to plate tectonics. The recent tectonic stress field of Asia is favorably simulated by using the finite element method on the assumption that the lithospheric thickness varies from 50 to 150 km corresponding to the age of crust, and that the direction of lateral load and displacement on the plate boundary is in accordance with that of the relative motion of plate. The results delineate the magnitude of stress vector on the Himalayan plate boundary about 3 times greater than that on the Pacific side plate boundaries, suggesting the effectiveness of this approach. Then, this approach is adopted to study the plate tectonics in the geologic past. Lineament analysis is done on the LANDSAT images in association with existing information the by land-survey, in order to reconstruct the tectonic stress fields of the Asiatic continent in the geologic past. On the basis of the regional characteristics of the directional dominancy of fault lineaments, distribution density, curvature and so on, the Asiatic continent is divided into the Pacific side, Tibet-south Mongolian, East Baikal and Three River domains, and eighteen fault lineament systems are distinguished. Each of these systems characterizes these respective domains. As a result, it is elucidated that the strong horizontal compression has acted alternatively from the Tethys (Indin) Ocean side and the Pacific Ocean side since Late Paleozoic time. In the Pacific side domain, the period when compression was strong ranges from Late Jurassic to Early Cretaceous. NS to NE-trending left-lateral faults (systems A_1, B_1, C_1) and ENE-trending thrusts with a left-lateral component (systems D and G) ("East Asiatic Left-lateral Fault Belt") were extensively developed. The principal axis of compressional stress gradually changed from NS to NW during this period. This suggests a corresponding change in direction of the relative motion of the plate in the Pacific. The horizontal compression in this period was very strong, and is estimated to be about 4 times greater than that on the Tethys Ocean side by the numerical simulation of the finite element method. Such a strong compression could not be caused simply by the accretion of the micro-continents of Japan and Sikhote Alin. Some other additional causes, including a very high convergence rate of the plates, are required. The fault lineament systems A_1, B_1 and C_1 were reactivated as normal faults during Eocene to Early Miocene. From such faulting, NW-trending horizontal tensional stress field is restored. Not only this tensional stress field, but also the contemporaneous back-arc spreading of the Japan Sea Basin and the South China Basin are supposed to have been caused by the northward motion slower than 7.2 cm/y of the eastern part of the Indian Plate and the Philippine Plate. Between the Siberian Platform and the Indian continent lie Baikalian, Caledonian, Variscan, Indosinian, Yanshan and Himalayan geosynclinal-orogenic belts which embrace some continental blocks. The mega-structure of this region is represented by two arches facing each other : the southward-convex arch which surrounds the southern margin of the Siberian Platform, and the north-northeastward-convex arch in front of the Indian continent. The faults parallel with these mega-structures are thrusts which originated at one of the tectonic phases mentioned above and were reactivated in later phases. They are represented by the lineament systems R, R_1 and R_2 in the Tibet-south Mongolian domain, and by the system U in the East Baikal domain which fringes the southeastern margin of the Siberian Platform. These fault systems include some paleo-subduction zones. Other dominant fault systems are NE to ENE-trending left-lateral faults (system S_1) and WNW to NNW-trending right-lateral faults (systems S_2 and S_3). These fault systems suggest that at any orogenic phases the western part of Asia was put under the control of a NS to NNE trending horizontal comperession. Such intermittent compressional stress fields correspond with successive stages of continental collision and accretion to the continent on the north : the collision of the Talimu and Sino-Korea continents with the Siberian Platform in the latest Permian or Early Triassic, the accretion of the Qiangtang-Indochina continental blocks in the Late Jurassic the accretion of the Iran-Afghanistan-Nyainqentanglha continental blocks in the Early Cretaceous and the collision of the Indian continent and its deep penetration into the Asiatic continent since the Eocene. The NS to NNW-trending faults in the Three River domain on the eastern wing of the Himalayan arc were formed as thrusts, thereafter they were converted to strike-slip faults by the deep penetration of the Indian continent into the Asiatic continent.
東北大学の論文
1985-03-30

Plate Tectonics of Eastern Eurasia in the Light of Fault Systems

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東北大学 | 論文

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