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SOFT COLLISION, SUPERPOSITION OROGENY AND POLYCYCLIC SUTURING

Ren Jishun; Niu Baogui; Liu Zhigang(Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037)  
Microcontinents that constitute the present East Asan continent were P8rt Of the in-terchange tectonic domain between Gondwana and Siberia (Laurasia) in the geological past. In Paleozoic Era these small masses were mostly located to the south of the palaeo-Asian ocean, be-longing to the structurally complicated northern margin of Gondwana; in Mesozoic Era they were mainly situated to the north of the Tethys, belonging to the structurally complicatedsouthern margin of the palaeo-Asian continent (eastern Laurasia) . In the course of the closureof the palaeo-Asian ocean the orogeny that happened there was not a violent collision orogenybetween the Gondwana and the Siberia continents proper but a slight or, say, soft collision be-tween the microcontinents on their complicated margins. Generally speaking, when microconti-nents or small masses collide with each other, the orogeny is usually slight because of lesser ki-netic energy. The tectonomagmmatic activity is mainly restricted to the overthruting continentalcrust, and the underthrusting one is only affected slightly. Afer colision, the oceanic basin dis-appear, but sea water does not definitely withdraw and mountains do not necessarily come into being, leading to such collision that calls forth no response of mountain building. Thus, the mi-crocontinents, though being connected, are not to be jointed into a single continent in dynamicrespect, that is to say, they are in a state of "connection rather than integrtion". These non-u-nited microcontinents may be subjected to another orogeny in a new tectonic cycle and new geo-dynamic conditions. The continental block lying in a low position will be again underthrust be-neath the high mass, resulting in the subduction orogeny of continental crust, i. e., the superpo-sition of the margin of one continent upon that of another, which is called continental superposi-tion orogeny. The continental masses can be geodynamically welded as a whole only after long-term and polycyclic continent-continent superposition or, say, polycyclic suturing. The weldingprocess between the Sino-Korean block and the Yangtze block, which was accomlished not on-ly by the Caledonian collisional orogeny, but also by the Variscan, Indosinian and Yanshanianpolycyclic superposition orogeny as well as the strike-slip-compressional orogeny, is an exampleof soft collision and polycyclic suturing.
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