东特提斯多弧－盆系统演化模式

MODELS FOR THE EVOLUTION OF THE POLYARC-BASIN SYSTEMS IN EASTERN TETHYS

自７０年代以来，以板块构造观点分析特提斯演化已有三种模式，即“剪刀张”、“传送带”和“手风琴运动与开合”模式。所有这些模式都是以一个联合古陆的形成和特提斯是泛大洋（古太平洋）中一个海湾的假设为前提，或以冈瓦纳大陆裂离、亚洲大陆增生为基点。随着对东特提斯（以青藏高原地区为主体）地质构造演化的认识深化，特提斯演化、造山作用的解释由两陆（劳亚和冈瓦纳）一洋（特提斯）模式转变为三陆群（劳亚、冈瓦纳、泛华夏）二洋（特提斯和古亚洲）的特提斯多弧－盆系统洋陆转换演化模式，即多岛弧造山模式。这一多岛弧造山模式源自大陆地质，尤其是在中国西部造山带－盆地长期的地质考察研究实践。运用多岛弧造山模式，反思青藏高原及邻区山盆系统的地质事实，深刻认识到在东特提斯发现的许多由消减洋壳和消减杂岩所组成的蛇绿混杂岩带中，“三位一体”的蛇绿岩多数是“小洋盆”、弧后盆地、岛弧边缘海型，既存在早古生代岛弧、陆缘弧和晚古生代的火山弧，又有中生代的陆缘弧、岛弧。多岛弧－盆系统的存在意味着大洋岩石圈的存在、消减和转换。特提斯大洋岩石圈至少从古生代到中生代历经发生、发展到萎缩、消亡的长期连续的复杂的演化过程。古特提斯是原特提斯的继承和发展，中生代东特提斯?

Three models for the Tethyan evolution,i.e.,'scissors spreading','belt transforming'and'accordion opening and closing'have been proposed according to the plate tectonic analysis since the 1970s.All these models rest on the basis of the assumption that the Tethyan Ocean is a gulf' of the Palaeo-Pacific Ocean,or on the basis of the forma- tion of Pangea,breakup of Gondwana land and accretion of the Asian continent.Our con- clusions represent a refinement of the interpretation of geological and tectonic evolution of eastern Tethys (dominated by the Qinghai-Xizang Plateau).The Tethyan evolution and orogenesis are interpreted from the'two continents'(Laurassia and Gondwana) and'one ocean'(Tethyan Ocean) model to'three landmass groups'(Laurassia,Gondwana and Pan- Cathaysian) and'two oceans'(Tethyan and Palaeo-Asian) model for the ocean-continent transformation,i,e.,the polyarc orogenesis model of the Tethyan polyarc-basin system. This polyarc orogenesis model is deduced from the continental geology,especially the geology of the orogenic zones and basins in western China.In the ophiolitic melange zones consisting of subdueting oceanic crust and subduction complexes in eastern Tethys,the 'three in one'ophiolites are composed substantially of'small oceanic basins',back-arc basins and island-are marginal sea type,implying that there are both Early Palaeozoic is- land ares,epicontinental arcs and Late Palaeozoic volcanic ares,and Mesozoic epicontinen- tal arcs and island ares in the study area.The presence of the polyarc-basin systems means the existence,subduction and transformation of the oceanic crust.The Tethyan oceanic crust at least underwent,from Palaeozoic to Mesozoic times,initiation,development, clousre and termination.The Palaeo-Tethys represents the succession and development of the Proto-Tethys;however the Mesozoic eastern Tethys didn't result from the reopening of the closed Palaeo-Tethyan Ocean;Much of the Tethyan oceanic crust might have been incorparated into the subsequent Indian Ocean. The Kunlun front arc and Kang-Dian continental-margin arc occurred on the western side of the Pan-Cathaysian landmass group during Palaeozoic time.The formation of the polyarc-basin systems in northern Kunlun during the Ordovican was constrained by the Palaeo-Tethyan and Palaeo-Asian Oceans,similar to the Southeast Asia polyarc-basin sys- tems which were controlled by the bidirectional subduction of the Indian Ocean and Pacific Ocean. The Tanggula-Taniantaweng remnant arc split from the Kunlun front arc and Kang- Dian continental-margin arc constituted the Late Palaeozoic front arc installed on the southwestern margin of the Pan-Cathaysian continent.The Late Palaeozoic to Mesozoic represent the evolutionary stages of back-arc spreading,polyarc basin system develop- ment,arc-arc collision and arc-continent collision in the Qiangtang-'Three Rivers'area. It is shown that the northern margin of Gondwana land on the southern side of the Tethyan Ocean began to develop into an active continental margin from the Carboniferous onwards.The Mesozoic also represents the periods of the arc-basin evolution on the Xizang Islands. On the basis of the coexistence of the rock associations and the polyarc-basin systems in the eastern Tethyan temporal and spatial units,the present paper has put forward a pol- yarc orogenesis model,which is believed to be the key for the interpretation of the trans- formation of the oceanic crust into the continental crust,We are sure that the model will has great vitality for the further research of the Qinghai-Xizang Plateau and global palaeogeographical reconstruction.