Abstract: 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 formation of Pangea,breakup of Gondwana land and accretion of the Asian continent.Our conclusions 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 epicontinental 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 systems 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 development,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 polyarc orogenesis model,which is believed to be the key for the interpretation of the transformation 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.