OPHIOLITE AND CONTINENTAL SUTURE
Chao Tsungpu (Institute of Geology, Academia Sinica, Beijing)
In this paper, the problems of ophiolite and continental plate tectonics are discussed, and some knowledges and notions are obtained. In the folded range the folded units and nappes involving the old Precambrian continental crust or crystalline basement are well known.When nappes are superposed, ophiolites are often found in the higher units. This means that their place of origin is relatively far back and it is consistent with their origin in the back-arc basin. Theoceanic flor hypothesis o'f ophiolite come from the general idea that oceanic floor are made up of mafic and ultramafie rocks just as ophiolites are. It is considered that oceanic crust can be generated in different tectonic setting: mid-ocean ridges, back-arc basins, spreading trough, and perhaps in immature island arc. After examining the chemical data of the lava in the typical ophiolites, such as Troodas and Vourinos, some recent authors show that the volcanic rock are quite different from those of mid-ocean ridge, but similar to those of island arc/ marginal basin system, and boninite-like magma often occur. In contrast, some famous ophiolites, such as North Appinne, Corsica show mid-ocean ridge basalt-like magma, regardless of the width of the original basin. But, the Alpine-Apennine Jurassic oceanic basin was formed through thinning and ri'fting of the former continental lithos-phere. However, an island or marginal basin origin has recently heen advocated for a number of ophiolites on the basis of observed petrographic, chemical and tectonic relations.It is certified that the volcanic magma types erupted in young back-are basin, such as the east Scotia Sea, shows a spectrum of composition from Island arc to ocean ridge basalts, while magma with island arc affinity has not so far been found in well developed back-arc basin, such as Parece Vela basin where magma type are chemically indistinguishable from mid-ocean ridge basalt.The Himalaya Indus-Tsangpo ophiolite suture is thought to represent the trace of the lost Tethyan ocean, but series of Gondwanian character have been recently discovered to the north of it, even containing the same upper Carboniferous glacial deposits as the famous spiti and salt Range sections. This suggests that this ophiolitic suture corresponds to a rift opened within the Gondwanian continental shelf.In accordance with the concept of fault-block system, tension and compression are closely related with each other and mutually interdependent two styles of fault-block movement, which are accounted for the major mechanism for mutual transformation of continental and oceanic crust. Continental crust stressed by tension is found to be disintegrated into rift or graben along fault, with further pulling apart and depression then transformed into abyssal basin through shallow seas. If the lithosphere is cut thoroughly by rift fault zone, accompaning with upswelling of upper mantle materials and a thick pile of basaltic and perhaps ophiolitic rocks, eventually an oceanic crust is thus developed. Owing to the action of compression, the oceanic crust may usually be folded and thrusted as well as crumbled togather generally remaining as suture zone of op-hiolites.