THE CHARACTERISTICS OF THE DEEP FRACTURES AND THEIR RELATED TECTONIC PROBLEMS

The Sanjiang area in Southwest China is a strongly fractured and folded area on the eastern margin of the Qinghai-Tibetan plateau where the crust is shortened for more than 500 km so that the deep fracture zones are closely spaced but their extension and connection are clear.Under such conditions the arcuate deep fracture zones of this area may be grouped into four deep fra- cture systems,i.e.:（1）the Bangong Lake-Nujiang River deep fracture system, （2）the North Lancang River-Shuangjiang deep fracture system,（3）the Jinsha River-Tengiao River deep fracture system.The principal deep fracture zone of each deep fracture system is a subduction zone of a plate of a certain order.The ophiolites left over by various subduction zones are just the evi- dence for the existence and evolution of Paleo-and Neo-Tethys.Paleo-Tethys is bounded on the east by the Longmen Mrs.-Red River subduction zone and on the west by the Bangong-Nujiang River zone.The areas to the west of the latter zone all belong to the domain of Neo-Terhys.The paleo-ocean was formed in C₁.The transformation from Paleo-Tethys to Neo-Tethys took place in the period of J₃-K₁.The boundary between Gondwana and Eurasia extends from the western sector of the Karakorum Mountains southeastwards to the North Lancang River-Shuangjiang River subduction zone.Therefore,this newly esta- blished Variscan-Indosinian subduction zone is by far more important than the Bangong Lake-Nujiang River suture zone,but the latter is the boundary fo transf- ormation from Paleo-Tethys to Neo-tethys,so its role should not be neglected.