The Kunlun, Qilian, and Qinling mountains already existed in embryonic form as coastal ranges in Triassic times. Marine conditions predominated to the south. The southern coastline is demarcated by the northern margin of the Tertiary Siwalik sediments. The paleo-landmass of Longmenshan–Sichuan–Yunnan separated this sea from that of southwest China. By Jurassic times, the coastline of the Eurasian continent within what is now China had already moved southwards to a line along the Kunlun and Hengduan mountains.Mainly on the basis of differences between continental and oceanic crust, several different seas can be distinguished: the Himalayan, Lhasa, Qiangtang, Hengduan, and Triangle seas, together with the Qaidam Peninsula and Xining Bay. Triassic and Jurassic transgressions and regressions of these seas are discussed in detail.During Carboniferous and Permian times the Lhasa and Himalayan seas were joined together with the main Gondwana Plate to the south. In Triassic times, when the India–Pakistan Subcontinent split from Gondwanaland, the Lhasa Sea was a small microplate, which separated from the Himalayan Sea along a deep fracture that follows the line of the Yarlung Zangbo (River). It drifted northeastwards relatively rapidly. The Triangle Sea was a similar microplate that drifted northwards. In Late Triassic times the Triangle Microplate collided with Eurasia along the margin of the Qaidam–Qinling fault belt. The Bangong Co – Nu Jiang deep fracture is regarded as the boundary between the northern and southern margins of the eastern Tethys, on the basis of Carboniferous to Jurassic paleobiogeographical provinces within the region studied.During Triassic and Early and Middle Jurassic times, an oceanic basin separated the Lhasa Sea from the Qiangtang and Hengduan seas. In the Late Jurassic Epoch, the Lhasa Microplate converged with the Qingtang Microplate, which had become attached to Eurasia. The former was subducted northwards. As a result, the boundary between the southern and northern margins of East Tethys was shifted southwards during Cretaceous times to the line of the Yarlung Zangbo (River) deep fracture. At the end of the Cretaceous and on into Eocene times, the Himalayan Sea closed with the collision of the India–Pakistan Subcontinent and the Lhasa Plate. Folding and faulting followed and the paleo-oceanic history of the Qinghai–Xizang Plateau came to an end.