Tectono-paleogeographic characteristics in northeastern Tibet, China and spatial-temporal transition of the northern boundary of Gondwana

The study region is located in the junction between the N-S-trending Sanjiang tectonic belt and E-W-trending Qiangtang tectonic belt. It is an important component part of the Tethys tectonic domain, including the Jinshajiang, Lancang River, Shuanghu-Changning and Bangong Co-Nujiang Paleo-Tethyan oceanic crustal belts and related parts of the Qamdo-Markam, Qiangbei, Sog-Zogang and Doima-Amdo microblocks. Four sutures sometimes were separated and sometimes converged and the microblocks between them were apparently separated but actually joined. The pre-Ordovician Jinshajiang belt is the northern boundary of the Pan-African soft basement of Pan-Gondwana, and in the early Paleozoic it was still the boundary of the Pan-African mixed biota and the Yangtze realm. In the Devonian to Carboniferous, the Paleo-Tethys diverged into the north and south branches, i.e. the Jinshajiang belt and Shuanghu-Changning belt, which, together with the median island block swarm of Yangtze affinity (Qamdo-Markam, Qiangbei and Sog-Zogang) between them, formed the framework of the "archipelagic sea". Then the Shuanghu-Changning belt was transformed in the northern boundary of Gondwana and the Doima-Amdo microblock was of Gondwana affinity. In the Early-Mid Permian, the median island blocks (block swarm) broke up into three island blocks: the Qamdo-Markam block of Yangtze affinity and the Qiangbei and Sog-Zogang blocks of Gondwana affinity, forming an archipelagic sea. The northern boundary of Gondwanaland was converted into the Langcangjiang belt. In the Late Permian, the Paleo-Tethya died out, forming Pangaea with an archipelagic sea in the study region. During the Late Triassic Indosinian orogeny, the Jinshajiang, Langcangjiang and Shuanghu-Qamdo belts were transformed into intracontinental orogenic belts. Meanwhile, the Bangong Co-Nujiang and Yarlung Zangbo belts formed the north and south branches of the Meso-Tethys, with a median microblock swarm named "Cimmerides" between them. In Late Jurassic to Early Cretaceous the Bangong Co-Nujiang belt was extinguished and the "Cimmerides" was accreted to the Eurasian continent in the Late Cretaceous. The Himalayan block was finally subducted beneath the Asian continent in the stage of the Himalayan movement.