On the Permian biogeographic boundary between Gondwana and Eurasia in Tibet, China as the eastern section of the Tethys

Abstract

As is well known, Glossopteris as a cool climate flora and the bivalve Eurydesma, the rugose coral Lytvolasma, the brachiopod Globiella and the fusulinid Monodiexodina as a cold-cool water fauna often occur with tillites and tilloids of glacial-glaciomarine origin or else closely following such rocks in the northern margin of the Godwanaland in Early Permina times (Asselian-Sakmarian). Therefore, they are usually considered as the indicators of the Gondwana Tethys facies. The case of the Gondwana Tethys is quite different from that of the Cathayan Tethys as the southern margin of Eurasia. The biota of south China and the Kunlun Mountains belonging to the Cathayan Tethys is characterized by tropical flora and warm water fauna. Evidently south China and the Kunlun Mountains area were covered by a warm water sea during the whole Permian period and perhaps from the late Middle Permian stage onwards the seawater became tropical. The Indus-Yarlung suture is the biogeographic boundary between Gondwana and Eurasia based on the discoveries of Glossopteris flora and Lytvolasma fauna in Qubu and Kangmar, southern Tibet. Recently, with the rapid advance of geological and palaeontological data the cool climate flora and the cold-cool water fauna with glaciomarine deposits have been found in Early Permian rocks in many localities of the Gangdise-Nyainqentanglha and Qiantang blocks. Therefore, they definitely belong to the epeiric sea of the marginal Gondwanaland. The biogeographic boundary between Gondwana and Eurasia should be moved from the Yarlung suture and the Bangongco-Nujiang suture northwards to the Longmuco-Yushu suture (i.e., the boundary passes between the Songban-Ganze block and the Qiangtang block.) It is very interesting that the Permian biota of Tibet such as that of the Salt Range shows the three developmental stages: the cold-cool water type, the mixture type and the tropical-warm water type. The change of flora and fauna might be caused by the Gondwanaland glacial action in Early Permian times (Asselian-Sakmarian), the worldwide warming of climate and the rifting and rapidly northwards drifting of Gondwanaland during the early Middle Permian (Artinskian). In the late Middle Permian the Gondwana Tethys became still warmer and even tropical and the typical warm tropical fauna of Neoschwagerina and Verbeekina quickly replaced the cool water one.