Identification of Gondwana tillite in the Bange County of Tibetan Plateau and its implications for the tectonic evolution of North Lhasa terrane—New evidence from the 1: 50000 thematic geological survey

This paper is the result of geological survey engineering.

Objective As the main terrane in the Tibetan Plateau, the division of the Lhasa terrane is significance for the spatial and temporal distribution pattern of the Tethys Ocean. Generally, it can be further divided into the North, Central, and South Lhasa terranes. However, it is still ambiguous about their basement nature, drifting, accretion, and tectonic affinity.

Methods In this paper, based on the 1:50000 geological survey in the Beila area, North Lhasa terrane, the widely exposed and typical tillite and seismite are newly identified from the Middle-Upper Jurassic Lagongtang Formation, northern Tibetan Plateau.

Results They can be compared with the Upper Carboniferous-Lower Permian Lagar Formation in the Xainza area, Central Lhasa terrane, indicating the Late Paleozoic glacial event in the northern margin of the Gondwana. The glacial marine gravels, formed by ice raft, are distributed in the sandy, silty and argillaceous matrix, with falling structure. Seismites are mainly seismic corrugated and shattering, with syngenetic or parasyngenetic seismic sedimentary records of microfolds, sand (mud) veins, coseismic joints, intraclastpara-breccia, steplike faults and flame structures.

Conclusions The identification of the tillite of Lagar Formation indicates that the North Lhasa terrane holds similar basement with the Central Lhasa terrane. It is probably anaccretion-induced micro-block formed gradually on the basis of local continental crust, which is closely related to the subduction process of the Bangong-Nujiang Tethys Ocean. This provides key evidence for further studying the tectonic affinity of the North Lhasa terrane.