FEATURES OF PLIOCENE-LOWER PLEISTOCENE SEDIMENTARY FACIES AND TECTONIC EVOLUTION IN THE ZANDA BASIN,NGARI AREA,TIBET

The Neogene Pliocene-Quaternary lower Pleistocene strata in the Zanda basin, Ngari area, Tibet, may be divided into the lower Pleistocene Qangzü (Q_p¹x) Formation and the Neogene upper Pliocene Guge Group (N₂gg) and the latter may be further divided into the Pliocene Zanda Formation (N₂²z) and the Pliocene Toling Formation (N₂¹t).The strata belong to graben-type lake-basin deposits and consist predominantly of alluvial-proluvial, paraglacial, glaciolacustrine, lacustrine and fluvial facies.The tectonic evolution of the basin went through five stages, i.e.:the early-rifting stage (N₂¹t), sustained rifting (N₂²z), lake basin outflow (Q_p¹x), steady uplift (Q_p²f) and rapid strong uplift (Q_p³-Qh).The activities of several normal faults on the southern and northern sides of the basin control the distribution pattern of the sedimentary facies:when fault activities were strong, the water bodies of the lake basin enlarged, the water became deep and mainly deep-and semi-deep lake deposits occurred; when fault activities weakened, the water bodies became shallow or dried up, the lake basin was filled up and there mainly occurred fluvial and alluvial-proluvial deposits.Since the beginning of the Quaternary, alpine glacial activities around the Zanda basin have intensified with the rapid uplift of Qinghai-Tibet Plateau, and then paraglacial and glaciolacustrine deposits have developed.