No abstract

Multiple lines of stratigraphic, geochemical, and fossil data suggest that fresh-mesohaline paleolakes were widespread in the Tengger Desert of northwestern China and underwent major fluctuations during the late Pleistocene. The paleolakes started to develop at ca. 42,000 14C yr B.P. The lake levels were the highest between 35,000 and 22,000 14C yr B.P., during which Megalake Tengger dominated the landscape. The climatic conditions at this time were unique for this area and have no modern analogue. After an episode of decline between 22,000 and 20,000 14C yr B.P. and an episode of rebound between 20,000 and 18,600 14C yr B.P., the paleolakes started to desiccate and completely disappeared around 18,000 14C yr B.P. The environmental proxy data indicate that the Megalake Tengger formed under warm–humid climates. The reconstructed climatic variations appear to be correlative with the abrupt climatic events reconstructed for the North Atlantic.

Summary A series of terranes were accreted to Eurasia in the region of what is now the Tibetan Plateau, including the Qaidam-Qilian, the Songpan-Ganzi, the Qiangtang, the Lhasa and the Tethyan Himalaya terranes. The drift history of the Qiangtang Terrane and the timing of the Lhasa-Qiangtang collision is controversial. To contribute to this topic, here, we paleomagnetically investigate the Middle-Upper Jurassic limestones of the Yanshiping group in the Zaduo area (32.5° N, 95.2° E), in the eastern Qiangtang Terrane. Twelve sites (133 samples) were processed. A major challenge in paleomagnetism is the possibility of remagnetization that interferes with paleogeographic reconstructions. Both thermal and alternating field demagnetizations were carried out to isolate the characteristic remanent magnetization (ChRM). Despite the positive reversals test, rock magnetic information points to a remagnetized ChRM. The ChRM is residing in stable single-domain (SSD) magnetite grains with cogenetic superparamagnetic (SP) particles. The co-occurreance of SSD and SP magnetites generate distinct rock-magnetic properties often referred to as the ‘remagnetized fingerprint’ in limestones. This remagnetization process is also manifested by the widespread occurrence of gypsum veinlets in the limestones. The site-mean direction of the 12 sites after tilt-correction is Ds = 30.6°, Is = 35.6°, κs = 182.9, α95 = 3.2°, corresponding to a paleolatitude of ∼19.7°± 2.8° N for the study area. The corresponding paleopole (59.8° N, 202.7° E with A95 = 2.8°) points to an NRM acquired after the India-Eurasia collision. The original sediments were likely anoxic because of the high organic carbon fluxes that prevailed during their deposition. After the India-Eurasia collision, it is envisaged that conditions became more oxic, giving rise to oxidation of iron sulphides to authigenic magnetite and the CRM acquisition. The Zaduo area in the Eastern Qiangtang Terrane has experienced ∼15.7° ± 3.2° (∼1740 ± 350 km) of latitudinal crustal shortening since the Eocene. In addition, the clockwise rotation responding to the India-Eurasia collision is also detected in the Zaduo area.