Cenozoic tectonic and geomorphic evolution of the Longxi region in northeastern Tibetan Plateau interpreted from detrital zircon

The Longxi region contains different kinds of Cenozoic sediments,including eolian deposits,reworked loess,fluvial and lacustrine deposits.The provenance evolution of these sediments is of great significance in exploring the uplift,tectonic deformation and associated with geomorphic evolution of the Northeastern Tibetan Plateau.In this paper,we used the single-grain zircon provenance analysis to constrain the provenances for the Paleogene alluvial conglomerates and for the Neogene fluvial-lacustrine sediments,and compared them with results from the loess deposits since the Miocene.The results show that:(1)the Paleogene alluvial conglomerates contain a large number of detrital zircons ranging from 560 to 1100 Ma that were derived from the Yangzi Block.However,the sediments of early Miocene have much fewer zircons of this age span,which are characterized by an abundance of zircon ages in the ranges of 200-360 Ma.This indicates that the Paleogene alluvial conglomerates mainly come from the middle and/or southern West Qinling,and the early Miocene sediments are primarily from the northern West Qinling;(2)Late Neogene fluvial sediments(11.5 Ma onward)in Tianshui-Qinan region are dominated by zircon ages of 3 80-450 Ma.This zircon population is similar to that of the exposed intrusive rocks of southern part of the Liupan Mountains,implying that the southern part of Liupan Mountains probably had already uplifted by 11.5 Ma;(3)Late Miocene lacustrine sediments in Tianshui region have a zircon age spectra that is remarkably different from coeval fluvial deposits,but is similar to the zircon age distributions of the Miocene loess in Qinan region,late Miocene-Pliocene Hipparion red clay and Quaternary loess.This indicates that fine particles within these Miocene lacustrine sediments in Tianshui region may be dominated by aeolian materials.This study reveals that provenance changes of Cenozoic sediments in Tianshui-Qinan region and its geomorphic evolution are closely related to the multi-stage uplift of the Northeastern Tibetan Plateau.In particular,the major uplift of the Northern Tibetan Plateau during late Oligocene-early Miocene may have not only provided the source areas and wind dynamic conditions for the deposits of the Miocene loess,but also provided the geomorphic conditions for its accumulation.

Spatial and Temporal Dynamics of Lakes in Nam Co Basin, 1991–2011

Lakes in Tibet Plateau with little effects of human activities serve as important indicators of climate change. This study analysed remote sensing data and long term climate variables to examine the hydrological response of lakes in Nam Co Basin. The area changes of lakes were extracted by Landsat TM/ETM＋ and analysed by SRTM 3 DEM. And the ICESat elevation data between 2003 and 2009 were used to observe the lake level of the Nam Co Lake. The results show that the number of new formed glacier lakes increased by 36% and the area of glacier lakes increased by 36.7%（0.97 km^2） from 1991 to 2011. At the same time, the surface area of the Nam Co Lake expanded by 3.71%（72.64 km^2） of the original size in 1991, with a tendency value of 3.63 km^2 per year. The lake level of the Nam Co Lake shows an increase tendency of 0.24 m per year during 2003-2009. These variations appear to be related to an increase in mean annual temperature of 0.06 oC per year, and an increase in annual precipitation of 2.1 mm per year in summer in the last two decades. The increased number of lakes and increased area of glacial lakes reached a peak at an altitude of 5 500-5 600 m a.s.l.. The number of new formed glacier lakes and the area of glacier lakes tend to higher altitudes. Climate change has an important impact on the variation of the glacier lakes and the Nam Co Lake.