Rapid incision events since 3.7 ka in the Anjihaihe Gorge along the northern Tianshan range, China

Abstract

Young river terrace levels distributed along active tectonic regions and major river channels record the histories of the most recent tectonic activities and geomorphological evolution. There are 18 terraces in the upper reach of the Anjihaihe River, along active zones of the northern Tianshan in China. The surface of these young terrace insets is covered by Aeolian loess and fluvial deposits. Consequently, optically stimulated luminescence (OSL) dating was used to reveal the burial ages of those sediments, and to calculate the tectonic deformation and fluvial incision rate. However, the ages of young quartz sediments are difficult to determine due to the weak and dim OSL signal characteristics, and therefore geological and geomorphological information was not forthcoming. Recently, however, a “simplified procedures” methodology was developed to overcome these dating problems. Loess deposits can constrain the minimum abandonment age and fluvial sands were used to provide the age limits of formation, which correspond to the maximum and minimum incision rates. The elevation of each terrace above the modern river bed was obtained using a Digital Elevation Map with 12-m resolution, Google Earth images, and portable Global Positioning System measurements. Together with the results of the OSL dating and the elevation measurements, we conclude that the Anjihaihe River has incised the gorge to a depth of ~260 m since 3.7±0.2 ka, with a maximum average incision rate of 7.2±0.1 cm/a. The incision record can be divided into four major stages: ~3.7−3.0, ~3.0−2.4, ~2.4−1.3 ka, and from 1.3 ka to the present; the respective average incisive rates are 3.1±0.3, 15±0.3, 8.5±0.2, and 4.6±0.4 cm/a. The results indicate that the incision occurred in a series of steps which coincided with environmental changes in the region. As a result, this set of 18 strath terrace levels has recorded the aggradational-degradational surface processes of the Anjihaihe River gorge since ~3.7 ka. Before ~3.7 ka, a thickness of ~260 m of fluvial sands was aggraded above the pre-Quaternary bedrock. Subsequently, the river incised these sand layers until T11, and then, before the formation of T11, younger fluvial sands were deposited on the top of older deposits after 2.4 ka. The AJH River then continued to vertically erode these younger sands until 1.3 ka, when the incision reached the pre-Quaternary bedrock. From 1.3 ka to the present, at least five strath terraces were formed in the gorge. These surface processes and rapid incisive rates are ascribed to the specific lithological and geomorphological characteristics of the area, under the influence of climate change. First, the bedrock of the terrace levels is composed of pre-Quaternary sandstones, and more homogeneous, less consolidated siltstones and sandy mudstones. In addition, the upper terraces are covered by mixed cobbles and sands with a thickness of ~100 m above the bedrock, and the lower terraces by a thin layer of fluvial sands above the pre-Quaternary bedrock. Second, the variations in the flux of river water and sediments during episodes of climate change influenced the slope of the terrace surface, resulting in changes in aggradational-degradational processes and in the energy of the incision. Climate change was also a driving force, because the abrupt changes in incision rate correspond well to anomalous climatic events. Overall, the specific geomorphological characteristics and location of the Anjihaihe River have resulted in its recording an unusually rapid incision rate within the Anjihaihe Gorge.