FLUVIAL TERRACE FORMATION AND TECTONIC UPLIFT RATEA CASE STUDY OF LATE QUATERNARY FLUVIAL PROCESS IN THE NORTH PIEDMONT OF THE TIAN SHAN, NORTHWESTERN CHINA
Tectonic geomorphology focuses on tectonic and surface processes and their effects on landform evolution, which is generally evaluated by the rate of tectonic activation. It is common to use the data on fluvial terrace (the surface height above the riverbed of a terrace and its formation age) to constrain the rate of river incision and further the rate of tectonic uplift. It is well known that both climate change and tectonic uplift can drive river incision to form the terrace sequence. Given the complexity of terrace formation, the results based on this method are likely incorrect. In the mountain front characterized by thrusting and folding, fluvial terraces preserved within an anticline are generally deformed as a result of fold growth. Beyond this structure, this terrace sequence is seldom affected by folding. Thus, the folded terrace within the anticline can be used to characterize the history of tectonic deformation of this structure and estimate the magnitude and rate of tectonic uplift. As one of the largest and most active mountain ranges in Asia, the Tian Shan exhibits rapid rates of deformation due to north-south convergence driven by the India-Asia collision system during the Cenozoic times. As a consequence, three successive roughly east-west-trending zones of fault-controlled folds have developed in the north piedmont of the Tian Shan, which are Belt I (Nan'anjihai-Nanmanas-Qigu-Changji-Kalazha Anticlines), Belt II (Huoerguos-Manas-Tugulu Anticlines) and Belt III (Dushanzi-Hala'ande-Anjihai Anticlines), respectively. Subsequently, transverse rivers incise into these anticlines and display well-developed alluvial fans and fluvial terrace sequences. By studying these units, we can better understand the mechanism of river incision and the pattern of tectonic deformation during Late Quaternary in this region. Here, we apply this approach to estimate the uplift rates of the anticlines of the three thrust- and-fold belts in the north piedmont of the Tian Shan based on these river terrace sequences of several main rivers (the Urumqi River, Taxi River, Jingou River, Manas River, Anjihai River and Kuitun River), and further analyze the temporal-spatial feature of Late Quaternary tectonic activation in the north piedmont of the Tian Shan Mountains. Our results indicate that the uplift rate of Tugulu anticline in Belt II since 13ka is 3. 52mm/a, Huoerguos anticline is 4. 8mm/a, higher than that of Dushanzi anticline (1. 2~1. 9mm/a) in Belt III and the east Manas anticline (<2mm/a) during the same period. These estimates on rates of tectonic uplift for the anticlines are regarded as indicative of a progressively weakening tectonic activation from the proximal to distal thrust- and-fold belt in the present study area, which is consistent with the pattern of crust shortening of these anticlines. For the single anticline of these belts,the similar temporal trend of tectonic activation can also likely be inferred from the uplift rates. In Belt II, the uplift rate of Tugulu anticline since 31ka is 4. 16mm/a, then decreasing to 3. 52mm/a since 13ka, the same to Manas anticline (from 1. 57mm/a to 1. 18mm/a) during this period (14~5ka). The uplift rates of Dushanzi anticline from 13ka to 11ka in Belt III indicate a similar decreasing trend (1.9-1. 2mm/a).