Quantitative Geomorphic Indices Characteristics Of The Urumqi River Drainage Basin In North Tianshan

Understanding the coupling relationship between climate change and tectonics, as well as their impact on landform evolution, has been one of the hottest issues in tectonic geomorphology. The most common objects used to carry out this research are orogenic zones. GIS spatial analysis on Digital Elevation Model (DEM) has been widely used to extract several quantitative geomorphic indices and can be applied in multiple spatial-scale, which, to some extent, has overcome the spatial limitation traditional geomorphic methods (eg. fluvial terrace analysis) possess that traditional geomorphic researches are normally confined to small catchments. Of these indices, Hypsometric Integral, Stream Length-gradient index, Stream-power incision model are the most frequently used indicators.The Tian Shan, as one of the largest and most active mountain ranges in Central Asia, has long been one focal research area in tectonic geomorphology. In this paper, we choose the Urumqi River drainage basin, one of the most important river systems situated in north Tianshan, as our research area. Tectonically and geomorphically, the Urumqi River drainage basin can be divided into four sub-areas, i.e. Tianshan uplift zone, Houxia graben, Nanshan uplift zone and piedmont depression, which are characterized by significantly different lithology and surface processes from each other. Previous work in this region was rather limited to discuss the forming mechanism of three east-west-trending zones of fault-controlled folds (Belt Ⅰ, Belt Ⅱ and Belt Ⅲ), relations between morainic landform and Quaternary glacial period, between fluvial terraces and tectonic uplift events in the north piedmont of Tianshan. However, the regional topographic evolution status of the Urumqi River drainage basin and evolutionary difference between its four sub-areas are still uncertain.In light of this, based on 30m ASTER GDEM data and GIS spatial analysis method, we first select four geomorphic indices, i.e. hypsometric integral, stream Length-gradient index, stream-power incision model and channel profile fitting function model, to discuss the regional landform evolution status of the Urumqi River drainage basin as well as four sub-areas. To figure out the evolutionary difference between different regions in north Tianshan, we then choose another four river drainage basins located in the Urumqi depression zone to compare with the Urumqi River, which are, from west to east, Kuitun River, Jingou River, Manas River and Taxi River, respectively.Our results show that, (1) the landform evolution of the Urumqi River drainage basin (HI=0.4884,θ=0.34) is, currently, at the stage of maturity (0.35<HI<0.6), during which period the magnitude of tectonic uplift is nearly equal to that of surface erosion based on analyses on hypsometric integral and stream-power incision model. And the topography of this region is close to steady-state (0.35<θ<0.6). Of these four sub-areas, only the topography of Houxia graben is near in steady-state. Channel steepness index analysis also indicates tectonic activity in Houxia graben is relatively weak compared to the rest three sub-areas.(2) Considering the sensitivity of river profile fitting function model and stream-length gradient index to several contributing factors (eg. lithology, branch afflux, local rupture structure) of channel profile, these two indices are actually not appropriate for landform evolution analysis. While in turn, this sensitivity to small-scale change of channel profile may be helpful to identify obvious or underlying active structures, like blind fault.(3) Tectonic uplift rates deduced from fluvial terraces located in anticlines show a northward decreasing trend in the north piedmont of Tianshan with a maximum in the second fault-controlled fold belt (Belt Ⅱ). Analyses on channel steepness index and concavity index, on the other hand, indicate that the most active tectonic zones, at present, are the Manas and Jingou River drainage basin. Besides, the strength of tectonic activity in the west, in general, are much higher than the east. Our results are in accordance with existing records of seismicity and crustal shortening amount, which has successfully proved the validity and feasibility of these geomorphic indices analysis in tectonic geomorphology.(4) Based on previous work, we have already known that the center of tectonic activity in the north Tianshan is moving to the north. Thus combined with the results which have been discussed above, we can infer that, of these three fault-controlled fold belts situated in the north piedmont of Tianshan, Belt Ⅱ is the most active zone at present. Besides, tectonic activity in Belt Ⅰ will be gradually weakening given its long history of topographic evolution, while that in Belt Ⅲ will be more and more strengthened.