Abstract:

The~1600km long, left-reverse strike-slip active Altyn Tagh fault system defines the northern edge of the Tibetan plateau, and serves as an important tectonic boundary in models describing the northward expansion of the plateau. The Altyn Tagh fault system has complex geometries, and consists mainly of the left-lateral South Altyn Fault to the south, the left-reverse or reverse-dominated North Altyn Fault to the north, and the intervening Altyn Shan. Most of the existing studies focus on the more active South Altyn Tagh Fault, but few has paid attention to the North Altyn Fault, which separates the Tarim Basin to the north from the Altyn Shan to the south, and figures importantly in understanding the tectonic evolution of the entire fault system. The kinematics of the North Altyn Fault in the Cenozoic remains disputed in whether it is a left-reverse or reverse-dominated fault. Herein, we used tectonic geomorphology analysis to systematically study the characteristics of active tectonics on the North Altyn Fault in the Quaternary. There are dozens of rivers in the Altyn Shan between the South Altyn Tagh Fault and North Altyn Fault, the majority of which originate near the South Altyn Tagh Fault and flow northward across the North Altyn Fault into the Tarim Basin. These rivers contain abundant information about the Quaternary tectonic activity of the North Altyn Fault. We used SRTM DEM data to extract the geomorphic features of 18 rivers and related catchment basins flowing across the North Altyn Fault. Geomorphic index, such as river longitudinal profiles, standardized river length-gradient index(SLK), normalized river steepness index(K_sn), area-elevation curves and their integrals(HI)of catchment basins, are analyzed. The conclusions are drawn as follows.
The geomorphological indexes show that the eastern part of the North Altyn Fault is geomorphologically more active than the western part. Along the western part of the North Altyn Fault, the river longitudinal profile and the area-elevation curves of the corresponding catchment basins are both concave upward, with many small knickpoints on the river profile and relatively low SLK, K_sn, and HI values. On the contrary, most of the river profiles in the eastern part of the fault are convex or linear, with much larger knickpoints on the hanging wall of the North Altyn Fault, coinciding with high SLK and K_sn values. The associated area-elevation curves are mainly S-shaped and convex, and the HI values are relatively large. Tectonic geomorphic index is generally affected by lithology, climate and tectonics. The lithology of the hanging wall of the North Altyn Fault is relatively simple, consisting mainly of Precambrian metamorphic rocks intruded by some granite. There is no obvious difference in rock strength between the entire eastern and western sections. In addition, since the rivers are all located in the Altyn Shan and the area involved is not large, there is also no significant climatic variation along the strike of the North Altyn Fault in the Quaternary. Therefore, the difference of geomorphological activities between the parts should not be caused by difference in lithology and climate. Instead, we found that the eastern part of the North Altyn Fault is located to the north of the Akato restraining double bend, which features intense crustal shortening due to change of the fault strike, on the active South Altyn Tagh Fault. As such, we infer that the strong geomorphic activity of the eastern part of the North Altyn Fault likely results from intense lateral contraction from the Akato restraining double bend to the south, suggesting intimate interplay between the South Altyn Tagh Fault and the North Altyn Fault.
Our findings also imply that the North Altyn Fault likely changed from a strike-slip-dominated fault to a reverse-dominated fault in the late Cenozoic. It can be seen from the extracted river morphology that all rivers are relatively straight when passing through the North Altyn Fault, without systematic left-lateral deflection. The geomorphic indexes, such as the locations of river knickpoint, high SLK and K_sn value, which reflect where the relatively rapid tectonic uplift has occurred, all appear in the hanging wall of the North Altyn Fault. Moreover, a south-dipping frontal fault is discovered in the north of the North Altyn Fault. This fault cut and uplifted the Quaternary alluvial fan in the hanging wall, and the amount of uplift decreases gradually from middle to both sides until it vanishes, forming a bilaterally symmetric anticline approximately parallel to the fault. The rivers across through the fault are straight and undeflected systematically. All these show typical characteristics associated with a thrust fault. We thus infer that the North Altyn Fault is dominated by reverse dip-slip in the late Quaternary. Together with the Cenozoic strike-slip motion on the North Altyn Fault by the measurement of kinematic indicators, a transition from strike-slip-dominated to reverse-dominated in the late Cenozoic is thus expected.

Key words: Altyn Tagh fault system, drainage characteristics, geomorphological analysis, along-strike variation, kinematic change

摘要：

位于青藏高原和塔里木盆地之间的北阿尔金断裂是青藏高原向N扩展的重要边界条件, 目前对其新生代以来的运动学特征还存在较大争议。文中利用DEM数据提取了穿过北阿尔金断裂的18条河流及相关汇水盆地的地貌信息, 分析了河流平、 纵剖面的形态、 标准化河流长度坡度指数(SLK)、 标准化河流陡峭指数(K_sn)以及汇水盆地面积-高程曲线和积分(HI)等多个构造地貌指标参数, 以研究北阿尔金断裂晚第四纪以来的活动性和活动方式。结果表明, 北阿尔金断裂西段的河流纵剖面和汇水盆地面积-高程曲线呈上凹型, 河流纵剖面上发育较多小规模的裂点, SLK、 K_sn和HI值相对较低。与之相反, 其东段的河流纵剖面大都呈上凸型和平直型, 在北阿尔金断裂上盘发育较大规模的河流裂点, 对应较高的SLK和K_sn值; 面积-高程曲线形态以S型和上凸型为主, HI值也相对较大。上述地貌指标反映北阿尔金断裂东段的地貌活动性较西段更强, 推测为东段正好位于南阿尔金断裂阿克图受阻双弯曲北侧, 受其强烈侧向挤压所致。此外, 河流穿过北阿尔金断裂没有发生系统的左旋位错, 新发现的一条错断晚第四纪冲积扇的前锋断裂也呈现出典型的逆冲特征, 表明北阿尔金断裂在晚第四纪以来以逆冲运动为主。结合前人的研究结果推测, 北阿尔金断裂在晚新生代存在从走滑向逆冲的构造转换, 且与南阿尔金断裂之间存在密切的相互作用。

关键词: 阿尔金断裂系, 水系特征, 地貌分析, 走向变化, 运动学转变