冲断带构造作用控制下的分支河流体系特征及其成因分析

构造运动是控制分支河流体系发育的重要因素之一，其对沉积体系的控制作用一直是研究的热点与难点。前人对挤压、拉张、走滑等类型的盆地均有研究，但在挤压盆地边缘的冲断带中，冲断带对于沉积体系的控制作用研究较少，特别是对冲断带中常发育的分支河流体系（冲积扇）。为研究逆冲作用对分支河流体系的控制作用，通过Google Earth提供的全球地貌影像结合数字高程模型（Digital Elevation Model，DEM）等数据对塔里木盆地西北缘柯坪地区发育的分支河流体系（Distributive Fluvial System，DFS）进行测量研究与沉积学解析，探究盆地边缘逆冲断裂带的DFS几何形态发育特征及影响因素。在研究区内识别DFS共256个。柯坪地区发育的分支河流体系延伸长度范围为0.26~24.58 km，DFS面积范围为0.044~221.987 km2，物源区面积变化范围为0.1~290 km2，DFS坡度范围为0.011~0.182。柯坪地区分支河流体系上发育的河道主要是辫状河，河道下切程度有明显差异。决定柯坪地区分支河流体系发育的主要影响因素有物源供给，气候水文以及构造背景。物源区大小与DFS规模呈正相关；气候通过降雨量与蒸发量来影响DFS发育特征；水文则通过流域分布来影响DFS发育，分布在研究区内柯坪河流域的DFS，发育规模远大于流域外的；构造作用控制沉积区分布、可容空间的大小、坡度等，影响着DFS的分布与发育。研究结果表明:构造作用作为主要因素影响着柯坪地区DFS的分布与发育。在柯坪逆冲带，构造作用通过控制推覆体逆冲高低来影响DFS的大小与坡度。物源区大小也受地层抬升程度控制，地层出露越多，物源区一般越大。将柯坪地区以构造特征分为三个区，每个构造区的DFS具有明显差异。逆冲山前区的物源区域大，DFS规模最大，以扇形为主，坡度低，下切程度低；逆冲前缘区的物源区域较小，DFS规模变小，以长条形为主，坡度增加，下切程度增加；走滑断层区受断层影响，物源区与DFS规模明显变小，以扇形为主，平均坡度最大，平均延伸距离最小，下切程度显著。

Characteristics and Factor Analysis of Distributive Fluvial Systems due to Tectonic Thrust Belt Activity: Example of Keping area,northwestern Tarim Basin

Tectonic movement is an important factor in the development of distributive fluvial systems （DFS） and their effect on sedimentary systems， and has been the object of numerous studies. Previously， basin types have been studied （e.g.， compressive， tensile， and strike-slip）. However， fewer studies have been carried out on the effect on sedimentary systems of thrust belts at the edge of compressive basins， especially alluvial fan DFS which are a common development in the thrust belt. To study the influence of the thrust on DFS development， global landscape images provided by Google Earth were combined with a digital elevation model and other data. Surveys and sedimentological analyses were carried out to investigate the factors influencing DFS development and geometry in the thrust fault zone. A total of 256 DFS were identified in the study area. The branched river system developed in the Keping area extends for 0.26-24.58 km； the DFS area ranges from 0.044 to 221.987 km2. The source area varies between 0.1 and 290 km2. The slope of the DFS ranges from 0.011° to 0.182°. In most cases， braided rivers have developed in the Keping area， with significantly different degrees of undercutting. The main influencing factors in the development of DFS in this area are source supply， climate， hydrology and tectonic history. The size of the source area is positively correlated with the size of the DFS. Climate （rainfall and evaporation） and hydrological properties （distribution of drainage areas） affect the development of DFS. In the study area the DFS is much larger than other drainage areas. Tectonism has determined the sediment deposition area and accommodation space， slope， etc.， and the distribution and development of the DFS. This study indicates that the tectonic effect was the main influence on the distribution and development of the DFS in the Keping area: the thrust of the nappe has affected the size and slope of the DFS. The size of the source area also depends on the amount of stratum uplift， being larger when more of the stratum is exposed， and a greater DFS area implies a larger source area. The Keping area was divided into three regions based on structural characteristics. The DFS of each structural region is clearly different. Where the source area at the front of the thrust zone is large， the scale of the DFS is correspondingly large. It is mainly fan-shaped， with small slope and slight undercut. Where the front of the thrust area is smaller， the DFS is also smaller and steeper， and the degree of undercutting increases. Areas with strike-slip faults are affected by the faults such that the source area and DFS are significantly smaller， and the DFS is mainly fan-shaped. Where the average slope is steepest， the average extension distance is smallest and the degree of undercutting is significant.