Abstract:

River networks, the backbone of most landscapes on Earth, record abundant information on the perturbations of tectonics, climate, and sea levels. Based on the fluvial shear stress model, the bedrock incision has a positive, monotonic correlation with fluvial shear stress. The correlation coefficient between the fluvial shear stress and the erosion rate is called rock erodibility, which mainly depends on lithology and erosion process. Therefore, the fluvial shear stress can be used to calculate the erosion rate, when the rock erodibility is known. It also can be used to calculate the rock erodibility, when the erosion rate is known.
With the improvement of DEM and satellite image resolution, we can extract the corresponding geomorphic parameters(such as channel gradient, discharge, and channel width)from the DEM and satellite image, and then calculate the fluvial shear stress. In this study, we calculated the fluvial shear stress values at 240 points along two channels in the Longmen Shan area. The Minjiang River flows through Wenchuan-Maoxian Fault and then enters Pengguan Massif. Along the Chu River, the lithology is relatively consistent, all are sedimentary rocks, including a ~5km reach distributed along the Shuangshi-Dachuan Fault. The results show that: 1)The average fluvial shear stress is ~401Pa at the footwall of the Wenchuan-Maoxian Fault, while it is ~280Pa along the Wenchuan-Maoxian Fault; 2)the average fluvial shear stress is ~161Pa and ~104Pa at the hanging wall and footwall of Shuangshi-Dachuan Fault, respectively, while it is ~50Pa along the Shuangshi-Dachuan Fault. Fault activities cause the rock more fractured, which causes the fluvial shear stress along the fault to decrease significantly.
Then, according to the empirical relationship between fluvial shear stress and erosion rates of sedimentary rocks(E=3.2(τ^*-0.03)mm/a) and granitoid (E=1.6(τ^*-0.03)mm/a) in the Longmen Shan area, we transform the fluvial shear stress for 139 survey points to erosion rate. The average erosion rate at the footwall of the Wenchuan-Maoxian Fault(i.e. the Pengguan massif) is ~0.43mm/a, and those at the hanging wall and footwall of the Shuangshi-Dachuan Fault is ~0.49mm/a and ~0.28mm/a, respectively.
Moreover, according to the known erosion rates and fluvial shear stress, we calculated the rock erodibility of each survey point. The average erodibility is 13.9mm/a along the Shuangshi-Dachuan Fault(Chu River, sedimentary rocks)and 3.2mm/a along the Wenchuan-Maoxian Fault(Minjiang River, granitoid). The results indicate that: 1)The erodibility of the sedimentary rocks along the fault increases by ~3 times caused by the fault activity, while that of granitoid increases by ~1 time; 2)the fault activity can increase the rock erodibility within ~2km from the active fault in sedimentary rocks and within ~5km from the active fault in granitoid, respectively. This study indicates that fault activities could significantly increase the rock erodibility along the fault, and further influence the evolution of river networks around the fault.

Key words: erodibility, fluvial shear stress, erosion rate, Longmen Shan

摘要：

河流作为主要的地貌单元之一, 在经历构造活动、 气候变化、 海平面升降后, 会记录丰富的相关信息。基于河流剪切力模型, 可应用地貌参数对河流侵蚀速率进行计算。文中利用已有的龙门山地区沉积岩类和花岗岩类的河流剪切力与侵蚀速率之间的经验关系, 计算了沿河139个点的侵蚀速率。结果表明, 汶川-茂县断裂下盘的侵蚀速率为0.43mm/a, 双石-大川断裂上、 下盘的侵蚀速率分别为0.49mm/a和0.28mm/a。另外, 文中根据经验公式计算了每个观测点的可蚀系数(Erodibility), 揭示出: 1)断裂活动使沉积岩的可蚀系数增加了约3倍, 而花岗岩的可蚀系数增加了约1倍; 2)断裂活动对沉积岩的影响(距离断裂约2km范围内)明显比花岗岩(距离断裂约5km范围内)更集中。研究表明, 断层活动使可蚀系数明显增大(即断裂附近的岩石较破碎), 从而对区域地貌演化产生了重要影响。

关键词: 可蚀系数, 河流剪切力, 侵蚀速率, 龙门山断裂带