活动构造对高原边界侵蚀速率空间分布的控制作用——以龙门山地区为例

利用穿过龙门山地区的河流水文与泥沙资料估算该地区现代侵蚀速率,进而研究探讨其空间分布的不均匀性。年代尺度的现代侵蚀速率空间分布表明,高侵蚀区分布于控制高原边界的龙门山活动断裂带和岷山活动断裂带附近且位于逆冲断裂上盘宽约50km范围内,其值达0.5~0.8mm/a。在地形上,该高侵蚀速率区对应于陡峭高原边界的地形陡坡带。自龙门山断裂带向高原内部,侵蚀速率快速降低,在距离断裂带北西200km的地区侵蚀速率小于0.05mm/a。河流输沙量估算的现代侵蚀速率值与河沙宇宙成因核素估算的千年时间尺度的流域平均速率和低温热年代学方法估算的百万年时间尺度的单点平均侵蚀速率基本一致。季风降雨在龙门山地区也存在自东南向北西递降的趋势,但年降水量自山前的1000mm到高原内部的600mm 的递减梯度较小,不足以解释侵蚀速率的一个数量级的减小。气候对侵蚀作用的影响更多地表现在时间域随季风降雨强度的年及季节性波动而震荡,最突出的表现为80% ~90%侵蚀产物在6~9月被季风降水输移出去。逆冲断裂上盘宽约50km的高侵蚀区以及与地形陡坡带的对应性,体现了断裂上盘物质在从较平的断坪向较陡的断坡区运移过程中抬升增加的侵蚀卸载均衡。0.5~0.8mm/a 的抬升剥蚀率与该地区已知的1.4~3.0mm/a的水平缩短速率匹配,说明龙门山高原边界地形处于通量平衡态,且上地壳缩短与龙门山边界的陡降地形是匹配的。

PATTERN OF MODERN DENUDATION IN THE LONGMEN SHAN,EASTERN TIBETAN PLATEAU: TECTONIC CONTROLS ON FOCUSED EROSION ALONG THE PLATEAU MARGIN

We use river sediment load data to characterize the magnitude and spatial pattern of modern denudation across the Longmen Shan margin of the Tibetan Plateau.Relatively rapid denudation occurs adjacent to,and in the hanging walls of the margin-bounding active Longmen Shan and Huya reverse faults,reaching 0.5~0.8mm/a in a ca.50km-wide band.This sector also corresponds to the steepest part of the topographic rise at the plateau margin.The rate decreases with increasing distance plateau-ward,to less than 0.05mm/a at a distance ca.200km northwest of the Longmen Shan foothill.Decadal modern denudation rate is similar to the cosmogenic derived ka-time-scale catchment-averaged denudation rate from river sand,as well as the Ma-time-scale point denudation rate from low-temperature thermochronology.Monsoonal precipitation decreases across this margin,from 1000mm at the foothills to 600mm on the plateau surface,but cannot explain the one-order-of-magnitude spatial difference in erosion.Precipitation influence in modern denudation is manifested in the temporal domain by annual and seasonal fluctuations in accord with the variations in monsoon intensity.Majority of sediment is transported in large storms,particularly 80%~90% of suspended sediments delivered during June-September of monsoonal rain.Enhanced denudation in a 50km-wide stripe in the hanging wall of the Pengguan fault may represent an erosional balance to tectonic uplift and advection above the ramp-up of decollement at depth,connecting to the Pengguan fault.Denudation of 0.5~0.8mm/a above a ca.20° NW-dipping ramp,as structurally inferred by a critical taper wedge model,implies a horizontal shortening rate of 1.4~3.0mm/a,consistent with that measured by GPS geodesy.This suggests that the Longmen Shan margin may have reached a flux steady-state.The consistency also indicates that the topography in the Longmen Shan could be maintained entirely by upper crustal shortening.