Field measurements of incision rates following bedrock exposure: Implications for process controls on the long profiles of valleys cut by rivers and debris flows
Field measurements of incision rates following bedrock exposure: Implications for process controls on the long profiles of valleys cut by rivers and debris flows
复制标题
基岩暴露后切割率的现场测量：对被河流和泥石流切割的山谷长剖面的过程控制的影响
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发表时间：
2005
期刊：
影响因子：
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通讯作者：
L. Sklar
中科院分区：
文献类型：
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作者：
Jonathan D. Stock;D. Montgomery;B. Collins;W. Dietrich;L. Sklar
直到最近，已公布的基岩峡谷的切割速率来自于对切割表面的间接测年。基于直接测量的少量但不断增长的文献报告了短期基岩以地质上不可持续的速度下降。我们报告了在华盛顿州、俄勒冈州、加利福尼亚州和台湾的10个山谷中监测到的1-7年侵蚀销钉的基岩下降情况，这些山谷切割了硬化的火山岩和沉积岩。历史上，这些水道中的大多数都被剥离了沉积物。它们的基岩暴露在床荷载的磨损、采摘和季节性的干湿中，使坚硬的、完整的岩石变成板块或等量的碎片，被更高的水流带走。顺向切口率与岩石抗拉强度的平方成正比，与其他实验结果一致。每年高达厘米的测量速率远远超过区域长期侵蚀速率的估计，即使对于明显较小的沉积物运输速率也是如此。在华盛顿和台湾相邻的梯田上的文物表明，至少几十年来，海平面一直在以这种极端的速度下降。由于缺乏沉积物覆盖，这些地点的岩性下降速度远远超过了岩石的长期抬升速度。这种速率差异使得这些河流的长剖面不太可能按照河流幂律预测的方式直接调整为基岩硬度或岩石隆升速率，尽管观察到它们的剖面很好地符合排水面积与坡度的幂律图。我们假设，在抗拉强度低于~ 3-5 MPa的弱基岩岩性中，控制河道坡度的不是基岩硬度或岩石隆升速率，而是薄沉积地幔的运动阈值。为了说明这一假设，并为面积与坡度的幂律图提供另一种解释，我们将Shields的阈值输运概念与测量的水力关系和下游捕集率结合起来。与洪水形成的河流相比，我们在陡峭的山谷中安装的数百个侵蚀销钉中，没有一个显示出泥石流冲刷到基岩的任何事件后的洪水下降。这些结果与河道网络最陡部分（坡度在~0.03 ~ 0.10以上）基岩下降的主要原因是幕式泥石流的观点一致。
Until recently, published rates of incision of bedrock valleys came from indirect dating of incised surfaces. A small but growing literature based on direct measurement reports short-term bedrock lowering at geologically unsustainable rates. We report observations of bedrock lowering from erosion pins monitored over 1–7 yr in 10 valleys that cut indurated volcanic and sedimentary rocks in Washington, Oregon, California, and Taiwan. Most of these channels have historically been stripped of sediment. Their bedrock is exposed to bed-load abrasion, plucking, and seasonal wetting and drying that comminutes hard, intact rock into plates or equant fragments that are removed by higher fl ows. Consequent incision rates are proportional to the square of rock tensile strength, in agreement with experimental results of others. Measured rates up to centimeters per year far exceed regional long-term erosionrate estimates, even for apparently minor sediment-transport rates. Cultural artifacts on adjoining strath terraces in Washington and Taiwan indicate at least several decades of lowering at these extreme rates. Lacking sediment cover, lithologies at these sites lower at rates that far exceed long-term rock-uplift rates. This rate disparity makes it unlikely that the long profi les of these rivers are directly adjusted to either bedrock hardness or rock-uplift rate in the manner predicted by the stream power law, despite the observation that their profi les are well fi t by power-law plots of drainage area vs. slope. We hypothesize that the threshold of motion of a thin sediment mantle, rather than bedrock hardness or rock-uplift rate, controls channel slope in weak bedrock lithologies with tensile strengths below ~3–5 MPa. To illustrate this hypothesis and to provide an alternative interpretation for power-law plots of area vs. slope, we combine Shields’ threshold transport concept with measured hydraulic relationships and downstream fi ning rates. In contrast to fl uvial reaches, none of the hundreds of erosion pins we installed in steep valleys recently scoured to bedrock by debris fl ows indicate any postevent fl uvial lowering. These results are consistent with episodic debris fl ows as the primary agent of bedrock lowering in the steepest parts of the channel network above ~0.03–0.10 slope.