Constraining Earth’s sediment mass balance over geologic time requires a quantitative understanding of how landscapes respond to transient tectonic perturbations. However, the mechanisms by which bedrock lithology governs landscape response remain poorly understood. Rock type influences the size of sediment delivered to river channels, which controls how efficiently rivers respond to tectonic forcing. The Mendocino triple junction region of northern California, USA, is one landscape in which large boulders, delivered by hillslope failures to channels, may alter the pace of landscape response to a pulse of rock uplift. Boulders frequently delivered by earthflows in one lithology, the Franciscan mélange, have been hypothesized to steepen channels and slow river response to rock uplift, helping to preserve high-elevation, low-relief topography. Channels in other units (the Coastal Belt and the Franciscan schist) may experience little or no erosion inhibition due to boulder delivery. Here we investigate spatial patterns in channel steepness, an indicator of erosion resistance, and how it varies between mélange and non-mélange channels. We then ask whether lithologically controlled boulder delivery to rivers is a possible cause of steepness variations. We find that mélange channels are steeper than Coastal Belt channels but not steeper than schist channels. Though channels in all units steepen with increasing proximity to mapped hillslope failures, absolute steepness values near failures are much higher (∼2×) in the mélange and schist than in Coastal Belt units. This could reflect reduced rock erodibility or increased erosion rates in the mélange and schist, or disproportionate steepening due to enhanced boulder delivery by hillslope failures in those units. To investigate the possible influence of lithology-dependent boulder delivery, we map boulders at failure toes in the three units. We find that boulder size, frequency, and concentration are greatest in mélange channels and that Coastal Belt channels have the lowest concentrations. Using our field data to parameterize a mathematical model for channel slope response to boulder delivery, we find that the modeled influence of boulders in the mélange could be strong enough to account for some observed differences in channel steepness between lithologies. At the landscape scale, we lack the data to fully disentangle boulder-induced steepening from that due to spatially varying erosion rates and in situ rock erodibility. However, our boulder mapping and modeling results suggest that lithology-dependent boulder delivery to channels could retard landscape adjustment to tectonic forcing in the mélange and potentially also in the schist. Boulder delivery may modulate landscape response to tectonics and help preserve high-elevation, low-relief topography at the Mendocino triple junction and elsewhere.

中文翻译：

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巨石作为门多西诺三结交界处对构造强迫的河流和景观响应的岩性控制

限制地球在整个地质时间内的沉积物质量平衡，需要对景观如何对瞬态构造扰动做出定量了解。然而，人们对基岩岩性控制景观响应的机制仍然知之甚少。岩石类型影响输送到河道的沉积物的大小，从而控制河流对构造强迫的响应效率。美国北加利福尼亚的门多西诺三联结区是一种景观，在该景观中，由于山坡失稳而向通道输送的巨石可能会改变景观对岩石隆起的反应速度。据推测，泥石流经常以一种岩性（方济各混杂岩）运出，从而使河道变陡，河道对岩石隆起的反应变慢，有助于保持高海拔，低起伏的地形。由于砾石的输送，其他单位（沿海带和方济各斯片岩）的航道可能几乎没有腐蚀抑制作用或没有腐蚀抑制作用。在这里，我们研究了通道陡度的空间格局，抗侵蚀性的指标以及它在混杂和非混杂渠道之间的变化情况。然后，我们询问对河道进行岩性控制的巨石输送是否可能是陡度变化的可能原因。我们发现混杂岩河道比沿海带河道陡峭，但不比片岩河道陡峭。尽管所有单元中的河道都随着与测绘的山坡破坏的接近程度的增加而变陡，但与沿海带单元相比，混杂岩和片岩中接近破坏的绝对陡度值要高得多（约2倍）。这可能反映了混杂岩和片岩的岩石易蚀性降低或侵蚀率增加，或由于这些单位的山坡破坏而使砾石输送量增加而导致的陡峭陡坡。为了研究与岩性有关的巨石输送的可能影响，我们在三个单元中的失败脚趾处绘制了巨石图。我们发现，混杂岩河道的巨石大小，频率和浓度最大，而沿海带河道的浓度最低。使用我们的现场数据对通道斜率对巨石输送的响应进行数学模型化处理，我们发现巨石在混杂岩中的建模影响可能足够强大，足以解释岩性之间通道陡度的一些观测差异。在景观尺度上，我们缺乏足够的数据来完全消除由于空间变化的侵蚀速率和原位岩石易蚀性而引起的巨石引起的陡峭化。然而，我们的巨石制图和建模结果表明，与岩性有关的巨石输送到河道可能会阻碍杂色山脉和片岩中对构造强迫的景观调整。巨石的运送可能会调节景观对构造的响应，并有助于在Mendocino三重交界处和其他地方保留高海拔，低起伏的地形。