Post-emplacement reworking of volcanic debris avalanches

Volcanic debris avalanches have deposited as much as 1000 km3 of largely unconsolidated material on landscapes and remodeled existing drainage networks. The landscape disturbances created by these events pose severe, cascading downstream sedimentation hazards that can require long-term societal management, as demonstrated by decades of observations and ongoing interventions after the deposition of the 1980 debris avalanche of Mount St. Helens (United States). There, post-emplacement sediment yields caused by deposit erosion remain several times above estimated background yield and lakes impounded by the deposit still pose threats of downstream flooding. Despite the length and quality of measurements of the geomorphic evolution and consequent sediment release at Mount St. Helens, the long-term trajectory of drainage network evolution across, and the associated sediment release from, large volcanic debris avalanches remain uncertain. Observations and modeling at Mount St. Helens, however, indicate channel instability can persist many decades and may persist for centuries to millennia. We examined potential influences on the erosion and preservation of volcanic debris avalanche deposits (VDADs) by mapping valley networks developed on 89 VDADs selected from volcanic arcs across the world and spanning a variety of topographic settings and climate regimes. Using the best available topographic data (1 m lidar to 30 m radar-derived data depending on location) and aerial imagery, we estimated the areas of deposits that have been reworked relative to initial deposit footprints as a proxy for post-emplacement erosion. We found that a primary influence on reworking is the topographic confinement of the VDAD: confined, valley-filling deposits are systematically more reworked than unconfined deposits. There is no apparent relationship between deposit age and reworking for valley-filling deposits, indicating that drainage networks on deposits in confined topographic settings like at Mount St. Helens reform rapidly post-emplacement. In contrast, our data indicate that the reworking of unconfined deposits has a monotonic positive relationship with age. This observation agrees with a conceptual model of channel formation at Mount Taranaki (New Zealand), which posits that an unconfined VDAD created a topographic high that initially (e.g., 2 – 8 ky for the Pungarehu formation at Taranaki) diverted erosion to the deposit margins. We found only a weak to moderate relationship between reworking and modern precipitation regimes, which may reflect differences between modern and paleo-precipitation conditions at many of our study sites. We also found no correlation between the size (surface area or volume) of deposits and the degree of reworking. The work presented here implies that downstream cascading sediment hazards from landscape-resetting processes like VDADs (such as thick, extensive pyroclastic flow deposits) depend on the relief and organization of the surrounding landscape. The dataset in this resource contains maps of the reworked area of 88 volcanic debris avalanche deposits worldwide. Each folder within the .zip file is data for an individual volcano and contains a shapefile of the volcanic debris avalanche deposit(s) at that volcano and a shapefile of the reworked area of each deposit, if applicable.火山碎屑雪崩（Volcanic debris avalanches）可在地表堆积多达1000立方千米的大体未固结物质，并重塑原有水系网络。此类事件引发的地表扰动会带来严重的链式下游沉积灾害，需要长期的社会治理应对——美国圣海伦斯火山1980年碎屑雪崩堆积事件后数十年的观测与持续干预已证实了这一点。在该区域，由沉积物侵蚀引发的就位后沉积物产出量仍数倍于背景产出量估算值，且由堆积物壅塞形成的湖泊仍对下游洪水构成威胁。尽管圣海伦斯火山的地貌演化与伴随的沉积物释放过程拥有长期且高质量的观测数据，但大型火山碎屑雪崩覆盖区域的水系网络长期演化轨迹，以及由此产生的沉积物释放过程仍不明确。不过圣海伦斯火山的观测与模拟结果显示，河道失稳状态可存续数十年，甚至可能持续数百年至数千年。 本研究针对火山碎屑雪崩堆积物（Volcanic debris avalanche deposits, VDADs）的侵蚀与保存的潜在影响因素展开分析：我们对全球火山弧带内选取的89处VDAD上发育的河谷网络进行测绘，这些堆积物涵盖了多样的地形环境与气候类型。我们借助现有最优的地形数据（依区位不同，分辨率从1米激光雷达数据至30米雷达衍生数据不等）与航空影像，以初始堆积范围为参照，估算了被改造的堆积物面积，以此作为就位后侵蚀程度的替代指标。研究发现，改造程度的首要影响因素为VDAD的地形约束性：被河谷约束的充填型堆积物的改造程度系统性高于非约束型堆积物。对于河谷充填型堆积物，其堆积年代与改造程度之间未显现明显关联，这表明类似圣海伦斯火山这类受约束地形环境中的堆积物上的水系网络会在就位后快速重构。与之相反，本研究数据显示非约束型堆积物的改造程度与年代呈单调正相关关系。这一观测结果与新西兰塔拉纳基山的河道形成概念模型相符：该模型提出，非约束型VDAD会形成地形高地，初期会将侵蚀作用引导至堆积物边缘（例如塔拉纳基的蓬加雷胡组沉积，其时间跨度为2~8千年）。我们还发现，改造程度与现代降水模式仅存在弱至中等程度的关联，这可能反映了多数研究区域的现代与古降水条件存在差异。此外，堆积物的规模（表面积或体积）与改造程度之间未发现相关性。本研究结果表明，诸如VDADs（如厚层大面积火山碎屑流堆积物）这类重塑地表过程所引发的链式下游沉积灾害，取决于周边地形的起伏与水系格局。 本资源中的数据集包含全球88处火山碎屑雪崩堆积物的改造区域分布图。压缩包内的每个文件夹对应一座单独火山的数据，其中包含该火山的火山碎屑雪崩堆积物（若有多个则为多个堆积物）的shapefile文件，以及各堆积物改造区域的shapefile文件（如适用）。