Emplacement process and timing of large volcanic debris avalanches, Montserrat, Lesser Antilles: implications for volcanic and tsunami hazards

国际开发计划署第681号提案提出在小安的列斯群岛的蒙特塞拉特、多米尼加和马提尼克火山岛周围钻探10个地点。六个地点将针对最近（< 250 ka）由大规模火山侧翼崩塌产生的碎片雪崩。位于碎屑雪崩沉积物之外的四个地点将恢复碎屑雪崩产生的浊积岩，以及更长期（2-10 Ma）的火山爆发和岩浆演化记录。IODP 681将是科学上的第一次，因为它将提供第一个岩心，以穿透火山侧翼崩塌造成的海底碎片雪崩沉积物。这些崩塌是地球上体积最大（1到1000立方千米）的物质流之一。即使是较小的（1至10 km2）侧翼崩塌事件，例如我们将研究的那些，也会将与河流每年向海洋输送的沉积物量相似。最近在火山岛周围绘制的地图表明，火山侧翼崩塌比以前想象的要频繁，尤其是我们将研究的规模上的崩塌。因此，崩塌事件对世界范围内的火山和海啸灾害具有重要意义。特别是，火山侧翼崩塌引起的海啸的大小争议很大。海啸的震级很大程度上取决于雪崩碎片的放置方式。更好地了解雪崩的位置是更准确地预测海啸震级的关键。对这些IODP岩心中的碎屑雪崩沉积物定年将有助于了解大规模火山侧翼崩塌的时间，以及引发这种崩塌的因素。我们将确定崩塌事件是否通常与火山爆发有关，还是与海平面快速变化的时期有关。IODP 681岩心将有助于了解碎屑雪崩是如何就位的，例如通过记录沉积亚单元的数量和组成（以及由此产生的起源）。岩心也将以非同寻常的细节记录这些火山的长期（2-10 Ma）喷发历史。这一喷发历史将指导对小安的列斯群岛未来火山活动的预测，并有助于了解世界其他地区火山岛建设和破坏的周期。喷发方式是否存在模式，这些模式是否受到侧翼塌陷时间的影响？什么条件会导致特别大的爆发？最后，IODP 681将记录长期的岩浆演化，以了解火成岩过程（如部分熔融、岩浆分化和运输）如何产生弧壳。岩浆演化与喷发历史、侧翼塌陷的时间和规模有何关系？IODP 681的所有10个地点都可以获得带状测深、反向散射和浅层岩心，但我们缺乏蒙特塞拉特岛两个地点和马提尼克岛一个地点的地震数据。UKIODP现场调查将为Montserrat附近的两个地点提供地震数据，而法国GWADASEIS游轮将提供马提尼克岛近海的地震数据。UKIODP现场调查本身将通过收集火山碎屑雪崩的第一个三维地震数据而成为科学上的第一个。虽然在其他地方（如加那利群岛或夏威夷群岛）的地震勘探一直未能穿透和成像火山碎屑雪崩沉积物，但之前在蒙特塞拉特岛、多米尼加和马尼克岛周围进行的二维地震勘探表明，由于相对较浅的水深和薄的特征，这些特殊的沉积物可以成功成像。结合现场调查的第一个三维地震数据，以及IODP 681的第一个岩心，将为他们的时间、触发和就位过程提供强有力的新见解。这次现场调查获得的独特的高分辨率地震数据将为IODP资助提案681提供关键理由，因为这些将是世界上唯一进行高分辨率地震调查的碎屑雪崩矿床。

IODP proposal 681 was submitted to drill ten sites around the volcanic islands of Montserrat, Dominica and Martinique in the Lesser Antilles. Six sites will target recent (< 250 ka) debris avalanches generated by large scale volcanic flank collapse. Four sites located beyond the debris avalanche deposits will recover turbidites generated by the debris avalanches, and a longer term (2-10 Ma) record of volcanic eruptions and magmatic evolution. IODP 681 would be a scientific first, as it would provide the first cores to penetrate submarine debris avalanche deposits resulting from volcanic flank collapse. These collapses are some of the largest-volume (1 to 1,000 km3) mass flows on Earth. Even smaller (1 to 10 km3) flank collapse events, such as those that we will study, transport a similar amount of sediment to that delivered annually by rivers to the ocean. Recent mapping around volcanic islands suggests that flank collapses are more frequent than previously thought, especially collapses on the scale that we will study. Collapse events therefore have important implications for volcanic and tsunami hazards worldwide. In particular, the magnitude of tsunami generated by volcanic flank collapse is highly controversial. Tsunami magnitude is strongly dependent on how debris avalanches are emplaced. A better understanding of avalanche emplacement is key for predicting tsunami magnitude more accurately. Dating debris avalanche deposits in these IODP drill cores will help to understand the timing of large-scale volcanic flank collapse, and the factors that trigger such collapse. We will determine whether collapse events are typically associated with volcanic eruptions, or with periods of rapid sea level change. IODP 681 cores will help to understand how debris avalanches can be emplaced, for instance by documenting the number and composition (and hence origin) of deposit sub-units. Drill cores will also document the long term (2-10 Ma) eruptive history of these volcanoes in unusual detail. This eruptive history will guide prognosis of future volcanic activity both in the Lesser Antilles, and help to understand cycles of volcanic island construction and destruction in other locations worldwide. Are there patterns in eruption style, and are these patterns influenced by the timing of flank collapse? What conditions precede particularly large explosive eruptions? Finally, IODP 681 will document long term magmatic evolution in order to understand how igneous processes (such as partial melting, magma differentiation and transport) generate arc crust. How is magmatic evolution related to eruptive history, and timing and scale of flank collapse? Swath bathymetry, backscatter and shallow cores are available for all ten sites in IODP 681, but we lack seismic data for two Montserrat sites and one Martinique site. The UKIODP site survey would provide the seismic data for the two sites around Montserrat, whilst a French GWADASEIS cruise would provide the seismic data offshore Martinique. The UKIODP site survey would itself result in a scientific first, by collecting the first 3-d seismic data for a volcanic debris avalanche. Although seismic surveys in other locations (e.g. Canary or Hawaiian Islands) have consistently failed to penetrate and image volcanic debris avalanche deposits, previous 2-d seismic surveys around Montserrat, Dominica and Martinique have shown that these particular deposits can be imaged successfully due to relatively shallow water depths and thin character. Combining the first 3-d seismic data from this site survey, and the first drill cores for debris avalanches from IODP 681, will provide powerful new insights into their timing, triggers and emplacement process. The uniquely high resolution seismic data acquired by this site survey will provide a key reason for IODP to fund Proposal 681, as these would then be the only debris avalanche deposits worldwide to have high resolution seismic surveys.