New Insights into Ridge Processes in the Lau Basin Back Arc from Amplitude Variation with Offset Studies of Multichannel Seismic Data

洋中脊负责形成覆盖地球表面约60%的地壳，是地球上最活跃的火山系统，每年产生约20立方公里的熔岩。洋中脊研究的主要目标之一是确定和量化控制地壳增生和洋壳结构的变量。主要的变量是扩张速率：快速扩张的海脊，如东太平洋隆起，以稳定状态的岩浆房和轴向高压为特征，而缓慢扩张的海脊则以轴向裂谷和间歇性岩浆房为特征。在中等扩张速率下，约50-100 mm/年的全速率，脊的状态对扩张速率和岩浆供应（由地壳厚度测量）都很敏感，并且可以在轴向高压和裂谷之间切换。劳弧后盆地位于新西兰北部，汤加海沟以西，太平洋板块俯冲之上。挥发物，如水，从俯冲板块驱动的温度和压力增加的触发器负责的岩浆喂养托富阿火山弧的盆地东部边缘。 Lau弧后的脊以中等速度（40-90毫米/年）扩展，允许调查第三个增生变量-接近火山弧。先前对劳盆地多通道地震（MCS）数据的分析揭示了上地壳结构的系统趋势-速度降低，喷发火山堆（2A层）的相对厚度增加-随着劳东部扩张中心接近火山弧。这些变化归因于向南的趋势，即更多的富含二氧化硅的岩浆，以及俯冲影响的岩浆中挥发物的增加，这驱动了更剧烈的喷发，并产生了更多的多孔岩石。通过系统地研究MCS数据中岩浆房反射的振幅随偏移的变化以及相关的P到S相位转换。调查的一部分将从岩浆库向上连接到海底的黑色和白色烟囱。以前对洋中脊的研究表明，热液喷口区可能优先与“富含熔体”的岩浆房有关，而不是与含有更多晶体的岩浆房有关。另一股将向下连接，并调查是否有溶出挥发物在岩浆房沿着部分的脊的影响弧岩浆。这项研究的结果将与其他劳盆地的结果从岭2000年计划和传播通过岭2000年，岭视图和斯克里普斯可视化中心的网站，主要是作为2-D和3-D可视化模块，可用于从K-12学生到专业科学家的一系列观众。

Mid-ocean ridges are responsible for creating the crust covering approximately 60% of the Earth's surface and are the most active volcanic system on Earth, producing approximately 20 km3 of melt per year. One of the primary goals of mid-ocean ridge studies is to identify and quantify the variables that control crustal accretion and the structure of the oceanic crust. The primary variable is spreading rate: fast spreading ridges, such as the East Pacific Rise, are characterized by steady state magma chambers and an axial high, while slow spreading ridges, are characterized by axial rift valleys and intermittent magma chambers. At intermediate spreading rates, ~50-100 mm/yr full rate, the state of the ridge is sensitive to both spreading rate and magma supply as measured by crustal thickness, and can switch between axial high and rift valley. Located north of New Zealand, the Lau back-arc basin lies west of the Tonga trench and above the subducting Pacific plate. Volatiles, such as water, driven from the subducting plate by increasing temperatures and pressures are the trigger responsible for the magmas feeding the Tofua volcanic arc on the eastern edge of the basin. The ridges within the Lau back-arc, which are spreading at intermediate rates, 40-90 mm/yr, permit the investigation of a third accretion variable - proximity to a volcanic arc. Previous analysis of multichannel seismic (MCS) data from the Lau basin revealed systematic trends in upper crustal structure - reductions in velocity and increases in the relative thickness of the erupted volcanic pile (layer 2A) - as the eastern Lau spreading center approaches the volcanic arc. These changes are attributed to the southward trends towards more silica-rich magmas and to the increasing presence of volatiles in the subduction influenced magmas, which drive more vigorous eruptions and create more porous rocks.In this study, the physical state of the magma chambers in the Lau basin will be investigated by systematically examining the amplitude variation with offset of the magma chamber reflection in the MCS data as well as related P-to-S phase conversions. One strand of the investigation will link upwards from the magma chamber to the seafloor black and white smokers. Previous mid-ocean ridge studies suggest that hydrothermal vent fields may be preferentially associated with "melt-rich" rather than more crystal-laden magma chambers. Another strand will link downwards and investigate whether there are exsolved volatiles in the magma chamber along the section of the ridge influenced by the arc-magmas. The results of this study will be synthesized with other Lau basin results from the Ridge 2000 program and disseminated via the Ridge 2000, Ridge View and Scripps Visualization center websites, primarily as 2-D and 3-D visualization modules that can be used by a range of audiences from K-12 students to professional scientists.