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

洋中脊形成了覆盖地球表面约60%的地壳，是地球上最活跃的火山系统，每年产生约20立方千米的融水。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海洋地壳结构的变量。主要变量是扩张速度，快速扩张的隆起，如东太平洋隆起，以稳定状态岩浆房和轴向高为特征，而缓慢扩张的隆起，以轴向裂谷和间歇性岩浆房为特征。在中间扩张速率（~50 ~ 100 mm/yr）时，脊的状态对扩张速率和岩浆供应（以地壳厚度衡量）都很敏感，并能在轴向高位和裂谷之间切换。劳弧后盆地位于新西兰北部，位于汤加海沟以西，位于俯冲的太平洋板块之上。由于温度和压力的升高，从俯冲板块中喷出的挥发物，如水，是岩浆流入盆地东部边缘的托富亚火山弧的触发因素。劳后弧内的脊，正以中等速率（40-90毫米/年）扩张，允许研究第三个吸积变量——靠近火山弧。先前对劳盆地多道地震（MCS）数据的分析揭示了上地壳结构的系统性趋势——随着劳扩张中心东部靠近火山弧，喷发火山堆（2A层）的速度减小和相对厚度增加。这些变化归因于向南的趋势，趋向于更富硅的岩浆，以及俯冲影响的岩浆中挥发物的增加，这些挥发物推动了更猛烈的喷发，创造了更多的多孔岩石。本研究将通过系统地研究岩浆房反射在MCS数据中的振幅随偏移量的变化以及相关的p - s相转换，来研究劳盆地岩浆房的物理状态。调查的一部分将从岩浆房向上连接到海底的黑色和白色烟囱。先前的海中脊研究表明，热液喷口区可能优先与“富含熔体”的岩浆室联系在一起，而不是与更多富含晶体的岩浆室联系在一起。另一条线将向下连接，并研究在受弧岩浆影响的山脊部分的岩浆房中是否存在溶解的挥发物。这项研究的结果将与Ridge 2000项目的其他劳盆地结果综合起来，并通过Ridge 2000、Ridge View和Scripps可视化中心网站传播，主要作为2-D和3-D可视化模块，供从K-12学生到专业科学家的一系列受众使用。

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.