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

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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.