Thermal and melt structure of the Juan de Fuca plate from ridge to trench to arc, inferred from seismic attenuation across the Amphibious Array

这项研究的主要目标是更好地了解构成海洋盆地底部的构造板块下的地幔。卡斯卡迪亚倡议在太平洋西北部的整个胡安德富卡板块和邻近的卡斯卡迪亚边缘部署了一个两栖地震站阵列。这是地震阵列首次覆盖了一个完整的板块，从扩张中心到海沟再到火山弧。海洋板块从形成到老化，直至最终在大陆边缘的俯冲，其冷却的刚性结构都是我们所熟知的特征。相比之下，下面部分熔融的延展性地幔的性质就不太确定了。熔融是否只发生在海洋扩张中心下方岩浆产生上升流带下方的地幔中，还是在老化板块下方有少量岩浆？当板块靠近海沟并向下弯曲俯冲时，压裂是否会让海水过滤到板块中，被矿物质吸收并带入地幔？如果是这样的话，这些挥发物后来是如何以及在哪里出现并影响缓慢循环的地幔的流动性的呢？本研究中进行的地震分析旨在回答这些问题。研究生将进行大部分的分析，在博士培训的最后一年扩展技能，而不是在早期的论文研究中已经获得的。虽然海洋岩石圈的热结构已经被很好地理解，但是关于软流圈的基本问题仍然存在。部分熔体和结合水都被用来解释低软流圈地震速度和粘度，但它们的分布没有得到很好的约束。目前尚不清楚软流层的基底是否由恒定压力/相应温度定义，或者软流层是否被带进俯冲带并被带到深处。一旦板块俯冲，关于温度、水分布和熔体的其他问题就会存在。这项研究的重点是通过体波测量卡斯卡迪亚两栖阵列的地震衰减，因为这些物理性质比单独通过地震速度研究可以更好地阐明。由于两栖阵列提供了如此全面的宽带地震数据，在这里可以比其他任何海洋区域更好地看到体波衰减的预期变化。与地球动力学模型和表面波衍生图像的比较将允许校准基于实验室的非弹性理论。

The primary goal of this study is to better understand the mantle beneath tectonic plates that floor the ocean basins. The Cascadia Initiative deployed an Amphibious Array of seismic stations across the complete Juan de Fuca plate and adjacent Cascadia margin, in the Pacific Northwest. This is the first time a seismic array has covered a complete plate from spreading center to trench to volcanic arc. The cooling, rigid structure that characterizes an oceanic plate from its formation, through aging, and its eventual subduction at the continental margin is well understood. In contrast, the nature of the ductile, partially molten mantle underneath is less certain. Does melt occur in the mantle only beneath the magma-producing upwelling zone beneath the oceanic spreading center, or are small amounts of magma beneath the aging plate? As the plate approaches a trench and bends downward to subduct, does fracturing let seawater filter down into the plate, to be absorbed in minerals and carried into the mantle? If so, how and where do these volatiles later emerge to affect the fluidity of the slowly circulating mantle? The seismic analysis carried out in this study aims to answer these questions. A graduate student will carry out most of the analysis, expanding skills in a final PhD training year beyond what has already been gained in earlier thesis research.Although the thermal structure of oceanic lithosphere is well understood, basic questions regarding asthenosphere still exist. Both partial melt and bound water are proposed to explain low asthenosphere seismic velocities and viscosities but their distribution is not well constrained. It is not known if the base of the asthenosphere is defined by a constant pressure / homologous temperature, or if the asthenosphere is entrained at a subduction zone and carried to depth. Additional questions about temperature, water distribution and melt exist once plates subduct. This study focuses on seismic attenuation across the Cascadia Amphibious Array measured from body waves, since these physical properties can be better elucidated than from seismic velocity studies alone. Because the Amphibious Array provides such comprehensive broadband seismic data, the expected variations in body-wave attenuation can be seen here better than any other oceanic region. Comparisons with geodynamic models and with surface-wave derived images will allow calibration of laboratory-based theories of anelasticity.