Correlating the 410 km discontinuity low-velocity layer occurrence with velocity tomograms using Pds imaging with high-fold arrays

美国西部上地幔的两个新的地震观测结果是：1)高褶皱pascal阵列的叠加接收函数数据经常在410 km不连续面上发现一个低速层；2)观测到大约8个低速异常，从上地幔延伸到410 km不连续附近（Schmandt and Humphreys, 2010）。对410公里不连续层顶部低速层的最佳解释是，它表明存在由水过滤器模型产生的部分熔融层（Bercovici和Karato， 2003）。我们认为，在410 km不连续面以上的大量低速异常可能是浮力熔融底喷岩的表现，这些底喷岩是从浮力410 km不连续面熔体层顶部成核的。对410熔体层底喷流上涌的情景进行了研究，并证明是合理的（Leahy和Bercovici， 2008）。我们之前对410 km不连续层的成像工作表明，需要具有1年记录和30 km站距的高折叠pascal阵列才能可靠地成像410 km低速层。我们建议使用以下PASSCAL/Flexible阵列对410 km不连续面进行成像：内华达山脉地镜阵列、柔性阵列门多西诺实验、科罗拉多落基山脉实验和地震样带阵列以及高熔岩平原阵列。采用网格搜索算法和简单的速度模型参数化对410 km间断面进行建模；模型的不确定性将通过计算一维和二维模型参数边际概率密度函数来评估。我们的假设是，410公里不连续层以上的低速异常表明410熔体层的上涌底槽，这一假设将通过410熔体层的产状/厚度与P/S体波层析图的相关性来验证。我们将使用Schmandt和Humphreys的层析图（在回顾中），因为该模型除了可运输阵列数据外，还包含我们打算分析的所有PASSCAL阵列的旅行时间数据。美国西部地区目前正经历着显著的火山活动，山脉隆起，并正在积极地扩展（例如，形成盆地和山脉）。这种构造活动的某些部分很可能是由地幔柱上涌和海洋岩石圈俯冲等深部地幔过程驱动的。该提案旨在测试一个新的假设，即深部地幔过程如何影响美国西部的构造活动：具体来说，这种构造活动可能是由位于410公里深地幔速度不连续之上的部分熔融熔融层脱落的上涌底辟所驱动的。这一速度不连续表现了地幔橄榄石组分的固态相变，由于橄榄石组分在这一相变上下的溶解度不同，一个部分熔融的熔体层（熔体孔隙率几个百分点）可能是由在410 km不连续以下的充分水合物的地幔向上通量产生的。为了验证这一假设，将利用美国西部高密度pascal和柔性阵列地震部署的地震数据绘制410公里熔融层的空间分布。将这张新的410熔体层图与该区域下新的速度结构层析成像图像进行比较，将量化许多上地幔低速管道之间的相关性。和410千米的熔体层发生。发现上地幔低速异常是否存在410熔融底喷井的社会效益将是提高对美国西部火山活动起源的理解。

Two new seismic observations of the upper mantle beneath the western US are: 1) a low velocity layer atop the 410 km discontinuity is often found from stacking receiver function data from high fold PASSCAL arrays; 2) about eight low velocity anomalies are observed that extend from the uppermost mantle to near the 410 km discontinuity (Schmandt and Humphreys, 2010). The best interpretation of the low velocity layer atop the 410 km discontinuity is that it manifests the presence of a partially molten layer created by the water-filter model (Bercovici and Karato, 2003). We suggest that the numerous low velocity anomalies above the 410 km discontinuity may be the manifestation of buoyant melt diapirs that nucleate from the top of a buoyant 410 km discontinuity melt layer. This scenario of upwelling 410 melt layer diapirs has been studied and shown to be plausible (Leahy and Bercovici, 2008). Our prior work imaging the 410 km discontinuity demonstrates that high fold PASSCAL arrays with 1 year of recording and a station spacing of 30 km are required to reliably image a 410 low velocity layer. We propose to image the 410 km discontinuity using the following PASSCAL/Flexible arrays: the Sierra Nevada Earthscope Array, the Flex-Array Mendocino Experiment, the Colorado Rockies Experiment and Seismic Transects array, and the High Lava Plains Array. The 410 km discontinuity will be modeled using a grid-search algorithm and a simple velocity model parameterization; model uncertainties will be assessed by calculating the 1- and 2-dimensional model parameter marginal probability density functions. Our hypothesis - that the low velocity anomalies above the 410 km discontinuity manifest upwelling diapirs from a 410 melt layer - will be tested by correlating the 410 melt layer occurrence/thickness with P/S body wave tomogram(s). We will use the tomograms of Schmandt and Humphreys (in review) as this model has travel-time data from all the PASSCAL arrays we intend to analyze, in addition to the Transportable Array data.The western United States is a region currently experiencing significant volcanic activity, uplifting mountain ranges, and is actively being extended (e.g., to make the Basin and Range). Some portion of this tectonic activity is mostly likely driven by deep mantle processes such as upwelling mantle plumes and subducting oceanic lithosphere. This proposal intends to test a new hypothesis for how deep mantle process maybe affecting the tectonic activity of the western United States: specifically, that some of this tectonic activity maybe driven by upwelling diapirs that are shed from a partially molten melt layer that resides atop the 410 km depth mantle velocity discontinuity. This velocity discontinuity manifest the solid-state phase transition of the olivine component of the mantle, and due to differences in the olivine component water solubility above and below this phase transition, a partially molten melt layer (few percent melt porosity) maybe produced by an upward flux of sufficiently hydrate mantle from below the 410 km discontinuity. To test this hypothesis, the spatially distribution of the 410 km melt layer will be mapped using seismic data from high density PASSCAL and Flexible array seismic deployments in the western United States. Comparison of this new 410 melt layer map with new tomographic images of the velocity structure beneath this region will quantify the correlation between the numerous upper mantle low velocity ?pipes? and the 410 km melt layer occurence. The societal benefit to discovering whether the upper mantle low velocity anomalies manifest 410 melt diapirs would be an improved understanding of the origin of volcanic activity in the western US.