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

使用高倍阵列 Pds 成像将 410 km 不连续低速层出现与速度断层图关联起来

• 批准号: 1045810
• 负责人: Kenneth Dueker
• 金额: $ 11.24万
• 依托单位: University of Wyoming
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-02-15 至 2015-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1045810&HistoricalAwards=false
• 关键词: Correlating; 410; km; discontinuity; low

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.

美国西部下地幔的两个新的地震观测结果是：1）从高褶皱PASSCAL阵列的叠加接收器函数数据中经常发现410 km不连续顶部的低速层; 2）观察到大约8个低速异常，这些异常从上地幔延伸到410 km不连续附近（Schmandt和Humphreys，2010）。 对410 km不连续面顶部低速层的最佳解释是，它表明存在由水过滤模型（Bercovici和Karato，2003年）产生的部分熔融层。我们认为，上述410公里的不连续性的众多低速异常可能是表现的浮力熔体底辟，从一个浮力410公里的不连续熔体层的顶部成核。 这种上涌410熔融层底辟的情景已经过研究，并被证明是合理的（Leahy和Bercovici，2008）。 我们以前的工作成像的410公里的不连续表明，高倍PASSCAL阵列1年的记录和30公里的站间距需要可靠的图像410低速层。我们建议使用以下PASSCAL/Flexible阵列对410 km的不连续性进行成像：Sierra内华达州Earthscope阵列、Flex-Array门多西诺实验、科罗拉多落基山脉实验和地震断面阵列以及高熔岩平原阵列。 将使用网格搜索算法和简单的速度模型参数化来模拟410公里的不连续性;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模型的不确定性。 我们的假设-低速异常以上的410公里的不连续性表现上涌底辟从410熔体层-将通过相关的410熔体层发生/厚度与P/S体波断层扫描（S）进行测试。我们将使用Schmandt和Humphreys的断层图（回顾中），因为除了可移动阵列数据外，该模型还具有来自我们打算分析的所有PASSCAL阵列的旅行时间数据。美国西部是一个目前正在经历重大火山活动、山脉抬升并正在积极扩展的地区（例如，使盆地和山脉）。 这种构造活动的一部分很可能是由深部地幔过程驱动的，如地幔柱上涌和大洋岩石圈俯冲。这项建议旨在测试一个新的假设，如何深地幔过程可能会影响构造活动的美国西部：具体来说，这种构造活动的一些可能是由上涌底辟，从一个部分熔融的熔体层，位于顶部的410公里深地幔速度不连续脱落。这种速度不连续性反映了地幔中橄榄石组分的固态相变，由于这种相变上下橄榄石组分的水溶性不同，从410 km不连续性下方向上流动的充分水化的地幔可能产生部分熔融的熔体层（熔体孔隙率为百分之几）。为了检验这一假设，410公里融化层的空间分布将映射使用地震数据从高密度PASSCAL和灵活的阵列地震部署在美国西部。这一新的410熔体层地图与新的层析成像的速度结构下，这一地区的比较将量化之间的相关性众多的上地幔低速？管道？410 km熔融层的出现。发现上地幔低速异常是否表现出410熔融底辟的社会效益将是对美国西部火山活动起源的更好理解。