Collaborative Research: Structure and Dynamics of ultralow-velocity zones at the core-mantle boundary

合作研究：核幔边界超低速区的结构和动力学

• 批准号: 0911215
• 负责人: Justin Revenaugh
• 金额: $ 16.23万
• 依托单位: University of Minnesota-Twin Cities
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-07-01 至 2013-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0911215&HistoricalAwards=false
• 关键词: Collaborative; Research; Structure; Dynamics; ultralow

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)The boundary between Earth's rocky mantle and metallic core is home to a patchwork collection of partially molten structures. These structures, characterized by sharp peaks and valleys, are relatively thin (10s of km in thickness), dense (up to 10% denser than the surrounding rocks), and are spatially correlated with regions of upwelling flow in the lower mantle. Seismic waves slow down as they pass through these structures. Consequently, the fingerprint of these structures appear as UltraLow Velocity Zones (ULVZ) in seismic signals traveling through Earth's deep interior. Besides their unique seismic signature, ULVZ are also likely to be chemically distinct from the surrounding mantle rock. Far from being passive, ULVZ are strongly coupled with the vigorous motion in the overlying mantle that dissipates Earth's internal heat, drives plate tectonics, and causes volcanism on the surface.This research will study seismic signals to unravel the fine scale structure within the ULVZ, simulate the structure arising from vigorous motion of the partially molten material within the ULVZ using principles of fluid mechanics, and create synthetic seismic signals traveling through the simulated ULVZ structure. The result of these investigations will answer a number of fundamental questions on heat and matter transport across Earth's core-mantle boundary and the chemical nature and physical properties of the ULVZ.High resolution seismic ScP and PcP waveform data provides a wealth of information regarding the local structure of the ULVZ. Such thin, high density layers at the base of Earth's mantle are likely to play a major role in determining the location and stability of mantle plumes generating from the core-mantle boundary. In addition, this region is also likely to serve as an extremely important stage in any ongoing mass transfer between Earth's outer core and the mantle.In this project we propose to develop a combined geodynamic-seismic investigation of the internal structure and dynamics of the ULVZ. We employ existing and new high resolution migration and modeling of ScP and PcP waveforms to better constrain wave velocities, layer topography, and density contrasts within ULVZ at the base of the core mantle boundary. We will develop a two-stage geodynamic model, beginning by modeling the ULVZ as a thin, self gravitating layer spreading at the bottom of the mantle. Using this gravity current model and the observed topography, we can predict the viscosity of the ULVZ and also infer the extent of melting within the ULVZ. In the second stage, we will develop a multiphase, multicomponent model of magma mixing and small scale convection within the ULVZ. The resulting model provides constraints on melt storage and mass transfer within the ULVZ both with the outer core and lower mantle. The resulting models will be built into 2D and 3D forward synthetic seismogram simulations to test consistency with observed data and to better guide future seismic investigation of ULVZ by identifying diagnostic waveform effects visible at the surface.

该奖项由 2009 年美国复苏和再投资法案（公法 111-5）资助。地球的岩石地幔和金属核心之间的边界是部分熔融结构的拼凑集合。这些结构以尖锐的山峰和山谷为特征，相对较薄（厚度为数十公里），致密（比周围岩石致密达10%），并且与下地幔的上升流区域在空间上相关。地震波穿过这些结构时速度会减慢。因此，这些结构的指纹在穿过地球内部深处的地震信号中显示为超低速区 (ULVZ)。除了其独特的地震特征外，超低电压区在化学上也可能与周围的地幔岩石不同。 ULVZ 远非被动的，它与上覆地幔的剧烈运动密切相关，从而消散地球内部热量，驱动板块构造，并引起地表火山活动。这项研究将研究地震信号，以揭示 ULVZ 内的精细尺度结构，利用流体力学原理模拟 ULVZ 内部分熔融物质剧烈运动所产生的结构，并创建合成地震信号 穿过模拟的 ULVZ 结构。这些研究的结果将回答有关穿过地球核心-地幔边界的热量和物质传输以及 ULVZ 的化学性质和物理特性的许多基本问题。高分辨率地震 ScP 和 PcP 波形数据提供了有关 ULVZ 局部结构的丰富信息。地幔底部如此薄的高密度层可能在确定从地核-地幔边界产生的地幔柱的位置和稳定性方面发挥着重要作用。此外，该区域也可能成为地球外核和地幔之间任何正在进行的质量转移的极其重要的阶段。在这个项目中，我们建议对 ULVZ 的内部结构和动力学进行地球动力学-地震联合研究。我们利用现有的和新的高分辨率偏移以及 ScP 和 PcP 波形建模来更好地限制核心地幔边界底部 ULVZ 内的波速、层位地形和密度对比。我们将开发一个两阶段的地球动力学模型，首先将 ULVZ 建模为在地幔底部扩展的薄自重力层。利用重力流模型和观测到的地形，我们可以预测 ULVZ 的粘度，并推断 ULVZ 内的熔化程度。在第二阶段，我们将开发 ULVZ 内岩浆混合和小规模对流的多相、多成分模型。由此产生的模型提供了对 ULVZ 内外核和下地幔熔体储存和质量传递的约束。由此产生的模型将被构建到 2D 和 3D 正向合成地震记录模拟中，以测试与观测数据的一致性，并通过识别表面可见的诊断波形效应来更好地指导未来的 ULVZ 地震调查。