Investigating what causes the morphology of Large low-shear Velocity Provinces (LLSVPs)

研究导致大低剪切速度省 (LLSVP) 形态的原因

• 批准号: 1849949
• 负责人: Mingming Li
• 金额: $ 23万
• 依托单位: Arizona State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2019
• 资助国家: 美国
• 起止时间: 2019-09-01 至 2022-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1849949&HistoricalAwards=false
• 关键词: Investigating; what; causes; morphology; Large

When earthquakes happen, the energy propagates through the Earth's interior in the form of seismic waves. It is found that seismic waves propagate remarkably slower in two regions of the lowermost mantle beneath Africa and the Pacific Ocean than the surroundings. These regions are called the "large lower shear velocity provinces", or LLSVPs, which cover ~30% area of the Earth's core. The height of the LLSVPs ranges from 100s of km to ~2,000 km at different locations, with the Pacific LLSVP taller than the African LLSVP. The steepness of the LLSVP edges also changes significantly from place to place. The shape of the LLSVPs is controlled by, and provides important information on the density and viscosity structures of the deep mantle, which are linked to the origin of the LLSVPs, the dynamics of the deep mantle and the evolution of the Earth. It is thus critical to understand how density and viscosity structures of the deep mantle control the shape of the LLSVPs. Human beings are not able to go into the deep Earth to examine the LLSVPs in detail, but the interaction between the LLSVPs and the surrounding mantle should be governed by the fundamental physics laws of conservation of mass, momentum and energy. In this research, the team will follow these physical laws and perform numerical simulations to quantify the relationship between the shape of the LLSVPs and the density and viscosity structures of the deep mantle. The results should not only provide new understanding on the causes of the complex shapes of LLSVPs, but also help constrain the density and viscosity structures of the deep mantle. The project will also lead to understanding about the fundamental physics in the Earth's deep mantle. This project provides support to an early-career scientist and a 2nd year graduate student. It also provides teaching materials that will be used in undergraduate and graduate classes, and promotes educational outreach to the public at outreach events and via the internet.The structure and dynamics of the lowermost mantle play a critical role in the Earth's evolution. Seismic studies have revealed two large low-shear velocity provinces (LLSVPs) in the lowermost mantle. Complex features of the LLSVPs have been suggested by seismic observations, with variable topography, internal structure and steepness of the LLSVP edges. The morphology of the LLSVPs is mostly a result of the physical interaction between the LLSVPs and the surrounding mantle, which is in turn largely controlled by the viscosity and density of the lowermost mantle. However, the viscosity and density of the lowermost mantle remain not well constrained. It has been suggested that the LLSVPs are caused by compositionally distinct thermochemical piles. In this project, the team will use numerical modeling to quantify the relationship between the morphology of thermochemical piles and the deep mantle viscosity and density structures, and to explore conditions that lead to structures in the deep mantle with similar features to the seismically observed LLSVPs. The results will help understand the morphology of the LLSVPs, and provide constraints on the density and viscosity of the deep mantle. They will also lead to understanding about the fundamental physical processes and mechanisms controlling the interaction between the LLSVPs and the surrounding mantle.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

当地震发生时，能量以地震波的形式通过地球内部传播。研究发现，地震波在非洲和太平洋下方地幔下层的两个区域的传播速度明显慢于周围地区。这些区域被称为“大的低剪切速度省”（LLSVP），覆盖了地核约 30% 的面积。不同地点的LLSVP高度从数百公里到约2,000公里不等，其中太平洋LLSVP比非洲LLSVP高。 LLSVP 边缘的陡度也因地点而异。 LLSVP的形状受控于深部地幔的密度和粘度结构，并提供了有关LLSVP的起源、深部地幔动力学和地球演化的重要信息。因此，了解深部地幔的密度和粘度结构如何控制 LLSVP 的形状至关重要。人类无法深入地球深处详细研究LLSVP，但LLSVP与周围地幔之间的相互作用应该受到质量、动量和能量守恒定律的基本物理定律的支配。在这项研究中，研究小组将遵循这些物理定律并进行数值模拟，以量化LLSVP的形状与深部地幔的密度和粘度结构之间的关系。这些结果不仅可以为LLSVP复杂形状的成因提供新的认识，而且有助于约束深部地幔的密度和粘度结构。该项目还将引导人们了解地幔深处的基础物理学。该项目为一名职业生涯早期的科学家和一名二年级研究生提供支持。它还提供本科生和研究生课程使用的教材，并通过宣传活动和互联网促进向公众的教育宣传。最低地幔的结构和动力学在地球的演化中发挥着关键作用。地震研究揭示了最下地幔中有两个大的低剪切速度区（LLSVP）。地震观测表明LLSVP具有复杂的特征，包括可变的地形、内部结构和LLSVP边缘的陡度。 LLSVP 的形态主要是 LLSVP 与周围地幔之间物理相互作用的结果，而这又在很大程度上受到最下地幔的粘度和密度的控制。然而，最下地幔的粘度和密度仍然没有得到很好的限制。有人认为 LLSVP 是由成分不同的热化学堆引起的。在该项目中，研究小组将利用数值模型来量化热化学桩的形态与深部地幔粘度和密度结构之间的关系，并探索导致深部地幔中的结构与地震观测到的LLSVP具有相似特征的条件。这些结果将有助于了解LLSVP的形态，并为深部地幔的密度和粘度提供约束。它们还将引导人们了解控制 LLSVP 与周围地幔之间相互作用的基本物理过程和机制。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

项目成果

Seismic anisotropy, dominant slip systems and phase transitions in the lowermost mantle

下地幔中的地震各向异性、主要滑移系统和相变

• DOI: 10.1093/gji/ggab278
• 发表时间: 2021-07-19
• 期刊: Geophysical Journal International
• 影响因子: 2.8
• 作者: B. Ch;ler;ler;Li;Mingming Li;B. Romanowicz;H. Wenk
• 通讯作者: H. Wenk

Vastly Different Heights of LLVPs Caused by Different Strengths of Historical Slab Push

历史板块推力强度不同导致 LLVP 高度差异巨大

• DOI: 10.1029/2022gl099564
• 发表时间: 2022-08-26
• 期刊: Geophysical Research Letters
• 影响因子: 5.2
• 作者: Q. Yuan;Mingming Li
• 通讯作者: Mingming Li

Evolving morphology of crustal accumulations in Earth's lowermost mantle

地球最下地幔中地壳堆积物的演化形态

• DOI: 10.1016/j.epsl.2021.117265
• 发表时间: 2022-01
• 期刊: Earth and Planetary Science Letters
• 影响因子: 5.3
• 作者: Li, Mingming;McNamara, Allen K.
• 通讯作者: McNamara, Allen K.

Internal structure of ultralow-velocity zones consistent with origin from a basal magma ocean

超低速带的内部结构与基底岩浆洋的起源一致

• DOI: 10.1038/s41561-021-00871-5
• 发表时间: 2022-01
• 期刊: Nature Geoscience
• 影响因子: 18.3
• 作者: Pachhai, Surya;Li, Mingming;Thorne, Michael S.;Dettmer, Jan;Tkalčić, Hrvoje
• 通讯作者: Tkalčić, Hrvoje

Formation of large low shear velocity provinces through the decomposition of oxidized mantle

通过氧化地幔的分解形成大的低剪切速度区

• DOI: 10.1038/s41467-021-22185-1
• 发表时间: 2021-03
• 期刊: Nature Communications
• 影响因子: 16.6
• 作者: Wang, Wenzhong;Liu, Jiachao;Zhu, Feng;Li, Mingming;Dorfman, Susannah M.;Li, Jie;Wu, Zhongqing
• 通讯作者: Wu, Zhongqing