Contraining the large-scale dynamics and structure of the lower mantle using observations of the geoid, dynamic topography and plate tectonics

利用大地水准面、动态地形和板块构造的观测来约束下地幔的大尺度动力学和结构

基本信息

批准号：
1645245
负责人：
Shijie Zhong
金额：
$ 35.5万
依托单位：
University of Colorado at Boulder
依托单位国家：
美国
项目类别：
Continuing Grant
财政年份：
2017
资助国家：
美国
起止时间：
2017-01-01 至 2020-12-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1645245&HistoricalAwards=false
关键词：
Contraining large scale dynamics structure

项目摘要

Analyzing seismic waves traveling through the inside of the Earth has revealed that the Earth's mantle has a predominately long-wavelength (5000 km) structure. Particularly, the lower mantle beneath the circum-Pacific regions has seismic wave speeds that are relatively fast, while seismic wave speeds in the lower mantle beneath Africa and Pacific are relatively slow. These two seismically slow areas beneath Africa and the Pacific are are considered anomalies, and are most likely hot regions within the mantle because seismic waves travel slower in hot materials. These anomalies are particularly strong near the boundary between the core and mantle (Core-Mantle Boundary or CMB) regions, and are often termed as large low shear-velocity provinces or LLSVPs. The LLSVPs encompass most volcanism and large igneous provinces for the last 200 million years. This pattern in mantle seismic structure also highly correlates with the gravity anomalies at the Earth's surface. Together with seismic studies, geochemical studies also suggest that the LLSVPs may be compositionally different from the bulk of the mantle, being enriched in heavy, incompatible elements such as radioactive elements U and Th. Considering the significant influence of volcanic degassing on Earth's climate and the important effect of mantle viscosity on post-glacial rebound and sea-level change, it is therefore important to understand the dynamics of the long-wavelength mantle convection, the generation of the LLSVPs, and the relationship between the LLSVPs and their surface expression including volcanism and gravity anomalies. The goal of this 3-year project is to seek understanding of these fundamental questions, by analyzing and modeling fluid dynamical processes of mantle convection. This project will also lead to significant improvement in computational techniques in modeling mantle convection via international collaboration with an Australian geodynamicist. The project includes training of a graduate student. The project seeks to address the following three specific questions: 1) Are the LLSVPs purely thermal or thermochemical features? 2) Can the observations of the geoid and dynamic topography be made consistent with stable, thermochemical LLSVPs? 3) Can the long-wavelength mantle structure and convection (e.g., degree-2) be generated dynamically self-consistently with dynamic plates and realistic lithospheric rheology including the low-temperature plasticity? Four tasks for the proposed three-year project include: 1) to seek buoyancy and viscosity structures that explain the geoid, CMB excess ellipticity, and stability of the chemically distinct LLSVPs, by formulating mantle flow models with buoyancy derived from seismic models with compositional effects from the LLSVPs and post-perovskite phase change; 2) to test the hypothesis that the LLSVPs are purely thermal or thermochemical anomalies, using time-dependent convection models with imposed plate motion history; 3) to understand the origin of long-wavelength mantle structures including the LLSVPs, by formulating fully dynamic models of mantle convection with realistic lithospheric rheology; 4) to improve solvers in CitcomS for efficiency and robustness. The project should significantly improve understanding on the origins of the LLSVPs, long-wavelength mantle convection and plate tectonics. The results should have direct implications for studies in other areas beyond geodynamics, including mantle geochemistry, seismology, supercontinent cycles, gravity anomalies, and volcanism.

分析穿过地球内部的地震波，表明地球披着主要具有长波长（5000公里）的结构。特别是，在太平洋区域下方的下层地幔的地震速度相对较快，而非洲和太平洋下层下地幔的地震速度相对较慢。这两个地震速度在非洲和太平洋以下的缓慢地区被认为是异常，并且很可能是地幔中的热区域，因为地震波在热材料中传播的速度较慢。这些异常在核心和地幔（Core-Handle边界或CMB）区域之间的边界附近特别强，并且通常被称为大型低剪切省或LLSVP。 LLSVP涵盖了过去2亿年的大多数火山和大型火成岩省。地幔地震结构中的这种模式也与地球表面的重力异常高度相关。与地震研究一起，地球化学研究还表明，LLSVP在组成上可能与大部分地幔有所不同，并富含沉重的，不兼容的元素，例如放射性元素U和TH。因此，考虑到火山脱气对地球气候的重要影响以及地幔粘度对冰川后反弹和海平面变化的重要影响，因此重要的是要了解长波长度对流的动态，LLSVPS的产生的动态，LLSVPS的产生以及LLSVPS和它们的表面表达和包括火山的表达和重力症之间的关系。这个为期三年的项目的目的是通过分析和建模地幔对流的流体动力学过程来寻求对这些基本问题的理解。该项目还将通过与澳大利亚地球动力学家的国际合作来建模对流，从而显着改善计算技术。该项目包括培训研究生。该项目旨在解决以下三个特定问题：1）LLSVP是纯粹的热化学特征吗？ 2）可以使Geoid和动态形态的观察与稳定的热化学LLSVP保持一致吗？ 3）长波长的地幔结构和对流（例如，度2）是否可以通过动态板和现实的岩石圈流变学自我一致地产生，包括低温可塑性？拟议的三年项目的四个任务包括：1）寻求浮力和粘度结构，这些结构解释了通过从LLSVPS和llsvps和-perovskite Perovskite阶段的构成效应的自动化模型得出的地幔流模型来解释化学上不同的LLSVPS的地震流量模型，从而解释了Geoid，CMB过多的椭圆性和稳定性； 2）测试LLSVP的假设是使用具有施加板运动历史的时间依赖的对流模型，纯粹是热化异常。 3）通过以逼真的岩石圈流变学制定全动态模型，了解包括LLSVP在内的长波长地幔结构的起源； 4）提高citcom中的求解器以提高效率和鲁棒性。该项目应显着提高人们对LLSVP，长波长对流和板块构造的起源的理解。结果应直接对除地球动力学以外的其他领域的研究具有直接影响，包括地幔地球化学，地震学，超大陆周期，重力异常和火山主义。