Collaborative Research: The role of viscosity heterogeneity in plate-mantle coupling

合作研究：粘度不均匀性在板块-地幔耦合中的作用

• 批准号: 0914712
• 负责人: Clinton Conrad
• 金额: $ 14.56万
• 依托单位: University of Hawaii
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-12-01 至 2012-11-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0914712&HistoricalAwards=false
• 关键词: Collaborative; Research; role; viscosity; heterogeneity

The earth's surface is broken into more than a dozen mobile "plates" that move relative to one another at rates of a few cm per year. Over millions of years, these plates deform at their boundaries as well as internally, producing plate-bounding shear zones such as the San Andreas Fault, basins, and mountain ranges. Much of this deformation occurs seismically, which generates an earthquake hazard in regions that are experiencing deformation. Ultimately, the forces that drive these dynamic surface processes originate within the Earth's deep interior, which flows as it convects. While Earth's surface plates are the surface expression of this convection, the details of how their motion and deformation is related to the motion of the convecting mantle is not well constrained. Yet, it is essential to understand this plate-mantle interaction, which ultimately controls plate tectonics and the surface deformation that we observe. This project is designed to constrain the interaction between mantle flow, tectonic stresses, and plate motions. PIs Conrad and Lithgow-Bertelloni are using spherical finite element models of viscous mantle flow to predict the forces that the mantle exerts on the base of Earth's tectonic plates. These forces are then used to predict two fields that can be observed from the Earth's surface. First, plate velocities are predicted by balancing the forces on each of Earth's tectonic plates. The comparison of predicted motions to observations lead to further improvements in the numerical model. Second, the PIs compute the stresses experienced by an elastic lithosphere that is subjected to basal shear tractions associated with mantle flow and plate motions. These stresses are statistically analyzed against observations of lithospheric stress made in boreholes, from moment tensor solutions for earthquakes, and from geological observations of lithospheric deformation. To quantitatively assess the degree and variability of plate-mantle coupling, the PIs focus on the influence of viscosity heterogeneity beneath the elastic lithosphere. The material strength of the rocks beneath the lithosphere is thought to vary between geological provinces. For example, oceanic and thin continental lithosphere may be underlain by low-viscosity asthenosphere that is weaker than the upper mantle while old continental shields may reside over thick, strong cratons that protrude deeply into the upper mantle. These extreme lateral variations in viscosity greatly influence the coupling between the mantle and the lithospheric plates. In doing so they affect plate motions, patterns of lithospheric stresses at the surface, and the long-term geological deformation of continents. The PIs are developing rigorous models of lithosphere-mantle interaction, calibrated against a variety of observations. Specifically, the PIs hope to further illuminate the range of material properties that characterize the sub-lithospheric mantle, and to gain a better understanding of how the mantle controls surface deformation and the background stresses associated with seismic hazards. This funded project involves the close collaboration between the two PIs with varying scientific and technical expertise ranging from numerical modeling to geology. Hence, the results of these experiments are likely to have a broad impact and to be of interest to diverse segments of the Earth Sciences community. Beyond the scientific collaboration, the project will involve the education of two PhD students (one female), the establishment of a new geodynamic research laboratory at Johns Hopkins University, and the continuing support of a Hispanic female PI.

地球表面被分成十多个移动“板块”，这些板块以每年几厘米的速度相对移动。数百万年来，这些板块在其边界和内部变形，产生板块边界剪切带，如圣安德烈亚斯断层、盆地和山脉。大部分变形是在地震中发生的，这会在变形区域产生地震危险。最终，驱动这些动态表面过程的力源自地球内部深处，它随着对流而流动。虽然地球表面板块是这种对流的表面表现，但它们的运动和变形如何与对流地幔的运动相关的细节并没有得到很好的约束。然而，了解这种板块-地幔相互作用至关重要，它最终控制着板块构造和我们观察到的表面变形。该项目旨在限制地幔流、构造应力和板块运动之间的相互作用。 PI Conrad 和 Lithgow-Bertelloni 正在使用粘性地幔流的球形有限元模型来预测地幔对地球构造板块底部施加的力。然后使用这些力来预测可以从地球表面观察到的两个场。首先，通过平衡地球每个构造板块上的力来预测板块速度。预测运动与观测结果的比较导致数值模型的进一步改进。其次，PI 计算弹性岩石圈所经历的应力，该岩石圈受到与地幔流和板块运动相关的基底剪切牵引力。根据对钻孔中岩石圈应力的观测、地震矩张量解以及岩石圈变形的地质观测，对这些应力进行统计分析。为了定量评估板块-地幔耦合的程度和变化，PI 重点关注弹性岩石圈下方粘度不均匀性的影响。据认为，岩石圈下方岩石的物质强度因地质省份而异。例如，海洋和薄大陆岩石圈下面可能是比上地幔弱的低粘度软流圈，而古老的大陆盾可能位于深入上地幔的厚而坚固的克拉通之上。这些极端的粘度横向变化极大地影响了地幔和岩石圈板块之间的耦合。在此过程中，它们会影响板块运动、地表岩石圈应力模式以及大陆的长期地质变形。 PI 正在开发严格的岩石圈-地幔相互作用模型，并根据各种观测结果进行校准。具体来说，PI希望进一步阐明表征地下地幔的材料特性范围，并更好地了解地幔如何控制表面变形和与地震灾害相关的背景应力。该资助项目涉及两位具有从数值建模到地质学等不同科学和技术专业知识的 PI 之间的密切合作。因此，这些实验的结果可能会产生广泛的影响，并引起地球科学界不同领域的兴趣。除了科学合作之外，该项目还将涉及两名博士生（一名女性）的教育、在约翰·霍普金斯大学建立一个新的地球动力学研究实验室，以及一名西班牙裔女性 PI 的持续支持。