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

地球表面被分成十几个移动的“板块”，它们以每年几厘米的速度相互移动。在数百万年的时间里，这些板块在其边界和内部变形，产生了板块边界剪切带，如圣安德烈亚斯断层，盆地和山脉。这种变形大部分发生在地震中，这在正在经历变形的地区产生了地震危险。最终，驱动这些动态表面过程的力量来自地球的深层内部，它在对流时流动。虽然地球表面的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有关的细节并没有得到很好的限制。然而，理解这种板块-地幔相互作用是至关重要的，它最终控制着板块构造和我们观察到的地表变形。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康拉德和Lithgow-Bertelloni正在使用粘性地幔流的球形有限元模型来预测地幔施加在地球构造板块底部的力。然后，这些力被用来预测从地球表面可以观察到的两个场。首先，板块速度是通过平衡地球各构造板块上的力来预测的。预测的运动与观测结果的比较导致数值模型的进一步改进。第二，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力。这些应力进行统计分析，对观测的岩石圈应力钻孔，从矩张量解决方案的地震，从地质观测的岩石圈变形。为了定量评估板块-地幔耦合的程度和变化，PI侧重于弹性岩石圈下的粘度不均匀性的影响。岩石圈下面的岩石的物质强度被认为在不同的地质区之间是不同的。例如，大洋岩石圈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深伸入上地幔的厚而强的软流圈之上。这些极端的横向粘度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。具体而言，PI希望进一步阐明岩石圈下地幔的材料特性范围，并更好地了解地幔如何控制地表变形和与地震灾害相关的背景应力。该资助项目涉及两个PI之间的密切合作，具有从数值建模到地质学的各种科学和技术专业知识。因此，这些实验的结果可能会产生广泛的影响，并引起地球科学界各阶层的兴趣。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，并继续支持一名西班牙裔女性PI。

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.