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

地球表面被分解成十多个可移动的“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都发生了变形，产生了像圣安德烈亚斯断层、盆地和山脉这样的板块边界剪切带。这种变形大多发生在地震上，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源自地球深处的内部，这些内部在对流时流动。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-地幔相互作用是至关重要的，它最终控制着我们所观察到的板块构造和地表变形。该项目旨在约束地幔流、构造应力和板块运动之间的相互作用。PIs Conrad和Lithgow-Bertelloni正在使用粘性地幔流动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。然后用这些力来预测两个可以从地球表面观察到的磁场。首先，通过平衡地球各构造板块的作用力来预测板块速度。预报的运动与观测的比较导致数值模式的进一步改进。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动和板块运动相关的基底剪切牵引。这些应力是根据钻孔岩石圈应力观测、地震矩张量解和岩石圈变形的地质观测进行统计分析的。为了定量评价板块-地幔耦合的程度和可变性，pi主要关注弹性岩石圈下粘度非均质性的影响。岩石圈下岩石的物质强度被认为在不同的地质区域之间是不同的。例如，海洋和薄的大陆岩石圈可能被弱于上地幔的低粘度软流圈所覆盖，而古老的大陆盾可能位于深伸入上地幔的厚而强的克拉通之上。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。在这样做的过程中，它们会影响板块运动、地表岩石圈应力的模式，以及大陆的长期地质变形。pi正在开发岩石圈-地幔相互作用的严格模型，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亚岩石圈地幔的物质特性范围，并更好地了解地幔如何控制地表变形和与地震危险相关的背景应力。该资助项目涉及两家具有不同科学和技术专长的pi之间的密切合作，范围从数值模拟到地质学。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部门的兴趣。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.