Collaborative Research: Quantifying Plume-Lithosphere Interactions with GNSS Geodesy, Seismology, and Geodynamic Modeling

第一部分：大陆裂谷是一个物理过程，它塑造了地球表面，并给当地居民带来了一系列地质灾害。东非裂谷系统（EARS）是地球上处于早期发展阶段的原型大陆裂谷系统，也是美国国家科学基金会地质系统计划的重点地点之一。了解更深层次的地幔与地球浅层大陆裂谷相互作用的物理原理，仍然是地球科学的一个突出挑战。在这个项目中，将解决地幔-岩石圈相互作用的物理问题，重点是使用计算模型和新的地震和全球导航卫星系统/全球定位系统（GNSS/GPS）精确定位数据来测试长期存在的地幔柱假设，以约束地幔和地表的运动。人们早就认识到东非裂谷系统（EARS）下的地幔是热扰动的。大量的地球物理和地球化学数据被用来论证热扰动的羽流起源。ear究竟是由单个、超级羽流还是多个羽流构成的，目前争论激烈。对超级地幔柱模型的测试是理解影响东非地质系统震源点（东部分支）的物理过程，以及更广泛地理解大规模地幔动力学的基础。为了充分评价超羽流和多羽流模型，进一步了解大陆裂陷过程，需要从地球地质系统焦点站点获得新的地震和GNSS数据，以及岩石圈-羽流相互作用的热力学模拟。提出的工作将允许解决两个地质问题：(1)上地幔羽流的存在或不存在如何影响伸展？(2)在整个岩石圈中应变是如何被容纳和划分的，对应变局部化和迁移的控制因素是什么？将安装乌干达东北部9个新台站的地震数据，以收集数据，生成地幔流动指标（地震横波分裂，SKS）和改进东部分支的地下图像（地震层析成像）。如果在大多数或所有地震台站的SKS测量中发现一致的偏北快速极化方向，则超级羽流模型将得到证实。如果观测到一个更复杂的快速极化方向模式，那么其他的解释将被检验，包括多重羽流假说。层析模型将约束所提出的地球动力学建模的初始条件。还将安装横跨肯尼亚（7个）和乌干达（3个）的东部分部的新GNSS站样带。如果GNSS数据具有与SKS观测一致的沿裂谷运动，那么具有强羽流-岩石圈相互作用的超级羽流模型更受青睐。如果SKS观测结果与超级羽流模型一致，而GNSS不一致，则表明地表-地幔解耦。这些和GNSS-SKS比较的替代结果将通过岩石圈-地幔系统的三维计算模型进行探索，该模型受到新的地震层析成像的限制。为了描述岩石圈与上地幔流动的耦合机制，将开发一套物质模型来确定岩石圈-软流圈边界（LAB）的耦合特性。模拟流将分别在地面和实验室使用新的GNSS和sks分裂观测值进行评分。除了上述科学目标外，该项目还将(1)通过非洲阵列支持的方式，为代表性不足的少数民族学生提供地震学研究生研究机会，从而增强多样性；(2)通过GNSS短期课程和AfricaArray教育计划为非洲科学家提供培训；(3)地球动力学建模扩展将作为ASPECT软件包的一部分向公众开放；(4)GNSS站将成为非洲阵列永久网络的一部分，并在项目结束后很长一段时间内向社区提供开放数据；(5)地震数据将向公众开放。该项目得到了美国国家科学基金会国际科学与工程办公室的部分支持。

Part 1: Continental rifting is a physical process that shapes the Earth's surface and causes a range of geohazards for local populations. The East African Rift System (EARS) is the Earth's archetype continental rift system in its early stages of development and one of the focus sites of the NSF GeoPRISMS program. Understanding the physics of deeper mantle interactions with continental rifting at the shallow parts of the Earth remains an outstanding challenge in the geosciences. In this project the physics of mantle-lithosphere interactions will be addressed, with a focus on testing long-standing mantle plume hypotheses using computational modeling and new seismic and Global Navigational Satellite System / Global Positioning System (GNSS/GPS) precision positioning data to constrain movements in the mantle and at the surface. Part 2: It has long been recognized that the mantle beneath the East African Rift System (EARS) is thermally perturbed. A host of geophysical and geochemical data are used to argue for a plume origin of the thermal perturbations. Whether the EARS is underlain by a single, superplume or multiple plumes is hotly debated. Fundamental to understanding the physical processes influencing the East African GeoPRISMS focus site (Eastern Branch), and more broadly of large-scale mantle dynamics, is testing the superplume model. New seismic and GNSS data are needed from the GeoPRISMS focus-site along with thermomechanical modeling of lithosphere-plume interactions to fully evaluate the superplume and multiple plume models and advance our understanding of continental rifting processes. The proposed work will allow two GeoPRISMS questions to be addressed: (1) How does the presence or absence of an upper-mantle plume influence extension? (2) How is strain accommodated and partitioned throughout the lithosphere, and what are the controls on strain localization and migration? Seismic data from 9 new stations in northeastern Uganda will be installed to collect data for generating mantle flow indicators (seismic shear wave splitting, SKS) and improved sub-surface images (seismic tomography) of the Eastern Branch. If a consistent northerly fast polarization direction in the SKS measurements is discovered for most or all of the seismic stations the superplume model would be corroborated. If a more complex pattern of fast polarization directions is observed, then alternative interpretations will be examined, including the multiple plume hypothesis. The tomography models will constrain initial conditions of the proposed geodynamic modeling. A transect of new GNSS stations across the Eastern Branch spanning Kenya (7) and Uganda (3 co-located) will also be installed. If the GNSS data have along-rift motions that align with SKS observations, then the superplume model with strong plume-lithosphere interactions is favored. If SKS observations are consistent with the superplume model, but GNSS are not, then surface-mantle decoupling is indicated. These and alternative outcomes of the GNSS-SKS comparisons will be explored with 3D computational modeling of the lithosphere-mantle system that is constrained by the new seismic tomography. To characterize lithospheric coupling mechanics to upper mantle flow a suite of material models will be developed to determine coupling properties at the lithosphere-asthenosphere boundary (LAB). Modeled flow will be scored with new GNSS and SKS-splitting observations at the surface and LAB, respectively. In addition to the above scientific objectives this project will (1) enhance diversity by providing graduate research opportunities in seismology for underrepresented minority students supported through AfricaArray; (2) provide training to African scientists through GNSS short courses and the AfricaArray education program, (3) geodynamic modeling extensions will become publically available as part of the ASPECT software package, (4) the GNSS stations will become part of the permanent AfricaArray network and provide open data to the community long after the project is over, and (5) seismic data will become publically available. This project has been supported in part by the Office of International Science and Engineering at NSF.