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

第一部分：大陆裂谷是一个塑造地球表面并给当地居民造成一系列地质灾害的物理过程。东非裂谷系统(EAR)是地球上处于早期发展阶段的原型大陆裂谷系统，也是NSF GeoPRISMS计划的重点地点之一。理解深部地幔与地球浅层大陆裂谷相互作用的物理学仍然是地球科学中的一个突出挑战。在这个项目中，将讨论地幔-岩石圈相互作用的物理问题，重点是利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(全球导航卫星系统/全球定位系统)精确定位数据来检验长期存在的地幔热柱假说，以限制地幔和地表的运动。第2部分：东非裂谷系统(EARS)之下的地幔受到了热扰动，这一点早就被认识到了。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。关于耳朵是由单一羽状物、超级羽状物还是多个羽状物构成的，人们争论不休。检验超烟羽模型是理解影响东非GeoPRISMS重点地点(东部分支)以及更广泛的大尺度地幔动力学的物理过程的基础。需要来自GeoPRISMS震源点的新的地震和GNSS数据，以及岩石圈-岩柱相互作用的热力学模拟，以全面评估超柱和多股岩柱模型，并加深我们对大陆裂谷过程的理解。拟议的工作将解决GeoPRISMS的两个问题：(1)上地幔热柱的存在或不存在如何影响伸展？(2)应变是如何在岩石圈中调节和分配的，以及对应变局部化和迁移的控制是什么？将安装来自乌干达东北部9个新台站的地震数据，以收集数据，以生成地幔流动指标(地震横波分裂)和改进的东部分支地下图像(地震层析成像)。如果在SKS测量中发现了大部分或所有地震台一致的偏北快偏振方向，则超烟羽模型将得到证实。如果观测到了更复杂的快速偏振方向模式，那么将检验其他的解释，包括多重羽流假说。层析成像模型将约束所提出的地球动力学模型的初始条件。还将安装横跨肯尼亚(7个)和乌干达(3个)的横跨东部分支的新全球导航卫星系统测站。如果GNSS数据具有与SKS观测相一致的沿裂谷运动，那么具有强烈烟柱-岩石圈相互作用的超烟柱模型是有利的。如果SKS观测与超羽模型一致，而GNSS不一致，则表明地幔去耦合。GNSS-SKS比较的这些和替代结果将通过受新的地震层析成像约束的岩石圈-地幔系统的3D计算建模来探索。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)的耦合性质。模拟的流动将分别在地面和实验室用新的GNSS和SKS分裂观测来评分。除上述科学目标外，该项目还将(1)通过为非洲阵列支持的未占名额的少数民族学生提供地震学方面的研究生研究机会来增强多样性；(2)通过全球导航卫星系统短期课程和非洲阵列教育计划对非洲科学家进行培训；(3)地球动力学建模扩展将作为方面软件包的一部分向公众开放；(4)全球导航卫星系统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时间向社区提供公开数据；(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.