NSFGEO-NERC Collaborative Research: Crust and mantle structure and the expression of extension in the Turkana Depression of Kenya and Ethiopia

板块构造论的基本思想是建立在一个简化的观点之上的，即岩石圈（固体地球的最外层）被分解成刚性的碎片，这些碎片相对于彼此漂移而内部不变形，就像拼图碎片四处移动以形成一幅画，但它们自己不会改变形状或大小。这种方法可以很好地捕捉到边界发育良好的碎片的运动，但无法解释这些碎片最初是如何形成的，比如非洲碎片是如何沿着东非裂谷系统分裂成几个部分的。本实验的重点是量化三种不同因素在影响板块破裂中的作用：岩石圈中预先存在的结构；现在地形;以及下面的对流地幔。肯尼亚北部和埃塞俄比亚南部的图尔卡纳凹陷是研究这些问题的理想地点：它可能是异常地幔第一次与非洲岩石圈相互作用产生岩浆的地方，它具有相当大的继承结构，与裂谷的邻近部分相比，它的地形很少。这个项目的科学和更广泛的影响是重大的。一个由美国国家科学基金会和英国自然环境研究委员会支持的国际团队，包括来自美国、英国、肯尼亚和埃塞俄比亚的科学家，将进行实地观测和科学计算。这项工作将为两所合作院校的研究生和本科生提供支持。了解新的板块边界是如何在空间和时间上形成的，可以让我们更好地了解地球在其漫长历史中的构造演化，识别过去、现在和未来的板块边界，并了解与构造边界相关的自然灾害，如地震和火山。力学能力、重力势、地幔动力学和岩浆作用之间的非线性相互作用决定了大陆板块边界如何随时间演化。东非裂谷系统（EARS）是裂谷过程的理想天然实验室。例如，由于整个ear的远场边界条件是相同的，因此系统地比较了熔体丰富和熔体贫乏部分的应变调节，阐明了加热和成分的作用。比较有和没有大横向物质非均质性的板块，揭示了先前存在的岩石圈结构的作用；比较具有不同总有限应变的扇形可以作为演化的代理。然而，还需要考虑的是重力势能（GPE）的作用，通过比较高地形和低地形的裂谷扇区。虽然图尔卡纳坳陷的地震和火山活动表现为低地形、高物质非均质性和高地幔地热的末端条件，但现代地球物理方法尚未对其进行详细研究。该项目涉及对图尔卡纳凹陷进行多方法地球物理调查，将地震和大地测量数据收集相结合，用于地震成像、震源机制、地表运动学、地壳应变率和构造结构。系统地比较数据产品，结合大地测量、构造和地震数据的反演模型，以及有限的裂谷地形和应变模式的正演数值模拟，将检验关于GPE和地壳结构在大陆裂谷中的作用的基本假设。这样做将有助于解决更长期的裂谷演化，特别是一个或两个地幔柱的作用，继承了中生代裂谷的大陆结构，以及地形反馈在促进和塑造大陆分裂中的作用。这种对裂谷动力学的限制反过来将使我们更好地理解岩石圈和地幔之间的质量和热量交换、长时间尺度的大陆构造板块和边界行为，以及与岩浆裂谷有关的危险和资源的时空分布。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The basic idea of plate tectonics is built on a simplified view that the lithosphere (the outermost layer of the solid earth) is broken into rigid pieces that drift around relative to each other without deforming internally, like puzzle pieces that are moved around to make a picture but don't themselves change shape or size. This works well to capture the motions of pieces once their boundaries are well-developed, but doesn't explain how the pieces are made in the first place, such as how the African piece breaks into several parts along the East African Rift System. This experiment is focused on quantifying the role of three different factors in influencing plate break-up: preexisting structures in the lithosphere; present topography; and the convecting mantle beneath. The Turkana Depression of northern Kenya and southern Ethiopia is an ideal place to investigate these issues: it is probably the location where anomalous mantle first interacted with the African lithosphere to produce magma, it has considerable inherited structure, and it has very little topography compared to adjacent parts of the rift. The scientific and broader impacts of this project are significant. An international team supported by the U.S. National Science Foundation and the U.K. Natural Environment Research Council and including scientists from the US, UK, Kenya, and Ethiopia will undertake both field observations and scientific computing. The work will support graduate and undergraduate students at the two collaborating US institutions. Knowing how new plate boundaries form in space and time allows us to better understand the tectonic evolution of the planet over its long history, to identify past, current, and future plate boundaries, and to understand the natural hazards associated with tectonic boundaries, such as earthquakes and volcanos.Nonlinear interactions among mechanical competence, gravitational potential, mantle dynamics, and magmatism determine how continental plate boundaries evolve over time. The East African Rift System (EARS), is an ideal natural laboratory for rifting processes. For example, because the far-field boundary conditions on the whole EARS are the same, systematic comparisons of strain accommodation in melt-rich and melt-poor sectors have illuminated the role of heating and composition. Comparing sectors with and without large lateral material heterogeneities has revealed the role of pre-existing lithospheric architecture; comparing sectors with different total finite strain can be used as proxies for evolution. What remains to be considered, however, is the role of gravitational potential energy (GPE) through a comparison of a rift sector in high topography to one in low topography. Although the seismically and volcanically active Turkana Depression appears to represent the end member conditions of very low topography, very high material heterogeneity, and elevated mantle geotherms, it has yet to be investigated in detail with modern geophysical methods. This project involves a multi-method geophysical investigation of the Turkana Depression, combining seismic and geodetic data collection for seismic imaging, earthquake source mechanisms, surface kinematics, crustal strain rates, and structural architecture. Systematic comparisons of the data products to one another, combined with inverse models of geodetic, structural, and earthquake data and limited forward numerical simulations of rift topography and strain patterns will test basic hypotheses about the role of GPE and crustal architecture in continental rifting. Doing so will help to resolve the longer-term rift evolution, especially the role of one or two mantle plumes, inherited continental structure from Mesozoic rifting, and topographic feedbacks in contributing to and shaping continental breakup. Such constraints on the dynamics of rifting will, in turn, enable a better understanding of the exchange of mass and heat between the lithosphere and mantle, long timescale continental tectonic plate and boundary behavior, and the spatial and temporal distribution of hazards and resources associated with magmatic rifting.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.