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

板块构造学的基本思想建立在一种简化的观点之上，即岩石圈(固体地球的最外层)被分解成相对彼此漂移的刚性碎片，而不会在内部变形，就像拼图碎片一样，它们被四处移动以形成一幅画，但本身并不改变形状或大小。这很好地捕捉了碎片的运动，一旦它们的边界被很好地开发，但不能解释碎片是如何制成的，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。这项实验的重点是量化影响板块解体的三个不同因素的作用：岩石圈中先前存在的结构；现在的地形；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：它可能是异常地幔最先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性结构，与裂谷的邻近部分相比，它的地形非常小。该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理事会支持的国际团队，成员包括来自美国、英国、肯尼亚和埃塞俄比亚的科学家，将进行现场观测和科学计算。这项工作将支持这两所合作的美国机构的研究生和本科生。了解新的板块边界在空间和时间上是如何形成的，有助于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。机械能力、引力势、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变。东非裂谷系统(EAR)是裂谷过程的理想天然实验室。例如，由于整个耳朵上的远场边界条件是相同的，系统地比较了富熔体和贫熔体部分的应变调节，说明了加热和成分的作用。比较具有和不具有较大横向材料非均质性的扇区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区可以作为演化的替代。然而，仍然需要考虑的是，通过比较高地形和低地形的裂谷扇区，引力势能(GPE)所起的作用。尽管地震和火山活动活跃的图尔卡纳凹陷似乎代表了极低的地形、极高的物质非均质性和高地幔地温的端元条件，但它尚未用现代地球物理方法进行详细的研究。该项目涉及图尔卡纳凹陷的多方法地球物理调查，将地震和大地测量数据收集结合起来，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。数据产品之间的系统比较，结合大地测量、构造和地震数据的反向模型，以及对裂谷地形和应变模式的有限正演数值模拟，将检验有关地球物理过程和地壳结构在大陆裂谷作用中作用的基本假设。这样做将有助于解决较长期的裂谷演化，特别是一到两个地幔热柱的作用，中生代裂谷作用继承的大陆结构，以及地形反馈对大陆解体的贡献和塑造。这些对裂谷动力学的约束反过来将使我们能够更好地了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，以及与岩浆裂解相关的灾害和资源的时空分布。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.