Collaborative Research: Testing the Role of Magma and Related Fluids in Early-Stage Rifting, East Africa

该项目的目标是以坦桑尼亚北方和肯尼亚南部东非裂谷系的一部分为目标，制定一种综合办法，以了解在厚大陆岩石圈中形成的初期和年轻裂谷带。这些裂谷的发展是一个基本的板块构造过程，但裂谷的启动和早期演化背后的驱动力是很难理解的，特别是岩浆和构造过程（岩脉侵入与断层）之间的应变适应的反馈和权衡。此外，热液流体和先存构造在弱化岩石圈、集中变形和岩浆活动方面的综合作用在裂谷系统的发育中几乎没有得到探索。该项目旨在解释初始裂谷下岩浆系统的相对作用，大型边界断层和横跨裂谷的分布正断层系统的发育，以及热液流体在弱化厚大陆岩石圈和促进集中裂谷带的发展和演化方面的地球化学影响。该分析将利用：（1）断层系统几何和动力学的实地测绘，以表征与岩浆系统相关的应变空间模式;（2）利用地球物理成像技术，对岩石圈中的岩浆和断层系统以及地壳和上地幔结构沿裂谷带段沿着的长度进行三维可视化（3）沿沿着边界断层系统对热液泉的气体排放进行取样，以确定可能导致岩石圈内弱化的流体的化学特征;以及（4）对断层系统暴露的火山产物进行取样，用于地质年代学分析，提供与岩浆幕有关的断层事件的时间限制，并允许量化时间平均应变率。这项研究将导致在裂谷早期阶段的岩浆和断层系统相互作用的综合模型的发展，可以作为评估全球大陆裂谷演化的基准。大陆裂谷带是神秘的，因为它们在厚，强大的大陆岩石圈的原因是知之甚少。它们的发育是板块构造过程的关键，但通过洋中脊扩张过程，促进大陆裂解，形成洋盆。世界范围内的裂谷带通常与大规模地震以及火山活动有关，要么集中在离散的火山，要么从数十公里长的裂缝中喷发。因此，它们带来了地质灾害，威胁着生活在世界各地裂谷带环境中的人类，包括美国。过去关于裂谷带发展高级阶段（大陆解体之前）的工作表明，大陆岩石圈通过裂谷作用逐渐伸展和变薄，使地幔在裂谷下方涌出并融化。然而，在年轻和发展中的裂谷系统，岩浆似乎是重要的裂谷，尽管很少地幔上涌在这个阶段。这表明，尽管存在厚厚的大陆岩石圈，但岩浆能够形成并上升到地壳中，使伸展成为可能，并有助于推动断层。岩石圈显然在某种程度上被削弱了，使得岩浆活动如此集中。该研究小组假设，岩石圈弱化是由与发展中的岩浆系统相关的集中的深部地幔流体辅助的，这些流体以化学方式改变和弱化岩石圈，使岩浆侵入并驱动裂谷发展。在东非裂谷的这项工作将使研究人员能够确定这些流体和侵入岩浆在削弱岩石圈和允许断层发展方面的相对作用。通过结合野外工作，地球物理学，地球化学和地质年代学，裂谷的发展和演变的模型将制定跨学科的集成，并将捕捉裂谷过程的多个方面。这项工作的结果将适用于全球裂谷带的发展，因此可以纳入现有的板块构造模型，为大陆分裂的可能性提供理论依据。

The goal of the project is to develop an integrative approach to understanding incipient and youthful rift zones developing in thick continental lithosphere by targeting a portion of the East African Rift system in northern Tanzania and southern Kenya. The development of these rifts is a fundamental plate tectonic process yet the driving forces behind rift initiation and early evolution are tenuously understood, particularly the feedbacks and trade-offs between strain accommodation by magmatic and tectonic processes (dike intrusion vs. faulting). Furthermore, the integrative roles of hydrothermal fluids and preexisting structure in weakening the lithosphere and focusing deformation and magmatic activity are virtually unexplored in developing rift systems. This project seeks to explain the relative roles of magma systems below incipient rifts, the development of large border faults and systems of distributed normal faults across rift valleys, and the geochemical influences of hydrothermal fluids in weakening thick continental lithosphere and promoting the development and evolution of focused rift zones. This analysis will utilize: (1) field-based mapping of fault system geometries and dynamics to characterize spatial patterns of strain in association with magmatic systems; (2) 3D visualization of magmatic and fault systems in the lithosphere, and crustal and upper mantle structure along the length of the rift zone segments, using geophysical imaging (a broadband seismic array); (3) sampling of gas emissions from hydrothermal springs along border fault systems to determine chemical signatures of fluids that may contribute to weakening within the lithosphere; and (4) sampling of volcanic products exposed by fault systems for geochronologic analysis, providing temporal constraints on faulting events in relation to magmatic episodes and allowing a quantification of time-averaged strain rates. This study will result in the development of an integrated model of magmatic and fault system interactions during the early stages of rifting that can serve as a benchmark for evaluating continental rift evolution globally.Continental rift zones are enigmatic in that they develop in thick, strong continental lithosphere for reasons that are poorly understood. Their development is a linchpin of the of plate tectonic process, however, facilitating continents breaking apart, forming ocean basins by the process of mid-ocean ridge spreading. Rift zones worldwide are commonly associated with large magnitude earthquakes as well as volcanic activity, either focused at discrete volcanoes or erupting from tens of kilometer long fissures. They thus present geologic hazards that threaten humans living in rift zone environments worldwide, including the U.S.A. Past work on rift zones in advanced stages of development (prior to continental breakup) has shown that the progressive stretching and thinning of the continental lithosphere by rifting allows the mantle to well up beneath the rift and melt. In young and developing rift systems, however, magma appears to be important for rifting despite very little mantle upwelling at that stage. This suggests that, despite the presence of thick continental lithosphere, magma is able to form and ascend into the crust, enabling extension and helping to drive the faulting. The lithosphere is clearly weakened in some manner to allow magmatic activity to be so focused. This research team hypothesizes that lithosphere weakening is assisted by focused, deep mantle fluids associated with developing magmatic systems and that these fluids chemically alter and weaken the lithosphere, allowing magma to intrude and drive rift development. This work in the East African Rift will allow researchers to determine the relative roles of these fluids and intruding magma in weakening the lithosphere and allowing faults to develop. By combining fieldwork, geophysics, geochemistry, and geochronology, a model for rift development and evolution will be formulated that is integrated across disciplines and will capture multiple facets of the rifting processes. The results of the work will be applicable to rift zone development globally and can thus be integrated into existing plate tectonic models to provide a rationale for why continental breakup is possible.