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

合作研究：测试岩浆和相关流体在东非早期裂谷中的作用

• 批准号: 1113346
• 负责人: Steven Roecker
• 金额: $ 6.43万
• 依托单位: Rensselaer Polytechnic Institute
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-08-15 至 2015-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1113346&HistoricalAwards=false
• 关键词: Collaborative; Research; Testing; Role; Magma

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.

该项目的目的是开发一种综合方法来理解在坦桑尼亚北部和肯尼亚南部的一部分东非裂谷系统的一部分，以理解厚厚岩石圈的初期和年轻裂谷区。这些裂痕的发展是一个基本的板块构造过程，但是裂谷开始和早期进化的驱动力是微不足道的，尤其是岩浆和构造过程之间的应变适应性之间的反馈和权衡（堤防入侵与故障）。此外，在开发裂谷系统中，几乎无法探索水热流体和先前存在的结构的综合作用。该项目旨在解释岩浆系统在起步裂谷以下的相对作用，跨裂谷谷的大边界断层和分布式正常故障的发展以及热液流体在削弱厚厚的大陆岩石圈中的地球化学影响以及促进聚焦动物的发展和演变。该分析将利用：（1）基于场的故障系统几何形状和动力学的映射以表征与岩浆系统相关的应变的空间模式； （2）使用地球物理成像（宽带地震阵列），岩石圈和上地幔结构的岩浆和断层系统的3D可视化以及地壳和上地幔结构沿着裂谷区域的长度（宽带地震阵列）； （3）对沿边界断层系统中水热弹簧的气体排放采样，以确定可能有助于岩石圈内弱化的流体的化学特征； （4）通过断层系统暴露的火山产品进行地球文学分析的采样，提供了与岩浆发作有关的故障事件的时间限制，并允许量化时间平均应变率。这项研究将导致在裂谷的早期阶段发展岩浆和断层系统相互作用的综合模型，该模型可以作为评估全球大陆裂谷进化的基准。洲际裂谷区是神秘的，因为它们在较厚，强大的大陆岩石圈中发展，原因是理解不足的原因。它们的发展是板块构造过程的关键，但是，促进大陆裂解，通过中海脊扩散的过程形成海盆。全球裂谷区通常与大量地震以及火山活动有关，该地震集中在离散的火山上，或者是从数十公里长的裂缝中爆发的。因此，他们提出的地质危害威胁着生活在全球裂谷环境中的人类，包括美国的过去在高级发展阶段的裂谷区（大陆拆分之前）的工作表明，大陆岩石圈的渐进式伸展和稀疏，可以使地幔在裂谷和裂谷下面良好。然而，在年轻和正在发展的裂谷系统中，尽管在那个阶段的地幔上升很小，但岩浆对于裂谷似乎很重要。这表明，尽管存在较厚的大陆岩石圈，但岩浆仍能够形成并升入地壳，从而延伸并有助于驱动断层。岩石圈显然以某种方式减弱，以使岩浆活性如此集中。该研究团队假设岩石圈削弱了与开发岩浆系统相关的聚焦，深层液体的帮助，并且这些液体在化学上改变了岩石圈并削弱了岩石圈，从而使岩浆侵入并驱动裂谷。东非裂谷的这项工作将使研究人员能够确定这些液体的相对作用，并侵入岩浆削弱岩石圈并允许断层发展。通过结合现场工作，地球物理学，地球化学和地质学，将制定裂谷发展和进化的模型，该模型跨学科整合，并将捕获裂谷过程的多个方面。这项工作的结果将适用于全球裂谷区的开发，因此可以集成到现有的板块构造模型中，以提供理由，说明为什么大陆拆分是可能的。