Collaborative Research: Geological Evolution of the Midcontinent Rift as a Hybrid Rift and Large Igneous Province

合作研究：作为混合裂谷和大型火成岩省的中部大陆裂谷的地质演化

• 批准号: 1549764
• 负责人: Tyrone Rooney
• 金额: $ 19.24万
• 依托单位: Michigan State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-06-01 至 2019-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1549764&HistoricalAwards=false
• 关键词: Collaborative; Research; Geological; Evolution; Midcontinent

The 1.1 billion year-old Mid-continent Rift is a 3,000 kilometer-long band of buried igneous and sedimentary rocks that is one of the most prominent features on gravity and magnetic maps of North America. One arm of the rift extends from Lake Superior to Oklahoma and the other through central Michigan to perhaps as far south as Alabama. Where exposed in the Lake Superior (or Keweenaw) region, surface observations and geophysical data indicate that the rift basin is filled with flood basalts, which are unusual landforms whose formation can have a devastating influence on climate and life. Most flood basalts around the world occur in so-called Large Igneous Provinces in which rapid eruption of a large volume of basaltic magma, created by deeper mantle processes (mantle plumes), flows great distances over existing topography to form a thick stack of lava flows. However, flood basalts of the Mid-Continent Rift depart significantly from this norm, occupying a relatively small space and erupted significant volumes for about 20 million years. These characteristics are difficult to explain solely through the traditional mechanisms proposed for the generation of flood basalts. This project explores the idea that the Mid-Continent Rift formed as a combination of rifting and interaction with deeper a mantle plume thus having characteristics of both a rift and a Large Igneous Province. Besides forming a fundamental structure of the North American continent, the Mid-continent Rift hosts important copper ore deposits in the Lake Superior region. The project would advance desired societal outcomes through: (1) improved STEM education and educator development through development of teaching modules and workshops for formal and informal educators; (2) increased public scientific literacy and public engagement with STEM through continuation of ongoing place-based outreach in national and state parks; and (3) development of a competitive STEM workforce through training of graduate students and by providing research opportunities for undergraduate students.In contrast to typical rifts that are filled with a mixture of clastic sediments, volcanoclastics, and lava flows, the 3000-km Midcontinent/Keweenaw Rift is filled with flood basalt with thicknesses that exceed even most large igneous provinces. These observations can be resolved by considering the Midcontinent/Keweenaw Rift as a hybrid rift and mantle plume-generated large igneous province. In this model, the volcanics were deposited during an initial rift phase where flood basalts filled a fault-controlled extending basin, and a post-rift phase where volcanics and sediments were deposited in a thermally subsiding sag basin without associated faulting. This project addresses the question of how the rift and plume interacted, including the possibility of lengthy interaction between an initially distant plume via material flow at the base of the lithosphere. Specifically, the project will use major and trace element analysis, isotopic characterization (Sr, Nd, Pb, and Hf), and argon geochronology of rift basalts to determine: (1) the geochemical signatures of the source reservoirs contributing to the basalts and their change over time; and (2) the temperatures, pressures, and source composition(s) of melt generation. The project employs a 2D parallel primitive variable particle-in-cell finite-difference method to study the behavior and melt formation of rifting under Precambrian mantle conditions in the absence or presence of a plume and geodynamic modeling to constrain the extent of lateral migration of plume material under Precambrian mantle conditions coupled with a model for melting and melt extraction. Rifting history is determined from the cusp in the apparent polar wander path and microplate kinematics from gravity modeling.

有11亿年历史的中大陆裂谷是一条3,000米长的埋藏火成岩和沉积岩带，是北美重力和磁力地图上最突出的特征之一。裂谷的一个分支从上级湖延伸到俄克拉荷马州州，另一个分支穿过密歇根州中部，可能向南延伸到亚拉巴马州。在上级湖（或基韦诺）地区，地表观测和地球物理数据表明，裂谷盆地充满了洪水玄武岩，这是一种不寻常的地貌，其形成可能对气候和生命产生破坏性影响。世界上大多数泛滥玄武岩都发生在所谓的大火成岩省，在那里，由更深的地幔过程（地幔柱）产生的大量玄武岩浆快速喷发，在现有地形上流动很远，形成厚厚的熔岩流。然而，中大陆裂谷的泛滥玄武岩明显偏离了这一标准，占据了相对较小的空间，并在大约2000万年的时间里爆发了大量的熔岩。这些特点是很难解释仅仅通过传统的机制提出的洪水玄武岩的生成。这个项目探讨了这样一种观点，即中大陆裂谷是裂谷作用和与更深的地幔柱相互作用的结合体，因此具有裂谷和大火成岩省的特征。除了形成北美大陆的基本结构外，中大陆裂谷还在苏必利尔湖上级地区拥有重要的铜矿床。该项目将通过以下方式推进预期的社会成果：（1）通过为正规和非正规教育工作者开发教学模块和讲习班，改善STEM教育和教育工作者的发展;（2）通过继续在国家公园和州立公园开展基于地方的外联活动，提高公众的科学素养和公众对STEM的参与;以及（3）通过培养研究生和为本科生提供研究机会，培养有竞争力的STEM劳动力。与充满碎屑沉积物、火山碎屑、和熔岩流，3000公里的中大陆/基韦诺裂谷充满了厚度超过大多数大型火成岩省的溢流玄武岩。这些观测结果可以通过将中大陆/基韦诺裂谷视为混合裂谷和地幔柱生成的大火成岩省来解决。在这个模型中，火山岩沉积在初始裂谷阶段，洪水玄武岩填充断层控制的延伸盆地，和后裂谷阶段，火山岩和沉积物沉积在一个热沉降的凹陷盆地，没有相关的断层。该项目解决了裂谷和羽流如何相互作用的问题，包括最初遥远的羽流之间通过岩石圈底部的物质流进行长时间相互作用的可能性。具体而言，该项目将使用裂谷玄武岩的主要和微量元素分析、同位素表征（Sr、Nd、Pb和Hf）和氩地质年代学来确定：（1）形成玄武岩的源储层的地球化学特征及其随时间的变化;（2）熔体生成的温度、压力和源成分。该项目采用二维平行原始变量粒子在细胞有限差分法研究的行为和熔体形成的裂谷在前寒武纪地幔条件下的存在或不存在的羽和地球动力学建模，以限制的程度的横向迁移的羽物质在前寒武纪地幔条件下，再加上熔化和熔体提取模型。裂谷的历史是确定从尖点在视极漂移路径和微板块运动学重力模拟。