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

11亿年前的中大陆裂谷是一条3000公里长的埋藏火成岩和沉积岩带，是北美重力和磁图上最突出的特征之一。裂谷的一端从苏必利尔湖延伸到俄克拉何马州，另一端穿过密歇根中部，向南延伸至阿拉巴马州。在苏必利尔湖（或Keweenaw）地区，地面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活产生破坏性影响。世界上大多数洪水玄武岩发生在所谓的大火成岩省，在那里，由更深的地幔作用（地幔柱）产生的大量玄武岩岩浆迅速喷发，在现有的地形上流动很远，形成厚厚的熔岩流堆。然而，中大陆裂谷的洪水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2000万年的时间里喷发了大量火山。这些特征很难仅仅通过洪水玄武岩形成的传统机制来解释。该项目探讨了中大陆裂谷是裂谷与更深的地幔柱相互作用的结合，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构造外，中大陆裂谷还在苏必利尔湖地区拥有重要的铜矿床。该项目将通过以下方式促进预期的社会成果：(1)通过为正式和非正式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)通过继续在国家和州立公园开展基于地点的推广活动，提高公众的科学素养和公众对STEM的参与；(3)通过培养研究生和为本科生提供研究机会，发展具有竞争力的STEM劳动力队伍。典型的裂谷充满了碎屑沉积物、火山碎屑和熔岩流的混合物，与之相反，3000公里的中大陆/基维诺裂谷充满了洪水玄武岩，其厚度甚至超过了大多数大型火成岩地区。这些观测结果可以通过将中大陆/基维诺裂谷视为裂谷和地幔柱生成的大火成岩省来解决。在该模型中，火山沉积在裂谷初始阶段，其中洪水玄武岩填充了断裂控制的扩张盆地；裂谷后阶段，火山和沉积物沉积在热沉降的凹陷盆地中，没有相关的断裂。该项目解决了裂谷和地幔柱如何相互作用的问题，包括通过岩石圈底部的物质流动，在最初遥远的地幔柱之间进行长期相互作用的可能性。具体而言，该项目将利用裂谷玄武岩的主要元素和微量元素分析、同位素表征（Sr、Nd、Pb和Hf）和氩年代学来确定：(1)对玄武岩有贡献的源储层的地球化学特征及其随时间的变化；(2)熔体产生的温度、压力和来源组成。本项目采用二维平行原始变颗粒单元有限差分方法研究了前寒武纪地幔条件下无或存在地幔柱的裂陷行为和熔体形成，并采用地球动力学建模来约束前寒武纪地幔条件下地幔柱物质横向迁移的程度，并结合熔融和熔体提取模型。根据视极漂移路径的尖端和重力模拟的微板块运动学来确定裂陷历史。

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.