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

有11亿年历史的中大陆裂谷是一条3000公里长的埋藏的火成岩和沉积岩带，是北美重磁地图上最突出的特征之一。裂缝的一只手臂从苏必利尔湖延伸到俄克拉何马州，另一只手臂穿过密歇根州中部，可能一直延伸到阿拉巴马州。在苏必利尔湖(或基韦诺)地区裸露的地方，地表观察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毁灭性的影响。世界上大多数泛滥的玄武岩都发生在所谓的火成岩大省，在那里，由更深的地幔作用(地幔热柱)产生的大量玄武岩岩浆迅速喷发，在现有地形上流经很远的距离，形成厚厚的熔岩流。然而，中大陆裂谷的溢流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2000万年内喷发了大量的玄武岩。这些特征很难仅通过传统的玄武岩形成机制来解释。该项目探索了这样一种观点，即中大陆裂谷是裂谷作用和与更深的地幔热柱相互作用的组合，因此既具有裂谷特征，又具有大型火成岩省的特征。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地区拥有重要的铜矿床。该项目将通过以下途径促进预期的社会成果：(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)通过继续在国家和州立公园进行持续的场所推广，提高公众的科学素养和公众对STEM的参与；以及(3)通过培训研究生和为本科生提供研究机会，发展一支具有竞争力的STEM劳动力队伍。通过将中大陆/基韦诺裂谷视为混合裂谷和地幔羽流产生的大型火成岩省，这些观察结果可以得到解决。在这个模式中，火山岩是在最初的裂谷阶段和后裂谷阶段沉积的，在裂谷阶段，溢流玄武岩填充了断层控制的伸展盆地，而在裂谷后阶段，火山岩和沉积物沉积在热下沉的凹陷盆地中，没有伴随的断层作用。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动进行长期相互作用的可能性。具体地说，该项目将利用主量元素和微量元素分析、同位素表征(锶、钕、铅和氢)以及裂谷玄武岩的Ar年代学来确定：(1)形成玄武岩的源岩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(S)。该项目采用二维原始原始可变质点有限差分方法研究前寒武纪地幔条件下的裂谷行为和熔体形成，并采用地球动力学模拟来限制前寒武纪地幔条件下羽流物质横向迁移的程度，并结合熔融和熔体提取模型。裂谷历史由视极漂移路径中的尖点和重力模拟的微板块运动学确定。

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.