ABSTRACT

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.

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