4A (2022-2024): Mapping the structure of the Duluth Complex & Animikie Basin

• United States Geological Survey (USGS), United States of America

The Mesoproterozoic Duluth Complex in Northeastern Minnesota is the second largest layered mafic complex in the world, second only to the Bushveld Complex. It is one of the major plutonic components of the Midcontinent Rift System and hosts a variety of copper-nickel sulfide and platinum group element deposits. Exploration drilling for basal mineralization rarely penetrated far into footwall rocks beneath the Duluth Complex and therefore data are lacking on possible location and attributes of footwall massive sulfides. New gravity modeling indicates thick lower crustal feeder systems are present beneath the Duluth Complex and igneous intrusions within the adjacent Animikie Basin, as well as conductors within the Duluth Complex, suggest that the Paleoproterozoic basin may have played a key role in localizing mafic intrusions and volcanics. We plan to deploy this broadband array in a grid along the southwestern contact between the Duluth Complex and Animikie Basin, with scattered stations extending into the both the basin and Duluth Complex, in order to further investigate the internal architecture of layered intrusions by identifying layering and estimating extent, as well as refining the regional geologic framework for these intrusions, emphasizing feeder zones and the relationship between basin metasediments and mafic intrusions. Using receiver functions, we plan to investigate the location and extent of staging chambers beneath the Duluth Complex as well as refine estimates of crustal thickness. We also plan to test the use of higher frequency receiver functions to identify igneous layering and upper crustal intrusion thickness. Using ambient noise tomography, we plan to map the thickness of the Animikie Basin in order to better understand its relationship to the Duluth Complex intrusions. The seismic velocity results are expected to shed light on depth to the base of intrusions and the likely composition of a prominent conductor adjacent to the Duluth Complex feeders at ~13 km depth, and thereby determine whether it is associated with Animikie metasediments, Archean granitic basement, or a deep-rooted shear zone. In addition to the requested broadband stations, we plan to deploy 20 in house SmartSolo nodal sensors within the broadband array for shorter durations, with the goal of mapping near surface structure and velocity contrast with higher frequency ambient noise.

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