120-4 Earth MRI aeromagnetic data over the western Spirit Lake tectonic zone, Iowa, Minnesota, South Dakota, and Nebraska

Abstract:

The Spirit Lake tectonic zone (SLTZ) forms the southern boundary of much of the Superior Province, juxtaposing Archean crust on the north with Paleoproterozoic crust on the south. The western Spirit Lake tectonic zone (WSLTZ) is informally defined as that portion of the SLTZ extending from the 1.1 Ga Midcontinent Rift System (MRS) on the east to the ~2.0-1.8 Ga Trans-Hudson orogen on the west. The WSLTZ region is thought to have significant potential for undiscovered deposits of critical minerals, including Ni-Cu-PGE-Co sulfides, Fe-Ti-V oxides, and REE-bearing carbonatites, among other deposit types. However, the Archean and Proterozoic rocks of interest are entirely concealed beneath hundreds of meters of sedimentary cover, and previous mapping studies were hampered by the low quality of public aeromagnetic data. To address mapping needs, a very large (274,100 line km, covering ~100,000 km²) aeromagnetic survey sponsored by the USGS Earth Mapping Resources Initiative (Earth MRI) was flown in 2023-2024 with 400 m line spacing and 100 m nominal terrain clearance.

Here, we highlight several aeromagnetic anomalies of interest. The WSLTZ is characterized by numerous subparallel, northeast-trending gradients, pattern breaks, and elongated anomalies that are likely produced by faults and crudely aligned dikes. Archean rocks north of the WSLTZ include many strongly magnetized intrusions and produce generally stronger anomalies than Paleoproterozoic rocks to the south. A suspected Mesoproterozoic massif-type anorthosite complex positioned along the WSLTZ near Spencer, Iowa, produces complex magnetic lows and highs. Along the South Dakota-Nebraska border a series of complex, high-amplitude anomalies are crudely aligned with the WSLTZ trend, likely produced by a large volume of poorly understood mafic/ultramafic intrusive rocks. The Sioux Falls area is characterized by smooth anomalies produced by thick, weakly magnetic Sioux Quartzite overlying deeply buried magnetic rocks. Across the survey region, Mesoproterozoic granitoids intruding Paleoproterozoic rocks south of the WSLTZ produce numerous sub-circular highs. Smooth anomalies lie over MRS flanking basins along the eastern and southeastern margins of the survey area, reflecting deep burial of magnetic crystalline rocks under weakly magnetized MRS sedimentary rocks. Rocks of the Trans-Hudson orogen produce relatively gentle, northwest-trending anomalies in the western part of the survey area, west of Yankton, SD.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-19363

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