120-3 Constraining the Three-Dimensional Geometry of the Duluth Complex, MN: Insights from Magnetic Fabrics and Paleomagnetic Data

Abstract:

The Midcontinent Rift developed within the interior of Laurentia during a period of extension and magmatism from 1109 Ma to 1084 Ma. Emplaced during the development of the Midcontinent Rift, the Duluth Complex is interpreted as the second-largest exposed mafic intrusive complex on Earth. The Duluth Complex comprises an anorthositic series and a layered series of gabbro and troctolite cumulates. Despite the wealth of data on the geology, mineralization, structure, timing, and emplacement mechanisms of the Duluth Complex, there remains some uncertainty surrounding its thickness and, therefore, overall volume. 

The tilt of the Duluth Complex is poorly constrained due to the absence of macroscopic igneous foliation over much of the complex, particularly in the anorthositic series. This research focuses on developing data on Anisotropy of Magnetic Susceptibility (AMS) parameters and paleomagnetic remanence directions compared with expected tilt-corrected directions. These data lead to new estimates of the Duluth Complex’s tilt and therefore its thickness, which exceeds 9 km in the center of the complex. Remanence directions estimate a consistent tilt of about 14 to 16 degrees to the southeast in the anorthositic and layered series, as well as the overlying volcanics, indicating that the Duluth Complex and overlying volcanics were tilted simultaneously as a coherent block rather than progressively during rift magmatism. AMS parameters are used in areas where igneous foliation is not measurable and provide tilt estimates consistent with those derived from remanence directions. Indication of uniform tilt from remanence directions justifies a slab simplification for thickness estimates, and consistency between AMS, remanence, and field-derived tilts provides confidence in these estimates. Variation in the geometry of the Duluth Complex along strike will require this method to be utilized across the complex when calculating an overall volume. 

Together, the new susceptibility and remanence data provide important petrophysical information for interpreting new USGS aeromagnetic surveys in northeastern Minnesota. Koenigsberger ratios calculated from Duluth Complex samples are dominated by remanent magnetization. This result provides important ground-truth data for modeling the aeromagnetic data and highlights the importance of considering remanent magnetization rather than induced magnetization alone.

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

doi: 10.1130/abs/2026AM-20983

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