PRELIMINARY 3D MODEL OF THE MIDCONTINENT RIFT SYSTEM IN THE WESTERN LAKE SUPERIOR REGION, WISCONSIN, MINNESOTA, AND MICHIGAN

We are constructing a three-dimensional (3D) digital model of the structure and configuration of sedimentary and volcanic basins related to the ca. 1.1 Ga Midcontinent Rift System (MRS) in the western Lake Superior region. The current efforts build on a 3D model previously developed in the 1990s by David Allen for his PhD thesis under the auspices of Bill Hinze. Technical advances in modeling capabilities, expanded and improved geophysical data coverage, and renewed interest in the mineral resources of the MRS provide the motivation for new attempts at 3D digital modeling of the MRS.  

Our preliminary 3D model is regional and intended to show broad variations in rift-related geology. Only three generalized, geologic packages are currently represented. The packages are 1) undivided Keweenawan plutonic and volcanic rocks, which accompanied rifting, 2) Oronto Group sedimentary rocks, which were deposited as thermal subsidence replaced magmatism, and 3) Bayfield Group and equivalent sedimentary rocks, which represent the youngest sedimentation at the end of rifting. In addition, three major fault systems are modeled: the Douglas, Lake Owen, and Keweenaw faults. These faults accommodated widespread reverse motion in response to a compressional regime that followed Bayfield sedimentation. 

The modeling strategy involves digitizing the bases of the geologic packages, fault locations, and general orientation data along 2D sections and from geologic maps. The 3D modeling software then connects the digitized points into surfaces and volumes in 3D space. Digitized points from the previous 3D model serve as a guide, but we are re-evaluating previous interpretations from mostly analog seismic-reflection sections and gravity and magnetic models. A few sections of legacy seismic data have been reprocessed, revealing more details and providing better velocity information for depth interpretations. For example, reprocessed data confirm the presence of angular unconformities between the volcanic and overlying sedimentary rocks at the sides of basement highs. The 3D model helps visualize these relations and the later reverse faults, suggesting that the basement highs influenced both rift formation and later contractional structures.