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Figure 1. (A) Bouguer gravity anomaly map for central North America. Anomalies related to the Midcontinent Rift (MCR), Southern
Oklahoma Aulacogen (SOA), and Reelfoot Rift (RR) are outlined. Dashed lines outline possible extensions of rift arms not included
in analysis. (B) Profiles used in calculating the average gravity anomalies. (C) Mean anomalies and standard deviations for rifts.

sequence of rifting, volcanism, sedimenta- 2020). The MCR likely formed as part of rift- because the west arm’s larger gravity anom-
tion, subsidence, compression, erosion, and ing of the Amazonia craton (now in north- aly indicates differences in magma volume
later effects (Stein et al., 2015; Elling et al., eastern South America) from Laurentia, and tectonic evolution. For simplicity, the
2020). They give insight into how rifts the Precambrian core of North America at models use average densities of the sedi-
evolve and are useful when studying other 1.1 Ga, after the Elzeverian and Shawinigan ment, igneous rift fill, underlying crust,
failed or active rifts elsewhere. orogenies and before the Grenville Orogeny underplate, and mantle. We began with
(C. Stein et al., 2014, 2018; S. Stein et al., GLIMPCE seismic reflection profiles across
MIDCONTINENT RIFT 2018). Surface exposures, seismic data, and Lake Superior that give the best available
The Midcontinent Rift (MCR), a 3000-km- gravity data delineate rift basins filled by image of structure at depth in the MCR
long band of more than 2 million km of bur- thick basalt layers and sediments, underlain (Green et al., 1989) and permit detailed mod-
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ied igneous and sedimentary rocks that out- by thinned crust and an underplate unit, pre- eling of its evolution (Stein et al., 2015). We
crop near Lake Superior, has been extensively sumably the dense residuum from the magma also considered prior gravity models across
studied, as reviewed by Ojakangas et al. extraction (Vervoort et al., 2007; S. Stein et parts of the MCR (Mayhew et al., 1982; Shay
(2001) and S. Stein et al. (2018). To the south, al., 2018). The rift was later massively inverted and Trehu, 1993). EarthScope data (Zhang et
it is buried by younger sediments, but easily by regional compression, uplifting the volca- al., 2016) provided values for the depth and
traced because the rift-filling volcanic rocks nic rocks so that some are exposed at the sur- thickness of the volcanics and underplate
are dense and highly magnetized. The west- face today. The MCR has little seismicity along the west arm that were used to update
ern arm extends southward to Oklahoma, as along most of its length, but portions in the models. These data showed that structure
shown by positive gravity anomalies and sim- Kansas and Oklahoma experienced seismic- below the west arm resembles that below
ilar-age diffuse volcanism (Bright et al., 2014). ity and Phanerozoic deformation (Burberry et Lake Superior, suggesting that the structure
The eastern arm extends southward to al., 2015; Levandowski et al., 2017). along the entire MCR is similar. On either
Alabama (Keller et al., 1983; C. Stein et al., We developed models for each arm (Figs. side of the central rift basin, basins ~5 km
2014, 2018; S. Stein et al., 2018; Elling et al., 2A and 2B), following Elling et al. (2020), thick resulting from post-rift sedimentation

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