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Figure 4. Gravity anomalies expected at various stages in rift evolution, based on model for the Midcontinent Rift under Lake Superior. During rifting, dense
volcanics cause a large positive anomaly. Subsequent deposition of low-density sediments and associated subsidence cause a gravity low. Inversion of the
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rift and erosion of low-density sediments cause the high observed today. Densities in g/cm . (After Elling et al., 2020.)
explanation of the differences between the CONCLUSIONS Braile, L., Hinze, W., Keller, G., Lidiak, E., and Sex-
SOA and RR, which formed about the same Traditionally, studies have considered the ton, J., 1986, Tectonic development of the New Ma-
time in similar events, is that the RR had less major failed rifts in central North America drid rift complex: Tectonophysics, v. 131, p. 1–21,
https://doi.org/10.1016/0040-1951(86)90265-9.
extension and inversion. separately. However, it is useful to consider Brewer, J., Good, R., Oliver, J., Brown, L., and
Although models without underplates them as similar although not identical enti- Kaufman, S., 1983, COCORP profiling across
could fit the gravity data, we include under- ties and to view them in the context of both the Southern Oklahoma aulacogen: Geology,
plates because seismic data both from the failed and active rifts worldwide. Although v. 11, p. 109–114, https://doi.org/10.1130/0091
-7613(1983)11<109:CPATSO>2.0.CO;2.
MCR (below Lake Superior and on its west they are grossly similar, with similar tec- Bright, R., Amato, J., Denyszyn, S., and Ernst, R.,
arm) and RR show them, and underplates are tonic origins and structural features, inter- 2014, U-Pb geochronology of 1.1 Ga diabase in
typically observed at presently spreading esting differences between them reflect the the southwestern United States: Lithosphere, v. 6,
rifts. Furthermore, underplates are thought to extent to which extension, magmatism, sub- p. 135–156, https://doi.org/10.1130/L335.1.
form from residual melt after extraction of sidence, and inversion by later compression Burberry, C., Joeckel, R., and Korus, J., 2015, Post-
low-density lavas and would be expected occurred. Further study of these and other Mississippian tectonics of the Nemaha Tectonic
Zone and Mid-Continent Rift System: The Moun-
given the volume of volcanic material in these failed rifts would provide additional insight tain Geologist, v. 52, no. 4, p. 47–73.
rifts. We expect their size to be proportional to into how many rifts transition from the final Burov, E., and Gerya, T., 2014, Asymmetric three-
the volume (cross-sectional area) of volcanics, stages of continental rifting to the early dimensional topography over mantle plumes:
as observed for rifted continental margins stages of seafloor spreading. Nature, v. 513, p. 85–89, https://doi.org/10.1038/
nature13703.
(Gallahue et al., 2020). Hence, the similar Calais, E., Freed, A., Van Arsdale, R., and Stein, S.,
underplates beneath the western MCR and ACKNOWLEDGMENTS 2010, Triggering of New Madrid seismicity by
RR are surprising, given that the MCR has We thank our many collaborators during a de- late-Pleistocene erosion: Nature, v. 466, p. 608–
roughly ten times more volcanics in cross sec- cade of rift studies synthesized here. In particular, 611, https://doi.org/10.1038/nature09258.
tion. One possible explanation is that in addi- Randy Keller provided his broad knowledge and Catchings, R., 1999, Regional Vp, Vs, Vp/Vs, and
deep insight about continental rifting both in North
Poisson’s ratios across earthquake source zones
tion to the volcanics in our RR model, another America and elsewhere. We also thank Nick from Memphis, Tennessee, to St. Louis, Missouri:
volcanic unit, a mafic high-density upper Swanson-Hysell and anonymous reviewers for Bulletin of the Seismological Society of America,
crustal layer, also exists. Liu et al. (2017, p. helpful suggestions, along with Jim Schmitt for v. 89, p. 1591–1605, https://doi.org/10.1785/
BSSA0890061591.
4581) suggest this possibility while noting that constructive discussions and timely editorial pro- Cox, R., and Van Arsdale, R., 2002, The Mississippi
cessing of this manuscript.
such a layer is not required by the data and embayment: Journal of Geodynamics, v. 34,
would be “rare, if not previously unrecog- p. 163–176, https://doi.org/10.1016/S0264-3707
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