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Three Major Failed Rifts in Central

North America: Similarities and Differences

Reece Elling, Seth Stein, Earth & Planetary Sciences, Northwestern University, Evanston, Illinois 60208, USA; Carol A. Stein, Kerri
Gefeke, Earth & Environmental Sciences, University of Illinois, Chicago, Illinois 60607, USA

ABSTRACT North America contains multiple impres- evolution are available only across the part of
The North American craton preserves sive, failed rifts (Fig. 1), preserving impor- the MCR below Lake Superior. Conversely,
nearly two billion years of geologic history, tant aspects of the fabric of nearly two bil- EarthScope local seismic array data showing
including three major rifts that failed rather lion years of geologic history in Laurentia, structure beneath the rift are available only
than evolving to continental breakup and its Precambrian core (Whitmeyer and across parts of the MCR’s west arm and
seafloor spreading. The Midcontinent Rift Karlstrom, 2007; Marshak and van der the RR.
(MCR) and Southern Oklahoma Aulacogen Pluijm, 2021). We focus on three major Using gravity data from the PACES
(SOA) show prominent gravity anomalies due failed rifts, covering ~10% of central North (Keller et al., 2006) and TOPEX data sets
to large volumes of igneous rift-filling rock. America (defined for these purposes as (Sandwell et al., 2013), we extracted profiles
The Reelfoot Rift (RR), though obscure in the area shown in Fig. 1A). One, the 150 km long and ~50 km apart across each
gravity data, is of interest due to its seismicity. Midcontinent Rift (MCR), is a prominent rift (Fig. 1B). Figure 1C shows each rift’s
The ca. 1.1 Ga MCR records aspects of the feature in geophysical maps of the region. mean Bouguer anomaly and standard devi-
assembly of Rodinia, whereas the ca. 560 Ma Due to its size and the availability of geo- ation. The mean profiles show differences
SOA and RR initiated during the later breakup physical and geological data, the MCR has between rifts, reflecting their tectonic ori-
of Rodinia and were inverted during the been the focus of many studies giving gin and subsurface structure. The MCR’s
assembly of Pangea. Comparative study of insight into its evolution, role in the assem- west arm shows large gravity highs (~80
these rifts using geophysical and geological bly of Rodinia, and processes of rifting and mGal) bounded by ~20 mGal lows on either
data shows intriguing similarities and differ- passive margin evolution (e.g., Green et al., side of the rift basin. In contrast, the MCR’s
ences. The rifts formed in similar tectonic set- 1989; C. Stein et al., 2018; Swanson-Hysell east arm has a positive anomaly half that of
tings and followed similar evolutionary paths et al., 2019). Two other failed rifts, the the west arm and lacks bounding lows. The
of extension, magmatism, subsidence, and Southern Oklahoma Aulacogen (SOA) and Southern Oklahoma Aulacogen has an ~60
inversion by later compression, leading to Reelfoot Rift (RR), have also been subjects mGal positive anomaly, similar to the MCR,
similar width and architecture. Differences of much interest. Parts of the SOA lie within whereas the RR shows only a minor (~10–
between the rifts reflect the extent to which the basement near and below the Anadarko 15 mGal) positive anomaly despite forming
these processes occurred. Further study of Basin, a major oil- and gas-producing basin. about the same time as the SOA.
failed rifts would give additional insight Thus, its oil-bearing upper crust is well The profiles are generally similar in
into the final stages of continental rifting studied (Brewer et al., 1983; Keller and width and form, but differ in amplitude,
and early stages of seafloor spreading. Stephenson, 2007; Hanson et al., 2013), but suggesting general similarities in crustal
the deeper structures in the lower crust and and uppermost mantle structure between
INTRODUCTION uppermost mantle are rarely the primary the rifts. We use the mean gravity profiles
Plate tectonics shapes the evolution of the target of study. The RR and its northern exten- augmented with seismic and other data,
continents and oceans via the Wilson cycle, sions, on the other hand, have little interest for combined with results from earlier studies,
in which continents rift to form new oceans. the energy industry but are of interest due to model the rifts’ general subsurface struc-
Many rifts evolve to passive continental mar- to their active seismicity (Hildenbrand and tures. We start with the hypothesis that the
gins. However, some rifts fail before conti- Hendricks, 1995; Calais et al., 2010). rifts are similar, and so when needed use
nental breakup and remain as fossil features These three failed rifts are grossly similar, inferences from one rift to gain insight into
within continents, which are largely buried with similar tectonic origins and structural the others, to the extent that the data permit.
beneath the surface and studied primarily features, but with interesting differences Although models from gravity data alone
with gravity and seismic surveys. Failed rifts highlighting aspects of their evolution. These are non-unique, augmenting them with
preserve a snapshot of the rifting process are shown by gravity data that are uniformly information from seismic, aeromagnetic,
before the beginning of seafloor spreading sampled across the central U.S. (Fig. 1). In surface mapping, and drill-hole data lets us
and thus give insight into late stages of conti- contrast, other data available differ from area characterize average structure along the
nental rifting and formation of passive to area. In particular, high-quality seismic rifts and illustrate similarities and differ-
continental margins (S. Stein et al., 2018; reflection data giving detailed structure at ences between them. The similarities and
Stein et al., 2022). depth that allows modeling of the rift’s differences reflect the combined effects of a

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4 GSA TODAY | June 2022