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arm contains 20–25 km of volcanics, whereas Our SOA model is modified from Keller location is constrained by Liu et al.’s (2017)
the east arm contains 10–15 km. The dense and Stephenson’s (2007) model based on results. An underplate has also been observed
igneous rocks affect the gravity anomaly gravity, seismic, aeromagnetic, surface map- along the RR’s northeastern extension (Aziz
much more than the underplate, so the geom- ping, and drilling data. Seismic reflection Zanjani et al., 2019). A feature of our model,
etry of the volcanics in the east arm was data were used to constrain the location and required to replicate the lack of a large gravity
adjusted to match the gravity profiles. thicknesses of the gabbroic and felsic intru- anomaly, is that the RR contains far less
sions producing the large positive anomaly. high-density volcanics than the other rifts,
SOUTHERN OKLAHOMA We simplified their model for comparison perhaps because it extended less. Low-density
AULACOGEN with the other rifts. Sedimentary basin rocks Quaternary sediments of the Mississippi
The Southern Oklahoma Aulacogen (SOA) were averaged into a few units, and bodies River basin overlying the rift rocks also con-
(Walper, 1977) is a linear alignment of exten- within the gabbroic intrusion that increased tribute to the minimal anomaly.
sively inverted rift structures perpendicular in density with depth in the original model
to the southern tip of the MCR’s west arm. Its were averaged to a single density. Keller and SIMILARITIES AND DIFFERENCES
main structures are the Wichita uplift (and Baldridge (1995) proposed the presence of Comparing the three rifts’ average gravity
associated igneous provinces) and Anadarko an underplate, which is consistent with the profiles and subsurface structures inferred in
Basin. Both the SOA and RR (discussed gravity data and included in our model, part from them illustrates similarities and
shortly) initiated as the Cuyania block, though seismic data adequate to confirm (or differences between the rifts.
also known as the Argentine Precordillera, disprove) its presence are not available.
rifted away from Laurentia (Thomas, 2011; Tectonic Setting
Whitmeyer and Karlstrom, 2007). Rifting is REELFOOT RIFT All three formed during rifting associated
thought to have begun in latest Precambrian, The Reelfoot Rift (RR) underlies the Upper with Laurentia’s interactions within the super-
but the oldest dates come from SOA igneous Mississippi Embayment, a broad trough with continent of Rodinia. The MCR formed after
rocks dated at ca. 540 Ma (Wall et al., 2021). a complex history of rifting and subsidence the Elzeverian and Shawinigan orogenies and
The SOA’s geologic and tectonic history (Catchings, 1999). The NE-trending graben of before the Grenville Orogeny that assembled
has three major phases. The first involved the RR is 70 km wide and more than 300 km Rodinia (e.g., Hynes and Rivers, 2010). Its for-
emplacement of the Wichita Igneous Province long. Reflection profiles and mafic alkalic mation was likely associated with rifting
during development of a rift beginning in the plutons suggest several episodes of faulting between Laurentia and Amazonia during a
Ediacaran to mid-Cambrian (Brewer et and intrusive activity (Mooney et al., 1983). plate boundary reorganization (S. Stein et
al., 1983; Perry, 1989; Wall et al., 2021). The RR is believed to have experienced al., 2014, 2018) (Fig. 3A), although details of
Extensional and transtensional tectonism multiple phases of subsidence (Ervin and Amazonia’s location and motion are not well
within the SOA developed during the latest McGinnis, 1975), with the earliest rifting in constrained at this time because of limited
Precambrian–Cambrian opening of the the Ediacaran associated with widespread paleomagnetic data (Tohver et al., 2006; Li et
southern Iapetus Ocean as part of Rodinia’s rifting along North America’s margins during al., 2008).
breakup (Robert et al., 2021). Following rift the breakup of Rodinia. The rift basin primar- Additional evidence for this view comes
failure, thermal subsidence allowed deposi- ily developed during this Cambrian event. from a change in Laurentia’s absolute plate
tion of thick sedimentary sequences, mark- Later subsidence, perhaps as late as the motion around the time of the formation of
ing the onset of the Anadarko Basin forma- Cretaceous, is associated with emplacement the MCR. A global plate model (Scotese and
tion (Perry, 1989; Johnson, 2008). Finally, of mafic igneous intrusives inside the rift and Elling, 2017), updated with a global compila-
Late Mississippian through Pennsylvanian deposition of several kilometers of sediments tion of paleomagnetic poles (McElhinny
compression inverted the SOA and formed a that bury them (Hildenbrand and Hendricks, and Lock, 1996; Torsvik et al., 2008, 2012;
NE-trending fold-thrust belt containing the 1995; Cox and Van Arsdale, 2002). Relative to Merdith et al., 2017; Scotese and Van der
Wichita and Arbuckle Mountains (Keller the MCR and SOA, the RR experienced Voo, 2017; Veikkolainen et al., 2017), was
and Stephenson, 2007). The compression is significantly less volcanic activity during rift- inverted to generate synthetic apparent polar
believed to be related to North America’s ing, and its subsidence influenced the sedi- wander (APW) paths that match the plate
collision with Africa and South America mentation and subsequent development of model. Comparison with global mean poles
during the Alleghenian Orogeny (Kluth and the drainage basins of major rivers, such as (GMP) revealed these synthetic APW paths
Coney, 1981) or tectonic activity along North the Mississippi. Climate-controlled erosion produce a good fit within the α95 error of the
America’s western and southwestern mar- and unloading of sediments that fill the rift GMPs. Laurentia’s APW path has a major
gins (Lawton et al., 2017; Leary et al., 2017). basin have been proposed to have triggered cusp, called the Logan Loop, recorded in
The SOA exposes only a fraction of its extent the present seismicity (New Madrid seis- part by the MCR’s volcanic rocks (Fig. 3C).
in the Wichita Mountains and contains more mic zone) on faults remaining from the Cusps in APW paths have been observed
than 210,000 km of buried mafic rocks up to rifting (Calais et al., 2010). elsewhere when continents rift apart (Gordon
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10 km thick along the entire rift (Hanson et We developed our model by modifying et al., 1984). A similar cusp appears ca. 600
al., 2013), along with a large volume of felsic one by Liu et al. (2017) based on their work Ma in this model (Fig. 3C), during opening
igneous rocks, including granitic intrusions and earlier models constrained by seismic of the Iapetus Ocean as the Argentine
and interbedded rhyolites. Emplacement and refraction, gravity, and magnetic data Precordillera microcontinent rifted from
subsequent inversion of the igneous rocks (Mooney et al., 1983; Braile et al., 1986; the Wichita embayment on Laurentia’s SE
yielded a positive gravity anomaly of ~60 Nelson and Zhang, 1991). Earlier studies iden- margin (Whitmeyer and Karlstrom, 2007;
mGal, similar to the average of the MCR arms. tified an underplate, or “rift pillow,” whose Thomas, 2011). Both the SOA and RR

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