1. Midcontinent Rift (MCR)
Comparative Riftology: insights into the evolution of passive continental margins and continental rifts from the failed
Midcontinent Rift (MCR)
Jonas Kley1, Seth Stein2, Carol Stein3, Randy Keller4, Michael Wysession5, Andrew Frederiksen6, Reece Elling2
1Georg-August-Universität Göttingen
2Northwestern University
3Univ. of Illinois at Chicago
4University of Oklahoma
5Washington University
6University of Manitoba

2. (which just happened to fail at an unusually late stage)
Is the 1 Ga-old Midcontinent Rift (MCR) a typical example of rift-to-passive margin evolution?
(which just happened to fail at an unusually late stage)

3. MCR: Long arms of buried dense & highly magnetized
1.1 Ga igneous rocks ~ 3000 km long, ~ 2 x 106 km3 magma
MCR: Gravity high
Rio Grande Rift: Gravity low OK AL
Stein et al., 2014
Gravity high due to filling by igneous rocks: thick volcanics like many rifted continental margins

4. Crustal thickening & underplating along west arm shown by surface wave tomography & receiver functions
Zhang et al. 2016

5. MCR has the linear geometry of a rift
and the magma volume of a Large Igneous Province (LIP)
Stein et al., 2015

6. Crustal structure data and surface geology allow reconstruction of rift evolution
Profile after GLIMPCE line C (Green et al.,1989)
Inset from Milkereit et al. (1990)
Stein et al., Geosphere, 2015

7. Rift evolution
1. Normal faulting and synrift volcanism, crustal thinning
2. Faults inactive, post-rift volcanism(!) and subsidence Ma Ma
4. Reverse faulting and uplift Additional crustal thickening
3. Volcanism terminated, post-rift subsidence and sedimentation,
crustal root
1086-? Ma
Between 960 and 540 Ma
Stein et al., Geosphere, 2015

8. MCR crustal models predict even stronger gravity high in rifting stage with mafic underplate
Stein et al., Geosphere, 2015
After Inversion & Erosion
Rifting Ends
About Ma
Volcanics at surface
Very large positive gravity anomaly
Present-day
Volcanics exposed
Gravity high bounded by gravity lows
Measured (Green et al 1989)

9. Became inactive once seafloor spreading was established
MCR likely formed as part of the rifting of Amazonia from Laurentia, recorded by APWP cusp at ~1.12 Ga
Became inactive once seafloor spreading was established
APWP for Laurentia poles
1.109 Ga
Stein et al., GRL, 2014

10. MCR was microplate boundary (Chase and Gilmer, 1973)
Similar to today’s East African Rift: microplates between diverging major plates
Laurentia
(Later collision)
Euler pole
Merino et al., GRL 2013

11. Successful half-graben rifting with MCR geometry yields asymmetric passive margin
South Atlantic
Blaich et al., 2011
Successful MCR

12. Is the failed Midcontinent Rift a frozen snapshot of a rifted volcanic margin early in its evolution?
Flood basalts, Isle Royale National Park
Dipping into rift axis below Lake Superior
Flows
Argentina / South Africa conjugate margins, onset of spreading
Becker et al., 2016
Incipient
MOR

13. What is your guess?

14. Archean crust
Plume
Depleted mantle
Nicholson et al. 1997