North America’s Midcontinent Rift: When Rift Met LIP Carol Stein1, Seth Stein2, Jonas Kley3, Randy Keller4, Trevor Bollman2, Emily Wolin2, Hao Zhang2, Andrew Frederiksen5, Kunle Ola5, Michael Wysession6, Douglas Wiens6, Ghassan Al-Equabi6, Greg Waite7, Eunice Blavascunas8, Carol Engelmann7, Lucy Flesch9, Jake Crane9, Tyrone Rooney10, Robert Moucha11, Eric Brown12, SPREE Project Team   1U. of Illinois at Chicago, 2Northwestern Univ., 3Georg-August-Universität Göttingen, 4Univ. of Oklahoma, 5Univ. of Manitoba, 6Washington Univ., 7Michigan Tech, 8Whitman College, 9Purdue Univ., 10Michigan State Univ., 11Syracuse Univ., 12Aarhus Univ.

Long arms of buried dense & highly magnetized 1 Long arms of buried dense & highly magnetized 1.1 Ga igneous rocks ~ 3000 km long, ~ 2 x 106 km3 magma MCR unusual – gravity high due to filling by igneous rocks below thick sediments, in contrast to usual rift low due to sediment fill

MCR volcanic rocks are much thicker than other LIPs MCR volcanic rocks deposited in subsiding basin [Stein et al., Geosphere, 2015]

Volcanics and postrift sediments show two-stage evolution Ojibwa Fault/ Lower volcanics truncate toward basin edge, indicating deposition during fault motion Upper volcanics and postrift sediments dip from both sides & thicken toward basin center, indicating deposition in subsiding basin [Manson and Halls, 1997] Profile after GLIMPCE line C [Green et al., 1989] [Stein et al., Geosphere, 2015]

Rift/LIP model for MCR evolution Rifting (extension) begins About 1120-1109 Ma NNW SSE [Stein et al., Geosphere, 2015]

Rifting and volcanism, crustal thinning Pre-Portage Lake volcanics Rift/LIP model for MCR evolution Rifting and volcanism, crustal thinning Pre-Portage Lake volcanics About 1109-1096 Ma [Stein et al., Geosphere, 2015]

Rift/LIP model for MCR evolution Faults inactive, volcanism and subsidence, Portage Lake volcanics, crustal thickening About 1096-1086 Ma [Stein et al., Geosphere, 2015]

Rift/LIP model for MCR evolution Faults inactive, volcanism ended Subsidence & sedimentation, crustal thickening About 1086-? Ma [Stein et al., Geosphere, 2015]

Reverse faulting and uplift Additional crustal thickening Rift/LIP model for MCR evolution Reverse faulting and uplift Additional crustal thickening Much later [Stein et al., Geosphere, 2015]

Net crustal thickening Rift/LIP model for MCR evolution Net crustal thickening Present [Stein et al., Geosphere, 2015]

Crustal thickening observed along west arm Surface wave tomography [Shen et al., 2013] Receiver functions [Moidaki et al., 2013]

Laurentia’s apparent polar wander path (APWP) has abrupt cusp at ~1 Laurentia’s apparent polar wander path (APWP) has abrupt cusp at ~1.12 Ga before major MCR igneous activity starts Cusps indicate change in direction (different pole of rotation) for plates [Stein et al., GRL, 2014] [Schettino and Scotese, 2005]

MCR likely formed as part of the rifting of Amazonia from Laurentia, recorded by APWP cusp & became inactive once seafloor spreading was established APWP for Laurentia poles [Stein et al., GRL, 2014]

East African Rift Africa rifting into 3 major plates & 3 microplates (Saria et al., 2013) If the EAR does not evolve to seafloor spreading & dies, in a billion years & additional continental collisions it would look like an isolated intracontinental failed rift - like the MCR. [Stein et al., GRL, 2014]

Other rifts give insight into how the MCR looked at different stages of its evolution Southern Oklahoma Aulocogen, a failed rift that opened in the Cambrian breakup of Rodinia and was inverted in the late Paleozoic, is similar to today's MCR, with a gravity high due to the igneous rocks filling the rift (Hanson et al., 2013), Presently-active East African rift, a good analogy to the MCR's early stages, shows crustal thinning beneath extending arms (Simiyu and Keller, 1997).

RIFT/LIP HYBRID Rifting requires tectonic stresses and faulting consistent with continental breakup. Volume and composition of the volcanic rocks are interpreted as requiring a mantle plume [Nicholson et al., 1997; White, 1997]. Large magma volume suggests that a rifting continent by chance overrode a plume or a shallow region of anomalously hot or fertile upper mantle. Initial modeling [Moucha et al., 2013] implies that the MCR's magma volume cannot have been generated by passive upwelling, even in Precambrian mantle hotter than today's, but required even hotter temperatures.

Paradox: the huge igneous fill shows up well in density – related data, poorly in velocity EarthScope video

Surface waves show the MCR's low-velocity sediments but not the underlying volcanic rocks Compared to the surrounding crust, the basalt rift fill is denser, but has similar or slightly lower S-wave velocity S- and P -wave structure at 100 km shows no clear anomaly beneath the MCR Melt extraction from the mantle left little velocity perturbation in the upper mantle [Al-Eqabi, Wiens, Wysession et al.]

Shear wave splitting also shows little effect below MCR [Ola et al.] Shows significant change going into the Superior province to the north, which may have been was so thick and strong that the MCR did not break into it.

See Stein et al. Friday morning poster T51E-2959 Grenville Orogeny (events from about 1.3-0.98 Ga culminating in assembly of Rodinia) did not cause MCR to fail (stop extending) Malone et al., 2015 MCR extension began after the Shawinigan compressive phase and ended before the Ottawan compressive phase. See Stein et al. Friday morning poster T51E-2959

We produced interpretive video and print materials to explain the MCR’s geology and effect on the Lake Superior region’s human history and development.

Conclusions   MCR combines the geometry of a rift and the huge igneous rock volume of a Large Igneous Province (LIP). Reflection seismic data show an initial rift phase where flood basalts filled a fault-controlled extending basin and a postrift phase where LIP volcanics and sediments were deposited in a thermally subsiding sag basin. MCR formation associated with a cusp in Apparent Polar Wander path, which are typically associated with changes in plate motion due to rifting, probably due to rifting of Amazonia from Laurentia. Rifting from plate motion and hotspot volcanism required to generate the magma volume seem related by coincidence rather than causally Melt extraction produced rift basalt with velocity similar to surrounding lower crust but had little effect on mantle seismic velocities and fabric. Grenville Orogeny did not cause MCR to fail (stop extending)