Transcript Rift/LIP model for MCR evolution

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 1.1 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
[Stein et al., Geosphere, 2015]
[Manson and Halls, 1997]
Profile after GLIMPCE line C [Green
et al., 1989]
Rift/LIP model for MCR evolution
Rifting (extension) begins
NNW
About 1120-1109 Ma
[Stein et al., Geosphere, 2015]
SSE
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]
Rift/LIP model for MCR evolution
Reverse faulting and uplift
Additional crustal thickening
Much later
[Stein et al., Geosphere, 2015]
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 ~1.12 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
Presently-active East African rift, a good
analogy to the MCR's early stages, shows
crustal thinning beneath extending arms
(Simiyu and Keller, 1997).
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),
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.
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)