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It is worth recognizing the uncertainty
in when and how the southern GAAI were
sutured to Laurentia. In one interpretation,
they were left behind during the 1.1 Ga
breakup between Laurentia and Amazonia.
Thus, they experienced the two last phases
of the Grenville orogeny (Ottawan and
Rigolet), assuming collisions in the eastern
U.S. were the same as in Canada.
Alternatively, they may have collided with
Laurentia during the last ~100 m.y. of the
Grenville orogeny, but were not on the
block that caused deformation in Canada.
Another possibility is that the southern
GAAI accreted during Rodinia’s breakup.
As observed elsewhere, continental frag-
ments can rift off before major breakup
(Veevers, 2004). Some GAAI show evi-
dence of rifting and volcanic events start-
ing ca. 760 Ma (McClellan and Gazel,
2014), so if these events are related to oth-
ers in Laurentia, the southern GAAI
accreted to Laurentia before this time. In
summary, during the Grenville orogeny
the southern GAAI need not have been
part of North America. Thus, they cannot
with confidence be used to support defor-
mation of the eastern U.S. during the
Grenville orogeny (ending ca. 980 Ma).
Grenville deformation south of
Amazonia may have involved the Rio de la
Plata craton (Fig. 3). Some reconstructions
place this block along the southern part of
Laurentia’s eastern margin during the
Grenville orogeny, but there is much
uncertainty about its position owing to the
limited paleomagnetic data (Li et al., 2008;
Teixeira et al., 2013; Rapalini et al., 2015). not prevented the rift’s east arm continuing
Gaucher et al. (2011) argue that detrital
zircon distributions of late Neoproterozoic
sandstones show that the Rio de la Plata
craton was in contact with Laurentia and
Amazonia ca. 1 Ga. However, much geo-
graphic uncertainty remains, and little is
known about the river system distributing
these sediments.
DISCUSSION
It seems implausible that the tradition-
ally mapped “Grenville Front” in the cen-
tral U.S. is the western edge of a Grenville-
age fold-and-thrust belt. Recent
reinterpretation of well data indicates that
this crust was attached to Laurentia before
the Grenville orogeny began. Precambrian
faulting occurs substantially west of the
“front” and may not be of Grenville age.
Although some Grenville-age deformation
may have occurred in the central U.S.,
8 GSA Today | May 2018
there is no evidence of a coherent front
anywhere, much the less where one has
been assumed to be. The situation may
have been analogous to the isolated pock-
ets of deformation identified in the
Midwestern U.S. far inland from the
Paleozoic deformation fronts (Marshak et
al., 2000; Craddock et al., 2017).
Discarding the “front” makes sense
given recent insights into the evolution of
the Midcontinent Rift. The “front’s”
assumed location near southeast Michigan
implies that the east arm of the MCR
ended there (Cannon et al., 1989) so rift
volcanism and extension did not continue
to the east and south. If so, the rift would
have been an isolated intraplate event,
rather than part of a plate boundary reorga-
nization as implied by paleomagnetic data
and similar to those observed today in East
Africa and in the geological record else-
where (Stein et al., 2014).
More precise dating of MCR rocks near
Lake Superior than available when the
“front” was proposed (McLaughlin, 1954;
Bass, 1960) shows that the rift-filling igne- work was supported by NSF grants EAR-1550108,
ous rocks formed ca. 1109–1085 Ma,
although rifting started perhaps ~10 m.y.
earlier. Seismic reflection data suggest that REFERENCES CITED
the extension ended ca. 1096 Ma, ~10 m.y.
before basaltic volcanism stopped (Stein et
al., 2015). Even if compression occurred
near the “front” during the Grenville orog-
eny, this deformation would be younger
than the MCR’s extension. Hence the
“front” would not have been there and thus
southward.
This timing is consistent with what is
known about the initiation of the front in
Canada and analogous fronts elsewhere. In
a continental collision, deformation starts
near the contact and then generally
becomes progressively younger toward the
interior final front site. Bethune (1997)
dates the oldest known GF deformation in
Canada at ca. 1035 Ma. Slightly to the east,
Rivers (2012) dates the oldest deformation
as Ottawan, whereas most metamorphic
ages near the front in Canada are from the
younger Rigolet phase (Rivers et al., 2012).
The “Grenville Front” issue illustrates
the complexity of orogenic belts in space
(~5000 km) and time (~300 m.y.). It seems
likely that deformational phases varied
along Laurentia’s south and east margins.
While matching long and fragmented oro-
genic zones allows reconstructions of the
past configurations of continents, orogenic
belts with the same age need not be from
the same event. A billion years from now,
parts of the Andes and Cascadia volcanic
arcs might look similar, and the Alpine and
Himalayan collisions might appear to have
been adjacent.
SUMMARY
New data and insights show that the
linear gravity anomalies used to infer the
position of the GF in the central U.S. are
part of the MCR, and should not be
mapped as a separate entity. There is lit-
tle evidence that this lineation is associ-
ated with the western edge of a Grenville
fold-and-thrust belt, and good reason to
expect that Grenville deformation in the
central U.S. would differ from that
observed in Canada. It is time to erase
the “Grenville Front” lineament in the
central U.S. from maps.
ACKNOWLEDGMENTS
We thank Scott King, John Weber, and an
anonymous reviewer for helpful comments. This
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