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Crustal thickness along the ECGH and
FWR is similar to that beneath the MCR.
Teleseismic P-wave studies in Tennessee
(Owens et al., 1984) show thick crust simi-
lar to that beneath the MCR’s west arm
(Moidaki et al., 2013; Zhang et al., 2016)
and Lake Superior (Green et al., 1989). In
southern Ohio, seismic reflection and drill-
ing data support a half-graben structure
similar to other parts of the MCR (Dickas
et al., 1992). Gravity studies also suggest a thrusting (Bethune, 1997; Rivers et al.,
thick crust (Keller et al., 1982; Mayhew et
al., 1982; Buening, 2013), similar to that on data at the western end of OH-1 as show-
the MCR’s west arm (Merino et al., 2013;
Levandowski et al., 2015).
Moreover, new interpretation of the data
from wells in Ohio used initially to define
the “front” (Bass, 1960) indicates that the
geophysical lineament defining the “front”
is not a Grenville-age tectonic front but
rather part of the MCR, with pre-Grenville
Laurentia to the east (Petersson et al.,
2015). In Ohio and Kentucky, many of the
wells bottom in mafic rocks (Drahovzal et
al., 1992; Buening, 2013) similar to MCR
rocks exposed near Lake Superior and in
the buried west arm (Walker and Misra,
1992; Lidiak, 1996).
DID DEFORMATION NEAR THE
“FRONT” OCCUR DURING THE
GRENVILLE OROGENY?
In SE Canada, seismic reflection pro-
files show parallel southeast-dipping
reflectors extending at least 100 km south-
eastward from the surface trace of the
front (Rivers et al., 2012) to at least 20 km younger than the Grenville orogeny. If the
depth. However, reflection data across the
presumed “front” in the central U.S. look
quite different.
Much of the Precambrian tectonic his-
tory of Ohio and the “front” is based on
the COCORP OH-1 and 2 lines (Fig. 1).
Several subsurface features had been inter-
preted as part of the deformation. In this
interpretation, the area to its west was part 2015).
of the ca. 1.5–1.4 Ga Granite-Rhyolite
Province of Laurentia, whereas rocks to
the east were similar to provinces of
Canada’s exposed Grenville orogen
(Culotta et al., 1990). However, recent
gravity modeling (Buening, 2013) and
analysis of rocks from wells (Petersson et
al., 2015) suggest that the Granite-Rhyolite
Province continues ~100–150 km eastward
beyond the “front,” indicating that this
area is not a terrane added during the
Grenville Orogeny.
The seismic lines crossing the “front”
lack the layered structure seen in Canada,
implying a different history. Although they
have been interpreted as suggesting
Precambrian compression similar to that in
Canada, it is unclear whether the compres-
sion is of Grenville age. In Canada,
Grenville-age metamorphism and shearing
of the front are superimposed on rocks
recording older events of west-directed
2012). Baranoski et al. (2009) interpreted
ing rift development followed by thrust
faulting at least ~100 km west of the front.
The inferred age of compressional fault-
ing depends on the age of the faulted
Precambrian sedimentary rocks. By
default, it has been assumed that the com-
pression must be Grenville in age, so the
faulted sediment must be older than 980
Ma (e.g., Drahovzal, 1997). The
Proterozoic sandstone Middle Run
Formation, observed only in wells, is
thought to have similar age to the
Jacobsville Sandstone and Bayfield
Formation around Lake Superior. The
Jacobsville and Middle Run have some
similar distributions of Grenville-age zir-
cons. However, detrital zircon dating
shows that the Jacobsville must be younger
than 959 ± 19 Ma and is probably several
hundred million years younger (Malone et
al., 2016). Schneider Santos et al.’s (2002) tions were involved (Dalziel et al., 2000;
zircon analysis for the Middle Run finds a
maximum age of 1048 ± 22 Ma, but they
also argue that it must be significantly
Jacobsville and Middle Run are about the
same age, much of the Middle Run faulting
must be younger than Grenville age. These
ages show that the “front” in Ohio is not
the western edge of the Grenville fold-and-
thrust belt and that the deformation near it the Laurentia margin. Petrologic analyses
may be younger, probably reflecting the
same post-Jacobsville event that inverted
the MCR near Lake Superior (Stein et al.,
GRENVILLE-AGE APPALACHIAN
INLIERS AND LLANO UPLIFT
COMPARED TO CANADA
The argument for a “Grenville Front” in
the U.S. assumes that the entire U.S. East
Coast was affected by collisions in the
Grenville orogeny. Grenville-age events
are recorded in Canada and the northeast-
ern U.S. and in exposures in the Llano
www.geosociety.org/gsatoday
uplift zone in Texas. It has thus been
assumed that similar events occurred
between these two areas in the eastern U.S.
However, the deformational events in
Texas and Canada/NE U.S. were not
caused by the same plate collisions and
were not necessarily synchronous.
Moreover, in the central U.S., although
some Grenville-age deformation may have
occurred, there is no clear evidence of col-
lisions or of a coherent deformation front,
much the less where one has been assumed
to be.
Although no Grenville-age fold-and-
thrust belt is exposed in the central U.S.,
Grenville-age features proposed to reflect
localized deformation are observed (Ruiz
et al., 1984; Bornhorst et al., 1988;
Petersson et al., 2015). Most crucially, the
Appalachian Mountains in the U.S. contain
blocks called Grenville-age Appalachian
inliers (GAAI) (Fig. 1). These fragments
were assumed to be part of Laurentia dur-
ing the Grenville orogeny that were later
uplifted and exposed at the surface during
Paleozoic orogenies (McLelland et al.,
2013). These rocks’ ages are usually
assigned to phases in the Grenville orog-
eny (Fig. 4A).
The Llano rocks record compressional
events overlapping in time with ones in
Canada. However, different plate interac-
Davis and Mosher, 2015). The fact that
Grenville-age deformational events were not
continuous along Laurentia’s eastern and
south margin raises the questions of when
the GAAI accreted to Laurentia and whether
they record the same events as in Canada.
It seems likely that different tectonic
events occurred at different times along
suggest that GAAI south of about the New
Jersey/Pennsylvania border have
Amazonian affinity (Fisher et al., 2010;
McLelland et al., 2013), implying that they
were not part of Laurentia before the
Grenville orogeny. In reconstructions of
Rodinia (Fig. 3), Amazonia’s southern
extent along Laurentia is often near a tran-
sition in petrology from northern GAAI
with Laurentian affinities to southern ones
with Amazonian affinities. Hence, given
the Grenville’s complex history and
Amazonia’s motion, the history of colli-
sional events in Canada probably does not
describe the history of the southern GAAI,
especially before the Ottawan phase.
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