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104 W 70 W ca. 1.3–0.98 Ga culminating in the assem-
53 N 53 N bly of the supercontinent of Rodinia (Li et
al., 2008) (Fig. 3). Studies in SE Canada,
Mapped Grenville Front where Grenville rocks are exposed, find
that the orogeny involved discrete contrac-
MCR tional phases, notably the Shawinigan from
ca. 1200–1140 Ma, Ottawan from ca.
MCR 1090–1030 Ma, and Rigolet from ca. 1010–
980 Ma (Rivers, 2012; McLelland et al.,
FWR 2013) (Fig. 4A). In SE Canada, erosion has
exposed deformed rocks from these oro-
ECGH genic events, from ~54°N to Lake Ontario.

27 N 27 N The orogeny’s phases presumably reflect
104 W 70 W a series of continental blocks and arcs col-
liding with and accreting to Laurentia at
0 50 100 various locations along its eastern margin.
kilometers However, the specifics of the plate interac-
tions remain unresolved because the lim-
Figure 2. Magnetic anomaly map of the region. Outlines of Midcontinent Rift (MCR), Fort Wayne Rift ited paleomagnetic data allow a range of
(FWR), and East Continent Gravity High (ECGH) are from gravity data (Fig. 1). Data source https:// scenarios. In one (Fig. 4B), Amazonia col-
pubs.usgs.gov/of/2002/ofr-02-414/ (Bankey et al., 2002). lided with Texas and then moved north-
ward by strike-slip motion relative to
Rodinia Australia Laurentia from ca. 1.18–1.12 Ga (Tohver et
reconstruction al., 2002, 2006). It then rifted from
Laurentia, leaving the MCR as a failed
at ca. 1.0 Ga third arm, with extension ending ca. 1.096
Ga (Stein et al., 2014, 2015). Amazonia is
Siberia thought to have recollided with Laurentia
somewhat later, causing the Ottawan phase
Greater South North in Canada (McLelland et al., 2013). The
India China China southern extent of this collision varies
between reconstructions (Li et al., 2008,
Laurentia 2013; Cawood and Pisarevsky, 2017;
Merdith et al., 2017).
Kalahari Rio MCR
de la Even greater uncertainties arise in infer-
Congo Plata ?? ring what occurred during the Grenville
orogeny in the U.S. Although the front
Sahara Amazonia Baltica does not outcrop in the U.S., it has been
assumed to extend southward into the U.S.
West ca. 1.0 Ga orogenic belts on geological and geophysical grounds.
Africa and continental-margin arcs McLaughlin (1954) proposed that it contin-
ued through Michigan and Indiana to the
? ca. 1.0 Ga collisions unknown New Madrid seismic zone because he con-
sidered features such as the Cincinnati
Figure 3. Rodinia reconstruction showing major blocks. After Li et al. (2008). Dome to be Grenvillian, which are now
MCR—Midcontinent Rift. considered to be much younger. Bass
(1960) suggested that the GF was located
to the east, in western Ohio, because data
from deep drill holes indicated high-grade
metamorphic rocks to the east and unmeta-
morphosed igneous and sedimentary rocks
to the west.

Geophysical data provide the other argu-
ment for southward extension of the
Grenville Front. In Canada the front is
associated with weak gravity and magnetic
anomalies (Figs. 1 and 2). Zietz et al.
(1966) noted that the proposed front in
Ohio coincided with the eastern edge of

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