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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

                                          www.geosociety.org/gsatoday                                                                             5
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