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Is the “Grenville Front” in the central United
States really the Midcontinent Rift?
Carol A. Stein, Dept. of Earth & Environmental Sciences, University of Illinois at Chicago, 845 W. Taylor Street, Chicago, Illinois
60607-7059, USA, cstein@uic.edu; Seth Stein, Reece Elling, Dept. of Earth & Planetary Sciences, Northwestern University, 2145
Sheridan Road, Evanston, Illinois 60208-3130, USA; G. Randy Keller, School of Geology and Geophysics, University of Oklahoma,
100 E. Boyd, Norman, Oklahoma 73019, USA; and Jonas Kley, Geowissenschaftliches Zentrum, Georg-August-Universität Göttingen,
Goldschmidtstraße 3, 37077 Göttingen, Germany
ABSTRACT and 2) record different aspects of the rifting event in a plate interior, it now
Wilson cycle. One, the Midcontinent Rift appears more likely that it formed as part
Two prominent Precambrian geologic (MCR), is a U-shaped band of buried igne- of the rifting of the Amazonia craton (now
features of central North America are the ous and sedimentary rocks that outcrops in northeastern South America) from
Midcontinent Rift (MCR) and Grenville near Lake Superior. To the south, it is bur- Laurentia, the Precambrian core of North
Front (GF). The MCR, an extensive band ied by younger sediments, but easily traced America (Stein et al., 2014, 2016). Hence
of buried igneous and sedimentary rocks because the igneous rocks are dense and the east and west arms were analogous to
outcropping near Lake Superior, records a highly magnetized (Hinze et al., 1992; the east and west branches of the East
major rifting event at ca. 1.1 Ga that failed Merino et al., 2013). The western arm African rift, the broad zone forming one
to split North America. In SE Canada, the extends at least to Oklahoma, and perhaps arm of the Nubia (west Africa)–Somalia
GF is the continent-ward extent of defor- Texas and New Mexico, as evidenced by (east Africa)–Arabia three-plate system.
mation of the fold-and-thrust belt from the similar-age diffuse volcanism (Adams and
Grenville orogeny, the sequence of events Keller, 1994, 1996; Bright et al., 2014). The A second major feature, east of the
from ca. 1.3–0.98 Ga culminating in the eastern arm extends southward through MCR, is the Grenville Front (GF), also
assembly of the supercontinent of Rodinia. lower Michigan to Alabama (Lyons, 1970; known as the Grenville Front Tectonic
In the central U.S., lineated gravity anoma- Keller et al., 1982; Dickas et al., 1992; Zone. The front is the continent-ward
lies extending southward along the trend of Stein et al., 2014). Although the MCR was boundary of deformation of the fold-and-
the front in Canada have been interpreted often viewed as two arms of a three-arm thrust belt from the Grenville orogeny,
as a buried Grenville Front. However, we the sequence of orogenic events from
use recent tectonic concepts and data anal-
yses to argue that these anomalies delin- 104°W 705°3W°N
eate the eastern arm of the MCR extending 53°N
from Michigan to Alabama, for multiple
reasons: (1) These anomalies are similar to Front (mapped) mGal
those along the remainder of the MCR and Grenville 56
quite different from those across the front 0
in Canada; (2) the Precambrian deforma- MCR Adirondacks -40
tion observed on seismic reflection profiles
across the presumed “front” appears quite -50
different from that across the front in
Canada, cannot confidently be assigned -60
to the Grenville orogeny, and is recorded
at least 100 km west of the “front”; and -110
(3) during the Grenville orogeny, deforma-
tional events from Texas to Canada were FWR Grenville Front
not caused by the same plate interactions Appalachian Inliers
and were not necessarily synchronous. OH 1/2
Hence the commonly inferred position of
the “Grenville Front” in the central U.S. is OH
part of the MCR, and should not be mapped
as a separate entity. ECGH Age
INTRODUCTION Grenville
Two prominent Precambrian geologic Llano Uplift
features of central North America (Figs. 1
400 KILOMETERS
27°N 27°N
104°W 70°W
Figure 1. Gravity map showing Midcontinent Rift (MCR), Fort Wayne Rift (FWR), and East
Continent Gravity High (ECGH). Grenville-age Appalachian inliers with Laurentia and
Amazonia affinities are shown as light and dark gray regions. Grenville Front shown by
solid line where observed and dashed line where inferred. OH 1/2 indicates location of
COCORP seismic profile (Stein et al., 2014).
GSA Today, v. 28, doi: 10.1130/GSATG357A.1. Copyright 2017, The Geological Society of America. CC-BY-NC.
4 GSA Today | May 2018
