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Grenville Orogeny

Shawinigan Ottawan Rigolet

A Midcontinent Rift

Extension Subsidence ?

Extension ends Volcanism
starts ends
Maximum JBE
Depositional Age

1200 1100 1000 900 800
Age (Ma)
B

Laurentia

1.2 Ga 1.15 Ga Figure 5. Gravity profiles across Midcontinent Rift (MCR), Fort Wayne Rift
Amazonia Amazonia (FWR), East Continent Gravity High (ECGH), and Grenville Front (GF) in
Canada at locations shown. The gravity highs along the FWR and ECGH
MCR seem to be the continuation of the east arm because they are similar in
Grenville Front dimensions and magnitude to those elsewhere along the MCR in showing
Schematic spreading center a distinct central high. No similar high occurs across the Grenville Front.

Figure 4. (A) Timeline for evolution of the Midcontinent Rift (MCR) and appears quite different from the SE-dipping layered struc-
major phases of the Grenville Orogeny (Malone et al., 2016). (B) Recon- tures at the front in Canada, and need not be Grenville age;
struction of plate positions before Laurentia-Amazonia separation, sche- and
matic spreading center geometry, and relevant features (Stein et al., 2014). 3. The Grenville-age events in the Llano uplift area of Texas
JBE—Jacobsville Sandstone, Bayfield Group, and other equivalent sand- and much of the eastern U.S. differ, involved different conti-
stones. nental fragments, and may have occurred at different times
from those in Canada.
magnetic and gravity anomalies. Subsequent studies inferred that
the GF extended along the East Continent Gravity High (ECGH) THE “FRONT,” THE EAST ARM OF THE MCR, AND
through Kentucky and Tennessee to southwest Alabama (Fig. 1). WELL DATA
As a result, the GF is often drawn accordingly, although its posi-
tion varies (e.g., Whitmeyer and Karlstrom, 2007; Baranoski et al., The often-assumed southward continuation of the “front” in the
2009; Bartholomew and Hatcher, 2010; Stein et al., 2014). The U.S. is based on gravity and—to a lesser extent—magnetic linea-
absence of the GF between Alabama and the Grenville-age Llano ments. Hence a key question is whether the gravity anomalies
uplift zone in Texas has been attributed to the front’s being rifted along the Fort Wayne Rift and ECGH are associated with the GF
away from Laurentia during the latest Precambrian/Cambrian or the east arm of the MCR. If they reflect the front, then its
rifting event (Thomas et al., 2012). assumed location near southeast Michigan implies that the east
arm of the MCR ends there (Cannon et al., 1989). However, the
REEVALUATING THE “GRENVILLE FRONT” IN gravity highs along the Fort Wayne Rift and ECGH seem to be the
THE U.S. continuation of the east arm (Lyons, 1970; Keller et al., 1982;
Dickas et al., 1992; Stein et al., 2014), because they are similar in
In this paper we argue that the inferred “Grenville Front” in the dimensions and magnitude to those elsewhere along the MCR
central U.S. is part of the MCR, rather than the western edge of (Figs. 1 and 5) in showing a distinct central high. No similar high
deformation from the Grenville orogeny. This interpretation is occurs across the GF in Canada.
based on several aspects:
1. Gravity anomalies in the “front” are similar to those along The GF exposed in Canada is severely eroded and represents a
deep level of the basal shear zone, not the deformation front
the remainder of the MCR and quite different from those observed in modern orogens. The actual deformation front of the
across the front in Canada; Grenville orogen must have been at least several tens of kilome-
2. Although seismic reflection data near the presumed “front” ters northwest of the front. As discussed, the gravity highs in the
show faults and possibly suture zones, this deformation U.S. reflect the MCR’s east arm, so it is unlikely that the GF lay
immediately to their east. Shallow-level thrusting of Grenville age
would have directly impinged on the recently formed MCR. This
seems unlikely given that the gravity data imply that the structure
of the MCR’s east arm is similar to that of the west arm, far from
any possible GF.

6 GSA Today | May 2018