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appear syn-folding and this informed their interpreta-
tion of diverging declinations around the curved
mountain belt. However, when Stamatakos & Hirt
(1994) plotted all the pre-folding magnetizations from
many different localities from the southern limb of the
salient, they did not observe the predicted pattern of
shallower inclinations being coincident with more
northwesterly declinations if grain-scale strain were
an important factor. One possibility is that grain-scale
strain did in fact affect some of the remanences, as
shown in the detailed studies of the folds, but is not
widespread enough to be used as an explanation for the
declination divergence between the limbs of the salient.
Gray & Stamatakos (1997) used these results to con-
struct a model for the formation of the Pennsylvania
salient that was compatible with both paleomagnetic
data and structural geology data. They argue that the
rocks deformed as two separate sheets: the lower
Cambro-Ordovician carbonates and the upper Siluro-
Devonian siliciclastics. Laterally varying amounts of
layer-parallel shortening in the lower sheet caused ver-
tical axis block rotations in the upper sheet recorded by
the pre-folding magnetizations in the siliciclastics.
Imbrication by thrusting in the lower sheet caused a
thickening toward the middle of the salient and diver-
gent shortening directions and folding in the upper
sheet due to gravitational spreading.
Cederquist
et al
. (2006) tested this model by measur-
ing the paleomagnetism of the Cambro-Ordovician
carbonates at 10 anticlines around the salient and
found that all were remagnetized during the Late Pale-
ozoic, as are many carbonates in the Appalachians.
The main fi nding of this study is that all directions had
essentially the same declination around the salient.
Gray & Stamatakos's (1997) lower sheet showed no
bending since the time when the folds were formed in
it. Ong
et al
. (2007) took the testing of the Gray &
Stamatakos (1997) model further not with paleomag-
netic data but with calcite twinning data that meas-
ures the earliest strain in the rocks, in this case the
layer-parallel shortening that occurs before buckling
into folds. Ong
et al
. (2007) applied the same test used
for paleomagnetic declinations around a curved moun-
tain belt to the shortening directions obtained from
calcite twinning and found evidence for an oroclinal
bending of shortening directions around the salient for
both the Cambro-Ordovician and the Siluro-Devonian
rocks, the two supposedly independently deforming
sheets of Gray & Stamatakos (1997). However, most of
Ong
et al
.'s data is from the southern limb of the salient,
particularly for the Siluro-Devonian data for which
there is only one site on the northern limb. If the
Cambro-Ordovician and Siluro-Devonian data are con-
sidered separately instead of combined, then the
Siluro-Devonian data has a consistently more south-
easterly declinations (by 15°) than the Cambro-
Ordovician rocks; this perhaps suggests a difference in
the deformation of the two sheets. However, the calcite
twinning data does suggest a simpler kinematic sce-
nario for the formation of the Pennsylvania salient
than that envisaged by Gray & Stamatakos (1997).
This case study shows that understanding the bends in
mountain chains requires more than paleomagnetic
data alone.
Grain-scale strain rotation of remanence has not
been considered as a signifi cant factor in subsequent
studies of oroclines. However, in their study of the
Wyoming salient Weil
et al
. (2010) did briefl y discuss
the possibility of remanence rotation due to layer-
parallel shortening observed in the rocks. High-strain
sites were avoided in the paleomagnetic analysis of the
salient, but strain-remanence relationships were not
studied in detail.
CONCLUDING THOUGHTS
After a good deal of interest in the effects of grain-scale
strain on the accuracy of paleomagnetic remanence in
the 1980s and 1990s, little work has since been done
on this important question. Work on the natural mag-
netizations of Paleozoic-age Appalachian folded rocks
suggests that remanence may rotate at the grain scale
as a rigid particle, but laboratory experiments and
other fi eld studies suggest that the importance of
passive line marker rotation versus rigid particle
behavior has not been fully resolved. Determining how
paleomagnetic remanence rotates at the grain scale (if
at all) during fl exural fl ow/slip strain, a common strain
mechanism during the folding of rocks typically
sampled for paleomagnetism, will be critical for inter-
preting the paleomagnetic fold test as well as the accu-
racy of paleomagnetic directions.
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