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Fig. 7.10
The paleomagnetic declination test for oroclinal bending (from Weil & Sussman 2004): (a) a true orocline in which
the bend of the mountain belt is revealed by paleomagnetic declination and (b) a primary arc in which there is no vertical axis
rotation of declination around the fold belt. A progressive arc would have declinations around the arc intermediate between
the two cases.
(2004) summarize the current thinking that there are
three kinds of bends in mountain chains as follows.
1.
Primary arcs have been curved in map view from
the beginning. They formed in a curve and the curve
has not tightened during deformation.
2.
At the other end of the spectrum is the true orocline
that formed as a linear mountain belt and was subse-
quently bent into a curved shape during deformation.
3.
In between the two end members are progressive
arcs that bend as they are forming; they acquire their
curvature during deformation of the mountain belt.
The tests needed to distinguish between these differ-
ent types of mountain chain bends depend primarily
on measuring the changes in paleomagnetic declina-
tion around the curved mountain belt. Plots of declina-
tion with respect to the strike of the bedding around
the curve should show a linear relationship if the
mountain belt is a true orocline (Fig. 7.10). The defl ec-
tion of the paleomagnetic declination should follow
the defl ection of the bedding strike. For progressive
arcs, the declination will be less rotated than the strike.
Primary arcs would not show any declination rotation
around the arc of the belt. The Cantabrian arc in Spain
is a good example of a true orocline (Weil
et al.
2001 ).
Plots of the magnetic declination versus strike show
evidence that 100% of the strike variations are due to
true oroclinal bending.
The results for the Pennsylvania salient, one of the
most-studied arcs, are much more complicated. The
main fi nding about the Pennsylvania salient revealed
by paleomagnetism is that secondary magnetizations
in the folded rocks around the arc show absolutely no
difference in declination. Many of these secondary
magnetizations are syn-folding in age and were
acquired during the large fl uid - driven Kiaman age
remagnetization event observed throughout the Paleo-
zoic rocks of the Appalachians. This observation is
critical since it means that once the folds were forming
the bend in the Pennsylvania salient already existed,
the folds formed in a bent geometry (Stamatakos & Hirt
1994 ; Stamatakos
et al.
1996). However, the pre-
folding components of magnetization of the red beds
of the Silurian Bloomsburg and the Mississippian
Mauch Chunk formations do show a declination differ-
ence around the salient, with the directions in the
northern limb of the arc pointing north and the direc-
tions in the southern limb pointing northwesterly
(Kent & Opdyke 1985 ; Miller & Kent 1986 ; Kent 1988 ).
Initially, Stamatakos & Kodama (1991b) argued that
the divergence in declinations could be the result of
grain-scale strain rotating remanence by thrusting to
the north in the northern limb of the salient and
thrusting to the northwest in the southern limb of the
salient. This would predict that inclinations would
shallow but stay northerly in the northern limb of
the salient due to top - to - the - north bedding - parallel
shear. It would also predict that inclinations would
shallow and declinations would move northwesterly in
the southern limb of the salient due to top-to-the-
northwest bedding - parallel shear. Stamatakos &
Kodama (1991b) saw this pattern at two sites, one
from the northern limb at Montour Ridge, Pennsylva-
nia and one from the southern limb at Round Top,
Maryland. Of course, they also observed rigid particle
rotation of remanence within fi rst - order folds around
the salient that rotated a pre-folding remanence to
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