Tectonic Strain Effects on Remanence: Rotation of Remanence and Remagnetization in Orogenic Belts - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 91

that could detect increasing levels of strain, while
samples with initial fabrics could not. Pertinent to our
discussion here, the post-deformation remanence of
the initially fabric-less samples was interpreted by the
authors to be a remagnetization based on a change in
magnetic properties, i.e. a piezo-remanent magnetiza-
tion. The remagnetization was parallel in direction to
the pre-deformation remanence for these samples. The
samples with initial fabrics had a post-deformation
remanence that was either parallel to the pre-
deformation remanence or scattered. Unfortunately,
these results do not inform our understanding of how
remanence deforms during non-coaxial simple shear
strain, but they do show that the initial strain state of
a sample will affect how well magnetic anisotropy can
measure subsequent deformation. These results point
to the complexity of using magnetic fabrics to measure
the degree of rock deformation. Borradaile & Jackson
(2004) detail these complexities and conclude that
AMS is not a reliable way to quantify rock strain;
however, it can determine the orientation of the strain
ellipse.

coincident formation of a spaced slaty cleavage in the
rocks.
Strain-induced remagnetization was the conclusion
of two detailed strain-remanence studies of fi rst - order
folds in the Appalachian mountains of West Virginia.
Lewchuk et al . (2003) studied the Patterson Creek anti-
cline and Elmore et al . (2006) studied the Wills Moun-
tain anticline. Both studies investigated the Silurian
Tonoloway Formation and Ordovician Helderberg For-
mation carbonate rocks that were folded in both anti-
clines and carried a Late Paleozoic remagnetization.
The pattern was basically the same in both studies; the
Tonoloway Formation rocks had either a pre-folding
(Patterson Creek anticline) or early syn-folding (Wills
Mountain anticline) magnetization, while the Helder-
berg Formation had a true syn-folding magnetization
in the folds. Magnetite is the magnetic mineral respon-
sible for the magnetizations. Detailed measurements of
strain in the rocks showed that the syn-folding mag-
netizations were in rocks with more pressure solution
strain, either compactional or tectonic in geometry.
The authors of the studies argue for a modifi cation of
the rocks' magnetization caused by strain or a strain-
induced remagnetization modifi ed by a piezo-remanent
magnetization. Mechanical rotation of pre-folding
magnetic grains by strain is ruled out in the Wills
Mountain anticline study because the difference in the
amount of strain between the pre-folded and syn-
folded rocks is not enough to explain the syn-folding
geometry of magnetization.

STRAIN REMAGNETIZATION

The Till et al . (2010) study does show that tectonic
strain can likely lead to remagnetization of a rock.
Hudson et al . (1989) argue that a piezo-remanent
magnetization has remagnetized the folded Preuss For-
mation sandstone in a Wyoming thrust sheet. Meas-
urements of strain in the folded rocks suggest that the
amount and geometry of the strain could not have
caused over-steepening of the fold limbs, grain bound-
ary sliding or fl exural fl ow strain. Although the mag-
netization of the Preuss is syn-folding in geometry,
detrital magnetite grains carry it. There is no evidence
of secondary magnetic grains that could carry a CRM.
Furthermore, the thermal history of the rocks does not
point to a viscous partial thermal remanence as a
cause of remagnetization. The authors are left with
a stress-induced cause of remagnetization: a piezo-
remanence. Housen et al . (1993) appeal to an entirely
different mechanism for a strain-caused remagnetiza-
tion of the Ordovician Martinsburg Formation shales
and slates in eastern Pennsylvania. In a detailed sam-
pling of a shale-slate transition in the Martinsburg
Formation, Housen et al . observe dissolution and neo-
crystallization of magnetite leading to a CRM remag-
netization mechanism caused by increasing strain and

OROCLINES AND PALEOMAGNETISM

Paleomagnetism is one of the best ways of studying
how oroclines form. Oroclines are the bends observed
in many mountain chains globally. Weil & Sussman
(2004) show 11 major oroclines throughout the world.
The Pennsylvania salient in the Appalachians is a
mountain chain bend that has been studied the most,
both by paleomagnetic and structural geology methods.
Pertinent to our discussion of strain and paleomagnet-
ism, grain-scale strain was considered at one time to be
the possible cause of divergent declinations around the
Pennsylvania salient.
As Weil & Sussman (2004) point out there is a long
history to the idea of an orocline, the bending of a
once-straight fold belt. The name orocline was fi rst pro-
posed by Carey (1955), and ideas about the formation
of oroclines have evolved since then. Weil & Sussman