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non-magnetic matrix of calcite bonded with Portland
cement. Borradaile achieved strains of up to 45%
shortening.
For simple coaxial strain with up to 45% oblate
shortening and one component of magnetization, Bor-
radaile (1993) observed simple passive line marker
behavior for the rotation of remanence with the mag-
netization rotating away from the shortening axis by
amounts predicted from the strain of the rock ana-
logue and Wettstein's equation. It is important to
realize that Borradaile's experiments did not examine
the effects of non-coaxial strain, as might be expected
for fl exural slip/fl ow folding. Non-coaxial strain was
what Kodama observed to cause rigid particle behavior
of the remanence-carrying magnetic grains.
For multi-component magnetizations the results of
coaxial strain were, in some cases, counter-intuitive.
For these experiments two perpendicular components
of magnetization were applied to the samples, either
using an ARM or an IRM. One component was high
coercivity and the other component low coercivity. For
the magnetite-bearing samples, strain caused a hard-
ening (increase) of the overall coercivity of the samples
reducing, preferentially, the low-coercivity component
of magnetization by presumably affecting the large
multi-domain magnetite grains. The magnetization
was observed to rotate into the direction of the initially
harder component of magnetization with no regard to
the direction of the shortening axis. If the initial harder
component of magnetization was parallel to the short-
ening axis, the magnetization was observed to rotate
into the shortening axis; this was not a result expected
for passive line rotation or rigid particle rotation of the
remanence. For hematite-bearing samples with multi-
component magnetizations the results were exactly the
opposite. This time the magnetization rotated into the
softer of the two imposed components of magnetiza-
tion, again despite the direction of shortening. In this
case Borradaile interpreted the result to mean that the
fi ne - grained high - coercivity hematite particles were
dispersed by the strain. Finally, Borradaile (1994,
1996) deformed some samples carrying a single com-
ponent of magnetization by strains of only 15%. These
samples were observed to pick up a soft magnetization
parallel to the 30 μ T fi eld in the triaxial rig, providing
an example of strain-induced remagnetization i.e. a
piezo - remanent magnetization.
Borradaile's work shows that the effects of rock
strain on remanence can be very complicated partly
due to the grain-scale effects of strain on the individual
magnetic particles either changing their magnetic
domain state, rotating particles heterogeneously or
causing their remagnetization. As he points out in his
review article (Borradaile 1997), his results should be
extended to the natural world with caution because the
strain rates in the laboratory were 10
6
times greater
than those found in nature.
FURTHER FIELD AND LABORATORY
OBSERVATIONS
There has been little subsequent work on remanence
rotation due to tectonic strain, but the work done has
not supported rigid particle rotation. However, the
geometry of the strain related to the remanence has
not been particularly conducive to unraveling the
effects of passive line versus rigid particle behavior.
Kirker & McClelland (1997) looked at the defl ection of
remanence during progressive cleavage development
in kink bands in the Devonian Pembrokeshire red sand-
stones of southwest Wales. Remanence was defl ected
related to the amount of kinking. Some samples' mag-
netizations were rotated through local bedding, a sign
of rigid particle behavior, but most were not. Since the
magnetization is clearly syn-folding, the magnetiza-
tions were not exposed to as much strain as they would
have been if they were pre-folding in age. Cioppa &
Kodama (2003b) looked at strain and remanence in a
zone of intensifi ed axial planar spaced cleavage in the
red beds of the Mississippian Mauch Chunk Formation.
Strain intensity was measured with center-to-center
techniques. The magnetization was observed to rotate
into the axial planar cleavage developed in the rocks as
the amount of strain increased, but the pure-shear
coaxial strain did not allow the distinction between
passive marker and rigid particle behavior.
Till
et al
. (2010) conducted a high - pressure (300 MPa)
high - temperature (500 ° C) deformation experiment to
see the effects of greenschist grade metamorphism on
magnetic remanence and magnetic anisotropy. The
deformation involved simple shear along a plane per-
pendicular to the weak thermal remanence applied to
the samples; however, the shear was induced by coaxial
stress at 45° to the shear plane. Some of the samples
were created with a weak initial fabric, and other
samples without any initial fabric. The effects on mag-
netic anisotropy were the main focus of the experiment
and the authors found that the samples without initial
fabrics developed magnetic fabrics during deformation
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