Tectonic Strain Effects on Remanence: Rotation of Remanence and Remagnetization in Orogenic Belts - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 84

important to realize when documenting bulk strain for
interpreting the fold test of the low-grade rocks. Par-
ticulate fl ow is what van der Pluijm (1987) considers
to be rigid body rotation of rock grains in his discussion
of the effect of internal deformation of rocks on the
paleomagnetic fold test. Van der Pluijm (1987) consid-
ers the other end member of grain-scale deformation
to be homogeneous strain in which the grains of the
rock change their shape. This type of rock strain can
be easily documented by Fry and R f - ϕ strain measure-
ment techniques.
Whether the non-magnetic grains of the rock and
the magnetic mineral grains that carry the remanence
of the rock deform by particulate fl ow or by homogene-
ous strain becomes the focus of attention when consid-
ering the effects of tectonic strain on remanence. If
paleomagnetic remanence rotates due to rock strain,
and this itself is a matter of contention, it is important
to understand how the magnetic grains behave. If they
behave as rigid particles and rotate due to simple shear,
the remanence can rotate through the shear plane fol-
lowing the equations of Jeffery (1923; Fig. 7.4). If the
magnetic particles passively follow the deformation of
the homogeneously deforming rock particles, the
remanence will rotate according to the equations of
March (1932) in what is called March or passive
marker strain. The magnetization will behave like a
line drawn on the deforming rock body, and will not be
able to rotate through the shear plane during simple

shear. The behavior of the magnetic mineral grains
during deformation, rigid particle versus passive
marker, will be very important to how the paleomag-
netic fold test is interpreted if internal deformation of
the rock has affected the remanence as well as the rigid
body rotation of the fold limbs. If the remanence can
rotate through the shear plane, which is the bedding
plane in fl exural fl ow/slip folding, then a syn-folding
confi guration of the magnetization can be created
from an initial pre-folding magnetization (van der
Pluijm 1987 ; Kodama 1988 ). The evidence (both
fi eld observations and laboratory experiments) for
passive marker versus rigid particle rotation was the
focus of remanence rotation due to tectonic strain
studies in the 1980s and 1990s (Kligfi eld et al. 1983 ;
Hirt et al. 1986 ; Kodama & Goldstein 1991 ; Cogne &
Perroud 1985; Stamatakos & Kodama 1991a, b; Bor-
radaile 1993); a satisfactory resolution has yet to be
obtained.
It is instructive to consider the sequence of deforma-
tion that sedimentary rocks typically experience in the
evolution of an orogenic belt, as the accumulated
strains could have an effect on the rock's remanence
and should be considered, in light of possible rema-
nence rotation due to rock strain, in interpreting a
rock's paleomagnetism. As an illustration, we will use
Nickelsen (1979) and Gray & Mitra (1993) fi ndings for
the Appalachian fold belt. Other orogenic belts can
have similar strain histories, but may differ in some

Fig. 7.4 Rigid particle versus passive marker rotation of paleomagnetic remanence.