Tectonic Strain Effects on Remanence: Rotation of Remanence and Remagnetization in Orogenic Belts - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 89

tion was consistent with rigid particle rotation of the
hematite magnetic particles.
The R f - ϕ and the normalized Fry center-to-center
structural geology techniques were used to quantify
strain in carbonates and siliciclastics because the mag-
netic measure of strain by anisotropy of magnetic sus-
ceptibility (AMS) has been found to be inadequate.
AMS is a complicated signal arising from both para-
magnetic grains and ferromagnetic grains that react to
strain by different degrees, so a straightforward corre-
lation between AMS and strain is not possible (Bor-
radaile & Jackson 2004 ). The R f - ϕ technique measures
strain from the ellipticity and orientation of initially
elliptical grains that are reoriented and further
deformed by strain (e.g. Ramsay & Huber 1983). The
normalized Fry center-to-center technique uses meas-
urement of the distances between the centers of neigh-
boring quartz grains in a siliciclastic rock. The
deformation and reorganization of the quartz grains is
a measure of bulk strain in the rock.

Fig. 7.9 Detailed fi nite strain measurements at a fold in
the Bloomsburg Formation at Delaware Water Gap shows
strain ellipse orientations consistent with fl exural fl ow
folding strain at the southern limb of the fold (Stamatakos &
Kodama 1991b). Magnetization vectors measured in detail
throughout the fold limb show orientations consistent with
rigid particle rotation because the magnetization exposed to
the greatest strain (strain ellipse 3.23) has rotated through
the bedding plane (shear plane). J Stamatakos and KP
Kodama, The effects of grain-scale deformation on the
Bloomsburg Formation pole, Journal of Geophysical Research ,
96, B11, 17919-17933. Copyright 1991 American
Geophysical Union. Reproduced by permission of American
Geophysical Union.

REMANENCE ROTATION IN THE
LABORATORY

Realizing the importance of determining whether a
paleomagnetic remanence deformed as rigid particles
or as a passive line during the bedding-parallel simple
shear strain during fl exural fl ow/slip folding, Kodama
& Goldstein (1991) conducted simple shear box experi-
ments with mixtures of silicone putty and magnetite
or kaolinite clay and magnetite. The mixtures were
magnetized by applying an ARM at high, moderate and
low angles to the shear plane and then deformed.
Twelve out of twenty runs unequivocally contradict
passive line behavior for the ARM. Modeling of the
results is in strong agreement with rigid particle rota-
tion of the magnetite grains carrying the remanence.
Graham Borradaile has also done extensive work
to determine how paleomagnetic remanence rotates
during rock deformation. Borradaile's laboratory
strain work (summarized in Borradaile 1997) provided
an entirely different perspective and results from those
found by Kodama and colleagues. Borradaile used
a computer-controlled triaxial pressure rig for his
experiments that imposed high confi ning pressures
(≥ 200 MPa) and low differential stresses (≤ 100 MPa)
at low strain rates (10 − 5 - 10 − 6 sec − 1 ). For his rock ana-
logues, Borradaile used a mixture of either magnetite
or hematite as the magnetic mineralogy in a

tion for three folds in Pennsylvania and found that
the inclination of the remanence was directly corre-
lated to the strain, measured by the Fry (1979) and
R f - ϕ center-to-center techniques, and rotated in the
sense predicted by rigid particle rotation due to the
bedding-parallel simple shear caused by fl exural fl ow
(Fig. 7.9). For the Mauch Chunk Formation, Stamata-
kos & Kodama (1991a) studied one fi rst - order fold in
Lavelle, Pennsylvania again measuring the strain with
the center-to-center techniques to compare it to the
remanence directions in the fold. The strain in the fold
was consistent with fl exural fl ow strain and concen-
trated in the fi ner-grained lithologies, and not in the
more competent coarser sandstone layers as expected.
For the mudstones there was a direct correlation
between the paleomagnetic inclination and the magni-
tude of strain, and the sense of rotation for the inclina-