Tectonic Strain Effects on Remanence: Rotation of Remanence and Remagnetization in Orogenic Belts - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 87

Fig. 7.6 The effects of fl exural slip/fl ow folding strain when the magnetization rotates as a passive marker and the initial
inclination is very steep in bedding coordinates. In this special case the magnetization rotating into the long axis of the strain
ellipse is in the same sense as a rigid particle magnetization rotating in the bedding-parallel simple shear strain. Since the
magnetization would rotate opposite to the rigid body rotation of the fold limbs (same sense of limb rigid body rotation as in
Fig. 7.5 ) a syn - folding confi guration would result. This would not be true for an initially fl at magnetization in bedding
coordinates. (See Colour Plate 11)

grain-scale rotation mechanisms. For a paleomagnet-
ist, the most important internal strain to consider
turns out to be the commonly observed folding strain
of fl exural slip/fl ow. Many folds show slickensides on
the bedding planes of their limbs so, at the very least,
fl exural slip is occurring in these folds. What Kodama's
numerical modeling showed is that the grain-scale
deformation of the paleomagnetic particles, either
rigid particle rotation or passive line marker rotation,
was critical in determining whether a pre-folding
remanence was rotated into a syn-folding confi gura-
tion by fl exural slip/fl ow strain. Workers therefore
looked to evidence from deformed rocks and from labo-
ratory experiments to help understand how rema-
nence rotates during deformation.
As part of his examination of folding strain in rema-
nence, Kodama (1988) also looked at rock strain data
from two folds - one in carbonate rocks and one in
siliciclastic rocks - to determine if strain could have
caused syn-folding behavior and to see how a magneti-
zation deformed at the grain scale. In the Cambrian
dolomite of the Allentown Formation, the magnetiza-
tion in a small fold from a quarry in downtown Beth-
lehem, Pennsylvania, had already been shown to be

Fig. 7.7 Strain confi guration for tangential-longitudinal
strain during folding. Since no simple shear occurs,
magnetizations would rotate into the long axes of the strain
ellipses by passive marker rotation.

REMANENCE ROTATION IN THE FIELD

Theoretical work of Facer (1983), van der Pluijm
(1987) and Kodama (1988) on the effects of folding
strain on a rock's magnetization showed all the possi-
bilities for different strain geometries and different