Tectonic Strain Effects on Remanence: Rotation of Remanence and Remagnetization in Orogenic Belts - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 83

Fig. 7.2 Vector displacement fi elds for simple shear and pure shear strain. Modifi ed from Ramsay & Huber (1983). Strain
ellipses showing rotation of strain ellipse for non-coaxial strain and no rotation of strain ellipse for coaxial strain.

tant for paleomagnetists to consider as they interpret
their fold test results. Spaced cleavage is caused by two
processes: the preferred alignment of platy minerals
(usually phyllosilicates) either by physical rotation or
growth in a differential stress regime, and the dissolu-
tion and removal of material by pressure solution from
the cleavage domains. In our discussion, cleavage is
formed by coaxial strain (pure shear) with the axis of
maximum shortening perpendicular to the cleavage
plane. The removal of material from the cleavage
domains can cause an apparent oversteepening of the
fold limb dips. The volume loss and resulting oversteep-
ening can be important to the paleomagnetic fold test.
Finally, for the very low and low metamorphic
grade rocks in mountain belts typically measured by
paleomagnetists, the grain-scale behavior of sedimen-
tary rocks during these different kinds of strain is
important. Borradaile (1981) has suggested that par-
ticulate fl ow (grain boundary sliding) in which the
individual grains in a sedimentary rock do not deform
but fl ow past each other as the rock deforms can be an
important grain-scale deformation mechanism. Of
course, Borradaile points out, this is simply one
end member of a continuum of behavior. Particulate
fl ow can be coupled with different amounts of indi-
vidual grain deformation. For particulate fl ow without
any grain shape changes however, the standard struc-
tural geology techniques of measuring bulk rock
strain, i.e. Fry center-to-center measurements or R f - ϕ
measurements, will not record any rock strain. This is

Fig. 7.3 Simple shear strain parallel to bedding results
from fl exural slip/fl ow folding. Strain is distributed
continuously through the bedding for fl exural fl ow folding.
Strain is concentrated at the bedding planes for fl exural slip
folding. Arrows show the sense of shear parallel to the
bedding of the fold limbs. The orientations of the strain
ellipses on each fold limb are the result of the bedding-
parallel shear non - coaxial strain.

fl ow/slip strain. Similar folds, in which the apparent
thickness of a bed parallels the trace of the fold's axial
plane and remains constant, can however have simple
shear parallel to the fold's axial plane and affect the
magnetization of a rock similarly to fl exural fl ow/slip
(Facer 1983 ).
The development of axial planar spaced cleavage at
a high angle to bedding in a fold should also be impor-