Tectonic Strain Effects on Remanence: Rotation of Remanence and Remagnetization in Orogenic Belts - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 81

7

Tectonic Strain Effects on Remanence:
Rotation of Remanence and
Remagnetization in Orogenic Belts

Paleomagnetists naturally gravitate to folded rocks
for paleomagnetic measurements. The reason is simple:
paleomagnetists can constrain the age of a rock's mag-
netization by using Graham's (1949) fold test, a very
powerful fi eld test that shows whether a rock's paleo-
magnetic vector is older, younger or the same age as
the folding of a rock (Fig. 7.1). Even Graham (1949),
in the fi rst exposition of the fold test in the literature,
recognized that two types of deformation can affect a
rock's remanence: the rigid body rotation of a fold's
limbs and the internal strain that folded rocks undergo
as the rocks are bent into folds. Historically, paleomag-
netists have concentrated mainly on the rigid body
rotations of fold limbs, undoing them mathematically
to see if the magnetization on both limbs will cluster
better before, during or after the rigid body rotations
are removed by simple rotation around the strike of the
bedding. With the fi rst observation of ' syn - folding '
magnetizations (Scotese et al. 1982), i.e. when mag-
netizations cluster best during partial untilting of the
fold limbs, paleomagnetists began to realize that the
internal strain of rocks could rotate a pre-folding

remanence to appear ' syn - folding ' (Facer 1983 ; van
der Pluijm 1987 ; Kodama 1988 ).
To understand the effects of rock deformation on the
magnetization of a sedimentary rock, we need to
understand what type of sedimentary rocks paleomag-
netists typically measure and how those rocks will be
deformed in an orogenic belt. Paleomagnetists have
obtained the best sedimentary rock results from fi ne-
grained clastic rocks and from carbonates. The fi ne
grain size of the clastics ensures a quiet depositional
environment that allowed the magnetization of the
magnetic particles to align easily with the ambient geo-
magnetic fi eld during settling. Fine-grained rocks,
however, tend to soak up the tectonic strain in an oro-
genic belt more easily because they are usually less
competent. Fine-grained clastic rocks are therefore
more susceptible to any rotation of paleomagnetic
remanence or strain-induced remagnetization that
might occur. Carbonate rocks are important rock
types in orogenic belts and usually sampled by paleo-
magnetists. The previous chapter (Chapter 6) showed
however that both of these rock types were affected by