Tectonic Strain Effects on Remanence: Rotation of Remanence and Remagnetization in Orogenic Belts - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 88

Observed Magnetizations in Bloomsburg Formation
sandstone

sandstone

mudstone

sense of rotation
predicted by rigid particle

sense of rotation
predicted by passlive marker

observed magnetization vectors in
either sandstone or mudstone

Fig. 7.8 Schematic representation of remanence rotation experiment in the Bloomsburg Formation red beds (Kodama 1988).
The mudstone magnetization was rotated with respect to the magnetizations in the surrounding sandstone beds. The relative
sense of rotation suggests rigid particle behavior since it is opposite to that predicted by passive line marker behavior of rotation
into the long axis of the strain ellipse that would result from fl exural slip/fl ow strain. (See Colour Plate 12)

syn - folding in confi guration (Stead & Kodama 1984).
Kodama (1988) measured the strain in the fold limbs
using the R f - ϕ technique (Ramsay & Huber 1983). The
most important results of this study are as follows.
1. The amount of strain was not the same in both
limbs; it was greater and oriented in the sense expected
for fl exural fl ow folding in one limb, but smaller and
not in the fl exural fl ow sense for the other limb.
2. The maximum magnitude of strain measured was
only about one-quarter of that predicted by Ramsay's
theoretical prediction that simple shear would equal
limb dip (in radians).
Based on modeling, the amount of strain in the
Allentown Formation fold was not enough to rotate the
remanence of these carbonate rocks into a syn-folding
confi guration. To understand the effects of folding
strain in siliciclastic rocks, the Silurian Bloomsburg
Formation red beds were studied in a fold near Palmer-
ton, Pennsylvania. Only one limb (the steeper dipping
southern limb) of the syncline was exposed so the
study consisted of comparing the remanence in two
different lithologies in the limb - two sandstone beds
that stratigraphically bracketed a mudstone bed - with
the assumption that the less-competent mudstone bed
would soak up more strain than the sandstone beds.
No strain was measured in the beds; only their rema-
nences were compared. The mudstone mean paleo-

magnetic direction was observed to be 10° more
easterly than the sandstone mean paleomagnetic
direction. If the mudstone experienced more fl exural
fl ow bedding-parallel shear strain than the bracketing
sandstone beds, its magnetization should have been
more westerly (Fig. 7.8) than the magnetization of the
sandstone beds for passive line marker strain and more
easterly for rigid particle rotation. The results therefore
support rigid particle rotation for a paleomagnetic
remanence carried by the hematite particles of the
Bloomsburg Formation red beds. Rigid particle mode-
ling of the 10° offset between the mudstone and
sandstone magnetizations further indicates that the
mudstone could have experienced only about one-third
of the predicted maximum shear strain based on the
bedding dip. For both carbonates and siliciclastic rocks,
the amount of strain experienced is quite a bit less than
the maximum predicted by Ramsay (1967) based on
the limb dip; rigid particle motion is therefore sug-
gested as the grain-scale mechanism for remanence
rotation.
Rigid particle rotation of remanence was also
observed in detailed studies of strain and remanence
for the Bloomsburg Formation and the Mississippian
Mauch Chunk Formation red beds (Stamatakos &
Kodama 1991a, b). Stamatakos and Kodama studied
the strain and remanence in the Bloomsburg Forma-