How to Detect and Correct a Compaction - shallowed Inclination - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 57

Fig. 5.9 Anisotropy of isothermal remanence (AIR) for the Mauch Chunk and Passaic Formations and chemically
demagnetized anisotropy of magnetic susceptibility (AMS) for the Kapusaliang Formation. These magnetic fabrics are observed
in typical red bed formations plotted in stratigraphic coordinates. Note that the magnetic fabrics are characteristic of
deposition and/or compaction with minimum axes (circles) perpendicular to bedding, indicating that the hematite carrying
the remanence has depositional/compactional fabrics. Squares: maximum axes; triangles: intermediate axes. All points lower
hemisphere.

trated acid. As the acid soaks into the samples, fi rst the
smallest then eventually the largest hematite particles
are dissolved. Rinsing and measuring the samples peri-
odically during the acid leaching monitors the removal
of the magnetization. The underlying assumption of
the chemical demagnetization of red beds is that the
smallest hematite grains are the pigmentary grains
and are secondary, while the largest hematite grains
are detrital and therefore primary. AMS is also meas-
ured at each chemical demagnetization step. Tan et al .
(2003) were interested in determining the AMS
removed at the same chemical demagnetization steps
that isolated the ancient magnetization direction. This
was done by fi tting a second-rank AMS tensor at each
chemical demagnetization step and determining, by
subtraction, the second-rank AMS tensor removed
between the same chemical demagnetization steps that
isolated the ancient magnetization direction in the
rock samples. In this way, the AMS of the hematite
grains that carries the ancient remanence can be used
to correct the inclination. This approach relies on the
ferromagnetic hematite grains dominating the AMS
and on the chemical demagnetization isolating the
same magnetic direction as thermal demagnetization,

the technique typically used to identify the ancient
magnetization in red beds.
These assumptions were fulfi lled beautifully with the
Kapusaliang Formation from China. Determining the
individual particle anisotropy for susceptibility, which
has a different value than the remanence particle ani-
sotropy for the same grains, was however more diffi -
cult. It could not be measured directly, but could only
be derived by fi nding the best fi t between the corrected
inclinations and the theoretical correction curves.
The value obtained was later compared to the directly
measured remanence particle anisotropy for the
Mauch Chunk red beds and found to be in good agree-
ment. An individual particle susceptibility anisotropy
of 1.06 led to a 32.5° decrease in inclination for an
f = 0.30, a very large amount of shallowing.
The Kapusaliang results were signifi cant because
they were the fi rst results to show that the anoma-
lously shallow paleomagnetic inclinations observed
in Cretaceous and Early Tertiary rocks from central
Asia were not due to standing non-dipole fi elds or
unrecognized continental-scale shear zones, as had
been previously proposed, but due to inclination
shallowing in red beds. The shallow inclinations in