How to Detect and Correct a Compaction - shallowed Inclination - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 65

the data. This effect is demonstrated in Hillhouse's
(2010) study of the Oligo-Miocene Sespe Formation
red beds from coastal California (Table 5.2). These Mid-
Tertiary rocks show a vertical axis rotation for the area
that is consistent with the area's tectonic history. The
paleomagnetic results also indicate an anomalously
shallow inclination that indicate 11° of post-Mid-Ter-
tiary poleward motion of the area, inconsistent with
the amount of movement predicted by motion along
the San Andreas fault by a factor of 3. Applied to these
rocks, the anisotropy technique increases the inclina-
tion from 39° to 47° and into statistical agreement
with the expected paleolatitude given by San Andreas
motion ( f = 0.73). In order to have enough sites for the
EI correction, additional sites from other studies were
added to Hillhouse's dataset increasing N to 158. Indi-
vidual sample directions had to be used instead of site
means, in order to reach a high enough number of
samples for the EI correction. Differential rotations
between sites were seen in the data and so, to minimize
this effect on the correction, each site mean was cor-
rected to a common mean declination of 0 before its
individual sample directions were used in the EI tech-
nique. Despite this modifi cation of the dataset, the EI
technique gave a greater corrected inclination to 51°.
The corrected inclination eliminates the necessity of
postulating any movement along the San Andreas
Fault, so could be interpreted as a slight overcorrection
(probably due to vertical axis rotation effects that were
not adequately removed).
The studies in Table 5.2 show that the EI correction
has been applied to both magnetite-bearing and
hematite-bearing sedimentary rocks and resulted in
corrected inclinations that give geologically or tectoni-
cally reasonable interpretations.

Fig. 5.12 The magnitude of inclination shallowing for
typical magnetite and hematite-bearing sedimentary rocks
as a function of initial geomagnetic fi eld inclination.
(See Colour Plate 10)

accurate use of the paleomagnetic inclination, prima-
rily to determine the paleolatitude of a rock unit,
must be checked for inclination shallowing and cor-
rected if it is detected. Either the anisotropy-based tech-
nique, as initially envisioned by Jackson et al . (1991)
and modifi ed for hematite-bearing rocks by Tan &
Kodama (2003), or the elongation-inclination tech-
nique (Tauxe & Kent 2004; Tauxe 2005) can be used
for the detection and correction of inclination fl atten-
ing, being mindful of the limitations and pitfalls of
each correction method. The tabulated fl attening
factors for magnetite and hematite-bearing rocks in
Chapter 4 don ' t change signifi cantly when the studies
in this chapter are added. Magnetite-bearing rocks
typically have fl attening factors near to 0.7 and
hematite-bearing rocks near to 0.6. Fig. 5.12 shows
that the maximum inclination error occurs at initial
inclinations of c. 45-50° and has magnitudes of c. 10 °
and 15 ° for magnetite - and hematite - bearing sedimen-
tary rocks, respectively.

CORRECTING FOR INCLINATION
FLATTENING

The experimental and observational results covered in
Chapters 4 and 5 show that inclination fl attening is a
reality in many sedimentary rocks, both marine sedi-
ments and terrestrial sediments. It has been observed
and corrected in both magnetite-bearing rocks and in
hematite-bearing rocks. It is therefore critical that