How to Detect and Correct a Compaction - shallowed Inclination - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 61

ing that the measurement of a complete tensor might
give a different result than simply IRMs applied parallel
and perpendicular to bedding.

The EI technique uses the distribution of the sedi-
mentary rock site means to detect and correct inclina-
tion shallowing. The technique assumes that each
site is a time-independent spot reading of the secular
variation of the geomagnetic fi eld. The fi eld model
used by the EI technique (TK03) has a circular distribu-
tion of virtual geomagnetic poles (VGPs). A VGP is
simply the paleomagnetic north pole that correlates to
a paleomagnetic direction somewhere on the globe,
assuming that the geomagnetic fi eld is caused by a
dipole at the center of the Earth. Because of the
non-linear transformation between directions and
VGPs, a circular distribution of VGPs transforms to an
elongate distribution of paleomagnetic directions. This
distribution is elongate along the magnetic meridian
(declination) and becomes more elongate as the obser-
vation site approaches the equator, where its elonga-
tion has the greatest value ( c. 3). At the north (or
south) geomagnetic pole the directions have a circular
distribution, just like the VGPs. The elongation is meas-
ured by the ratio of the maximum and intermediate
eigenvalues of a tensor fi t to the distribution. To visual-
ize this, the directional distribution can be fi t to a 3D
ellipsoid. The shortest axis of the ellipsoid is vertical
and the maximum and intermediate axes are horizon-
tal. The eigenvalues are proportional to the axes of the
ellipsoid.
The EI correction uses this geomagnetic fi eld behav-
ior to correct for inclination shallowing. If the inclina-
tion of a sedimentary rock is fl attened by compaction
or a syn-depositional process, the directional distribu-
tion fi rst becomes circular and then elongate in an
east-west direction perpendicular to the magnetic
meridian, as the amount of fl attening increases. The EI
technique assumes that the fl attening is caused by the
King (1955) tan I c = f tan I 0 relationship where f is the
same fl attening factor introduced in Chapter 4 and
used throughout the topic to describe inclination shal-
lowing. In the EI technique, no rock magnetic data are
used to directly measure the fl attening factor f . Differ-
ent f factors are used to numerically unfl atten the site
mean distribution. The elongation of the unfl attened
distribution and its mean inclination are plotted for
different fl attening factors usually starting with f = 1
(no fl attening) and then decreasing toward f = 0 (com-
plete fl attening). Each fl attening factor creates a differ-
ent elongation-inclination pair that plots as a point on
the elongation-inclination graph. These points are
connected to form a curve. When the curve crosses the
elongation-inclination curve of the geomagnetic TK03

ELONGATION-INCLINATION
CORRECTION TECHNIQUE

Subsequent to the introduction and development of
the anisotropy - based inclination - shallowing correc-
tion, Tauxe & Kent (2004) were inspired to approach
shallow inclinations in sedimentary rocks from a
totally different perspective. They reasoned that incli-
nation shallowing would affect the distribution of site
means in sedimentary rocks signifi cantly fl attened by
either depositional or compaction processes. Tan et al .
(2003) observed a fl attened site mean distribution in
their uncorrected site means for the red beds of the
Kapusaliang Formation in Central Asia and pointed
out that the fl attened distribution was probably an
indication of inclination shallowing. Tauxe and Kent's
elongation-inclination method of correcting for incli-
nation shallowing is a technique for detecting and cor-
recting shallow inclinations totally independently of
anisotropy measurements and has gained much popu-
larity in the paleomagnetic community. Table 5.2
shows 14 studies in the literature where the EI method
has been used to check for and correct any inclination
shallowing detected in sedimentary rocks.
The EI method is both a clever and relatively easy
technique that has given essentially the same magni-
tude inclination correction as the anisotropy-based
correction in some important test cases (Tauxe et al .
2008 ).
The EI technique assumes that the Earth's secular
variation behavior has remained essentially the same
through geological time as it has been for the past 5
million years. This assumption has been tested (Tauxe
et al . 2008 ; Tauxe & Kodama 2009 ) using the geomag-
netic secular variation recorded by large igneous prov-
inces from the Precambrian to the Tertiary, and the
results show relatively nice agreement with the geo-
magnetic fi eld model used for the EI inclination correc-
tion (Fig. 5.10 ).
The large igneous province (LIP) data do appear to
support the EI fi eld model, particularly at intermediate
and high inclinations and very fl at inclinations (equa-
torial fi elds). So far, not much data are available to
check the validity of the model in the low, intermediate
inclination range.