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ling evidence supporting the anisotropy-inclination
correction and the validity of the anisotropy correction
approach.
TEST OF THE ANISOTROPY-
INCLINATION CORRECTION
The best test of the anisotropy-inclination correction
for magnetite-bearing rocks is the study of the Pale-
ocene Nacimiento Formation (Kodama 1997). In this
study, the reality of rock magnetically-caused inclina-
tion shallowing was well established by a 104 site mag-
netostratigraphic study (Butler & Taylor 1978) that
had a tight (3.0°) 95% confi dence limit. Tectonic paleo-
latitudinal transport of the site could not be used to
explain its 7 - 8 ° anomalously shallow inclination since
the rocks were located in the Colorado Plateau in the
Early Tertiary, deep in North America's interior. Fur-
thermore, the cratonic paleomagnetic pole for North
America for this period was well determined by igneous
rocks (Diehl
et al
. 1983 ; Besse & Courtillot 2002 ) that
would not be affected by inclination shallowing. The
age of the Nacimiento Formation was also well con-
strained by the magnetostratigraphy measured for the
unit, so the comparison to the Paleocene cratonic pale-
opole wasn't affected by age inaccuracies.
Paleomagnetic and ARM anisotropy measurements
were made on the mudstones and siltstones of the
Nacimiento Formation and the individual particle ani-
sotropy was determined by three independent tech-
niques: laboratory compaction experiments, direct
measurement of a magnetic separate and examination
of the shape of the magnetic grains by SEM. All three
approaches gave an individual particle anisotropy
of
a c.
2.2-2.8. The inclination of each individual
sample was corrected by its anisotropy, and the cor-
rected directions (using the sample declination) were
averaged by Fisher statistics (Fisher 1953) as was typi-
cally done in paleomagnetic studies. The resulting
anisotropy - based inclination - shallowing correction
perfectly removed the 7-8° of observed shallowing
to bring the Nacimiento direction into agreement
with the Paleocene cratonic fi eld for North America
(Fig. 5.7). This study clearly showed the power and
accuracy of the anisotropy correction. It also sup-
ported one of the assumptions used in inclination cor-
rections that the ARM anisotropy should be applied
only to the magnetic grains that carried the demagnet-
ized remanence. This can be done relatively easily with
an ARM applied to relatively low-coercivity magnetite
Fig. 5.5
Location of the Perforado Formation on the
Vizcaino Peninsula of Baja California (modifi ed from
Vaughn
et al
. 2005). The Vizcaino Peninsula is separated
from Baja California by a possible tectonostratigraphic
terrane boundary. The Pz (Paleozoic) and Pz/Mz (Paleozoic/
Mesozoic) contacts show the geologically determined extent
of paleolatitudinal offset for Baja California. Reprinted from
Earth and Planetary Science Letters
, 232, J Vaughn, KP
Kodama and DP Smith, Correction of inclination shallowing
and its tectonic implications: The Cretaceous Perforada
Formation, Baja California, 71-82, copyright 2005, with
permission from Elsevier.
graphic terranes of California were identifi ed. Since we
had shown that the main Peninsular Ranges terrane
of western North America had been in place with Tan
& Kodama's (1998) study, the question was now: could
terranes outboard of the Peninsular Ranges terrane,
located in the Vizcaino Peninsula, show latitudinal
transport even after compaction had been accounted
for? The Perforada Formation showed beautifully the
effects of different amounts of compaction on the pale-
omagnetic inclination (Vaughn
et al
. 2005 ). Rocks that
had strong oblate compaction fabrics for ARM anisot-
ropy and for AMS had the shallowest inclinations.
Rocks with very weak magnetic fabrics had inclina-
tions about 10° steeper (Fig. 5.6). Furthermore, when
the rocks with strong fabrics had their inclination cor-
rected by the anisotropy technique, the corrected incli-
nation agreed nicely with the uncorrected inclination
from the rocks with little or no fabric. This was compel-
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