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Fig. 6.18 Reconstruction
of a continent through PEP
analysis. A small circle arc
about a PEP, E ,isusedto
fit a sequence of
paleopoles ( red dots ). For a
track that ends at the North
Pole, a paleomagnetic
reference frame for time t
is found rotating P ( t )tothe
North pole about the PEP E
explain how to set up such an “absolute” frame.
Therefore, although PEP analysis furnishes an
intriguing way to represent the motion of a root
continent, it has not received much favor from the
community of paleomagnetists (e.g., Butler 1992 ;
Va n d e r Vo o 1993 ), possibly as a consequence of
the difficulties to define precisely the reference
frame.
of the reference frame where the rotation can
be observed. Unfortunately, the definitions that
can be found in the published literature are often
vague or inadequate. For example, Steinberger
and Torsvik ( 2008 ) built a reference frame assum-
ing that the z -axis was aligned with the spin axis,
while the y -axis was chosen in the Equatorial
plane, passing through the meridian where the
“continents' centre of mass” is placed. Apart
from the difficulty to find the “continents' centre
of mass”, this definition is somewhat confusing,
because the centre of mass of a system of plates
(or continents) is not independent from global
plate motions, thereby, it should not be used to
set up a reference frame for the detection of TPW.
As shown in the previous section, a suitable way
to define a coordinate system that is independent
from plate motions requires using hot spot tracks,
which are expressions of processes occurring
somewhere below the lithosphere.
For example, if a hot spot track exists on the
root plate, we can eventually fit a small circle
6.6
True Polar Wander
True polar wander (TPW) can be defined as a
rotation of the whole outer shells of the solid
Earth (crust plus mantle) with respect to the spin
axis, although this rotation would be indistin-
guishable from a motion of the lone lithosphere.
Such definition, which can be found in several
papers about this subject (e.g., Jurdy and Van der
Vo o 1974 ; Gordon 1987 ;Evans 2003 ), requires
a precise definition of what is intended by “ro-
tation of the whole Earth”, and a specification
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