Geology Reference
In-Depth Information
Fig. 5.11
Local
coordinate system for a
magnetized prism at
location
Q
along a
projection line.
R
is the
present ridge axis. (œ
p
,¥
p
)
is the palaeopole location
as seen for the prism. ™ is
the paleocolatitude of
point
Q
the conjugate plate. However, we shall see soon
that knowledge of just one APW path is sufficient
to calculate the field components.
A declination value can be used, together with
the local strike, “,ofthe
y
axis (main axis of the
magnetized prism), to calculate the parameter '
that appears in (
5.49
)and(
5.50
). Assuming that “
is measured clockwise from the North (Fig.
5.11
),
with 0
ı
“<360
ı
, it can be easily shown that in
any case ' is given by:
from the paleolatitude of the spreading segment
identity will hold also in the future, because the
separated blocks can only rotate about vertical
axes and be displaced in so far as sea floor
spreading proceeds. In fact, we know that the two
magnetized prisms will move apart in opposite
directions by a sequence of rotations about stage
this process the
direction
of magnetization will
change, and it will change differently for the
two blocks, so that after some million years the
declination
D
of the two magnetization vectors
will be significantly different. However, it is eas-
ily realized that the strike ' of the two prisms
with respect to their directions of magnetization
remains invariant, so that if “
0
is the modern strike
of the conjugate block and
D
0
'
D
D
“
C
180
ı
(5.61)
Let us consider now the oceanic crust that
is created along a ridge segment during a po-
larity chron. At the end of the chron, this is a
crustal block formed by two adjacent prisms with
identical GAD magnetization and strike ',which
are placed along the opposite flanks of the ridge
segment. Therefore, at this time the two prisms
have the same inclination,
I
0
D
I
, which depends
is its declination,
we have:
D
0
D
'
C
“
0
180
ı
(5.62)
