Geology Reference
In-Depth Information
Fig. 6.22
Accelerations at stage transitions.
e
k
and
e
k
C1
are two successive stage poles. Euler vectors
¨
k
and
¨
k
C1
are built using the versors through
e
k
and
e
k
C1
and the
angular velocities of the corresponding stages. Although
the variation
¨
can be quite large, the magnitude of
the angular velocity variation,
j
¨
jDj
¨
k
C 1
j
-
j
¨
k
j
is
generally small
stage transitions are associated with discontin-
uous variations of the stage pole about which
a tectonic plate moves, so that we can define
an
angular acceleration
vector ¨
¨
k
C 1
¨
k
at the end of the
k
-stage (Fig.
6.22
). Just as in
the example of Fig.
6.22
, these transitions do
not generally involve large variations of plate
velocity, because in most cases we have only a
change in the location of the stage pole, such that
j
¨
jj
¨
j
. In principle, phases of accelerated
plate motion should be associated with a series of
very short stages with increasing (or decreasing)
velocity. In practice, the analysis of marine mag-
netic anomalies often allows to determine only
an
average
stage velocity, whereas the details of
the spreading process are obscured. The reason
is that stage boundaries are usually associated
with sharp variations of curvature in the trend of
oceanic fracture zones (corresponding to changes
of stage pole location), only exceptionally with
variations of plate velocity. Therefore, we will
be able to identify phases of accelerated mo-
tion, which must not be confused with stages
of elevated but constant velocity, only by the
individuation of
short asynchronous pulses
of fast
(or slow) velocity of individual plates, not neces-
sarily associated with plate boundary processes
(e.g., collisions). In this case, we will observe
a significant positive (or negative) peak in the
magnitude of the angular acceleration of a
single
plate at a stage boundary, followed by a compara-
ble peak of opposite sign at the successive stage
boundary (Fig.
6.23
).
Plots of magnitude of the angular acceleration
vector,
jP
¨
j
, and of variations of angular velocity,
!, for nine large plates since the late Triassic are
illustrated in Figs.
6.24, 6.25
,
6.26, 6.27
,
6.28,
6.29
,
6.30, 6.31
,and
6.32
. Peaks in the magnitude
of the angular acceleration vector are associated
with large displacements of the stage pole during
stage transitions, but not necessarily with non-
equilibrium states. Conversely, alternate pairs of
large peaks in the plots of scalar acceleration
are indicative of non-equilibrium states of
trac-
tion
for the lithosphere, if they represent asyn-
chronous events. For example, a striking peak of
