Geology Reference
In-Depth Information
Fig. 6.21
Smoothed APW
path of central Africa
since the early Jurassic
(modified from Schettino
and Scotese
2005
).
Numbers represent ages
along the path
where
T
is the time and
r
is the position vector
of the reference point. To determine the absolute
velocity
v
, we perform a traversal of a rotation
tree from the selected plate to the root node,
calculating and summing at each step the relative
velocity of the current node with respect to its
node in a rotation tree represents a
conjugate
plate for the parent node, thereby, the velocities
are always calculated from stage poles. Conse-
quently, the relative motion of any plate with
respect to the root node can be represented by a
sequence of stage rotations. This is not sufficient
to explain the geometry of motion illustrated in
Fig.
6.20
, because the absolute motion of the root
plate could be a random sequence of instanta-
neous rotations, at least in principle. However, as
we have seen in section Sect.
6.5
, paleomagnetic
data suggest that even the motion of the root
plate can be described by a sequence of rotations
about PEPs. This observation results not only
from the study of Gordon et al. (
1984
)butalso
from an independent analysis of the geometry of
the smoothed APW paths proposed by Schettino
and Scotese (
2005
). For example, it can be proved
rigorously that the smoothed African APW path
can be divided into four small circle tracks, as
suggested by a visual inspection of Fig.
6.21
(Schettino and Scotese
2000
).
Therefore, we infer that the absolute motion
of any plate can be described by a sequence of
rotations about fixed Euler poles, as illustrated
in Fig.
6.20
by the paths travelled by refer-
ence sites. Such conclusion has important con-
sequences when we try to link plate kinematics
with mantle geodynamics, because it implies that
plate motions proceed for long time intervals
with constant angular momentum, so that the
total torque exerted on a tectonic plate is zero.
This in turn implies that in normal conditions
the lithosphere moves in equilibrium conditions,
such that the resistive forces opposing the motion
over the fluid asthenosphere are balanced by
active plate boundary forces. More specifically,
the existence of stationary plate motions that are
maintained for long time intervals implies that
the dominant driving force represented by the
gravitational pull of subducting slabs is always
balanced by the resistive viscous drag exerted
by the asthenosphere (Chase
1979
). However, in
Chap.
13
we shall see that episodes of accelerated
plate motion are possible and result from currents
in the asthenosphere, anomalous ridge push, or
continental collisions.
Now we are going to give a precise character-
ization of the global tectonic stages since the late
Triassic, as well as of possible events of acceler-
ated motion during this time interval. In general,
