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will remain neighbor points. Thus, the motion of
points
x
and
y
away from the ridge will leave
a linear track of age discontinuities,
T
,which
crosses the whole oceanic part of the plate and
reaches the continental margin, as illustrated in
Fig.
2.5
.
Furthermore, a specular trace will form on
the conjugate plate, reaching the opposite con-
tinental margin. Generally, fracture zone tracks
are easily identified on bathymetric or gravity
anomaly maps (Fig.
2.4
), because age disconti-
nuities are always associated with bathymetric
to the sea floor increases with the crustal age,
so that an age discontinuity always implies a
bathymetric gap. Despite the invariance of the age
discontinuity,
T
, along a fracture zone track, the
difference of depth across the two sides changes
with time, because the rate of sea floor subsidence
is not a linear function of time, especially dur-
ing the first
100 Myrs. This implies a lateral
discontinuity in the amount of subsidence, so
that fracture zones can be assimilated to vertical
faults characterized by vertical slip. Therefore,
as suggested by their name, transform faults are
converted to a different class of faults, fracture
zones, which are not associated with horizontal
slip and do not represent plate boundaries, but
simply are active bathymetric gaps associated
with discontinuities in the age of the sea floor.
Finally, it is necessary to keep in mind that
although transform faults and ridge segments are
very different tectonic features, they are part of
unique plate boundaries, namely the mid-ocean
spreading centers. In other words, they cannot be
considered as distinct classes of plate boundaries.
Now let us consider the kinematics along mid-
ocean spreading ridges. In principle, these fea-
tures should be orthogonal to the relative velocity
field between two plates. However, the exam-
ple of Fig.
2.4
shows that the azimuth of the
segments composing a mid-ocean ridge is not
necessarily 90
ı
from the direction of spreading.
This phenomenon is called spreading obliquity,
and is quantified measuring the angle between
the normal to the ridge trend and the direction of
a transform fault. Observation suggests that the
spreading obliquity is particularly strong in the
case of slow-spreading ridges (e.g., Southwest
Indian Ridge and North Atlantic Ridge), where
it could be as high as
80
ı
(Whittaker et al.
2008
). It is always necessary to take into ac-
count of this parameter when interpreting marine
to deal with oblique spreading. In general, plate
kinematics studies require an accurate prelimi-
nary mapping of the plate boundaries through
GIS software, especially in the case of mid-
ocean ridges. In this instance, the location and the
geometry of the segments forming a spreading
center, as well as the trace of transform faults,
can be established by close inspection of the
axial valley topography and by the analysis of
gravity anomalies (Fig.
2.4
). However, in most
cases a precise definition of the ridge segments
will require a successive refinement, based upon
the analysis of marine magnetic anomalies, as we
ocean ridges is not constant through the geologi-
cal time. It is subject to changes, even in absence
of variations of relative motion, as a consequence
of three basic mechanisms: spreading asymmetry,
ridge jumps, and ridge segment reorientations.
Figure
2.6
illustrates these three possibilities.
Spreading asymmetry occurs when the rate
of accretion of new crust is not uniform across
the two sides of a spreading segment (Fig.
2.6
a).
Let
v
be the full spreading rate along a ridge
segment. This quantity clearly coincides with the
local magnitude of the velocity vector of a plate
A
with respect to another plate
B
.
We can introduce a quantity
1 <'<
C
1,
such that the widths of the crust accreted to the
right and left sides of a spreading segment in a
time interval
t
are:
1
2
.1
C
'/
v
t
I
x
L
D
1
2
.1
'/
v
t
(2.32)
x
R
D
The quantity ' is an expression of the asym-
metry of spreading across a mid-ocean ridge
segment. In normal conditions ('
D
0), a spread-
ing segment moves at velocity v/2 with respect
to each of the conjugate plates. In the case of
spreading asymmetry, the segment will move at a
