Geology Reference
In-Depth Information
Fig. 2.4
Geometry of the
Mid-Atlantic ridge in the
central Atlantic area (
red
line
).
Dashed lines
are
major fracture zones.
Black
dots
bound transform faults
and spreading segments
along the ridge. Numbers
label the eight segments
identified along this tract of
the ridge.
Double arrows
indicate directions of
spreading. Also shown is
the full spreading rate. The
background image,
showing free-air gravity
anomalies (Sandwell and
Smith
1997
), enhances the
location of the axial zone
of the ridge and the track of
the fracture zones (
FZ
)
are going to consider all these faults and systems
of faults from the point of view of plate kine-
matics. Mid-ocean ridges are extensional bound-
aries in the oceanic domain. We have seen in
Chap.
1
that these features are spreading centers,
where new oceanic crust is passively accreted
as a consequence of divergent motion between
two tectonic plates. These boundaries are formed
by sequences of ridge segments that are linked
together by transform faults, as illustrated in
Fig.
2.4
.
Transform faults are faults with a pure strike-
slip kinematics and a strike that reflects the local
direction of instantaneous motion between two
plates. Therefore, these faults are always paral-
lel to velocity vectors of relative motion. This
relative motion is clearly left-lateral strike-slip
in the case of dextral offset of the spreading
segments and vice versa. For example, all the
transform faults shown in Fig.
2.4
imply left-
lateral strike-slip motion. The adjective “trans-
form” that is attributed to these tectonic features
arises from the fact that they generate active
bathymetric discontinuities, as far as the two
plates move apart. Such discontinuities are called
fracture zones and represent linear features that
apparently pursue the transform faults toward
the continental margins (Fig.
2.4
). Therefore, the
latter seem to be “converted” into a different
Fig. 2.5
Age discontinuities across fracture zones. Two
points,
x
and
y
, on plate
A
, having very different ages, are
at contact through a transform fault. When the two points
are moved to locations
x
0
and
y
0
, their difference of age,
T
, is conserved
class of faults, characterized by vertical slip. The
explanation of this phenomenon is quite simple.
Let
v
and
L
be, respectively, the relative ve-
locity along a transform fault and its length, and
consider a point
x
close to one of the ends of
the fault (Fig.
2.5
). The age of formation of the
crust at
x
is
T
(
x
)
D
2
L
/
v
, because the velocity
of accretion on both sides of the ridge is on
average
v
/2. This point is clearly at contact with
another point, say
y
, which is close to the ridge
and has age
T
(
y
)
D
0. Therefore, the difference
of age between
x
and
y
is
T
D
2
L
/
v
.Atany
successive time
t
, these two points will be dis-
placed away from the ridge by the same offset,
to two close locations
x
0
and
y
0
, independently
from any change of
v
with time. Therefore, they
