Geoscience Reference
In-Depth Information
30
20
10
0
10
20
Figure 9.32.
An idealized
map of a transform fault
offsetting two ridge
segments. Numbers are
ages in Ma. Age provinces
are progressively shaded.
Note that the motion on
the fault occurs only
between the ridge offsets
and is sinistral (to the left
looking across the fault)
not dextral (to the right)
as it would have been
described in the
pre-plate-tectonic,
pre-magnetic-stripe
literature.
10
0
10
20
30
40
100 km
Figure 9.33.
A
bathymetric map of the St
Paul, Romanche and
Chain transform faults in
the equatorial Atlantic
(Table 9.4). Dots indicate
earthquake epicentres.
The large dots are those
for which the fault-plane
solutions are shown.
Notice how closely the
epicentres follow the
ridge axis and transform
faults. Because the foci
were all shallow (less than
5 km), and the depth
range of faulting is about
0-10 km, the foci were
above (shallower than)
the 600
◦
C isotherm.
(From Engeln
et al
.
(1986).)
axis. Had this been the case, the entire fault would be active, not just the portion
between the two offset ridge axes, and the slip motion on the fault would be in
the opposite direction to that shown in Fig. 9.32. This gave rise to the question
'What happens to the fault on the continent?' or 'Where do transcurrent faults
end?' Plate tectonics neatly solved the problem.
There are several reasons why there are transform faults offsetting ridge axes.
First, the major faults are determined by the geometry of the initial break between
the two continents; this break occurs along zones of weakness, which are probably
partly associated with old geological structures. The break need not be a straight
line and may be quite irregular. Transform faults tend to develop in locations
where the relative motion is approximately parallel to the break and are neces-
sary in order to accommodate this geometry with the pole of rotation. Thus, the
preferred spreading-geometry configuration consists of straight ridge segments
offset by orthogonal transform faults. That segmentation of slow-spreading ridges
