Geoscience Reference
In-Depth Information
Figure 9.31.
Shaded
bathymetry of the
Clipperton transform fault
(Table 9.4)onthe East
Pacific Rise at 10
◦
N. The
image is 100 km
(north-south) × 175 km
(east-west). Depths are
metres below sea level.
The transform fault, a
straight narrow cleft that
cuts through the rugged
transform valley, has had
this configuration for
9 Ma. The northern ridge
segment is starved of
magma and deepens
steadily for 70 km towards
the transform. In contrast,
the southern ridge
segment is elevated and
is underlain by a magma
chamber right up to the
transform. Colour version
Plate 21. (Reprinted with
permission from
Nature
(Macdonald
et al.
1998,
Nature
,
335
, 217-25)
Copyright 1988 Macmillan
Magazines Ltd.)
more detailed the bathymetric, magnetic and reflection surveys become, the more
numerous small discontinuities are seen to be. In the FAMOUS area at 37
◦
Non
the Mid-Atlantic Ridge, the ridge segments are 20-60 km in length and are offset
by small discontinuities about 20 km in length. Even on this small scale, the
regular geometrical pattern is maintained.
Figure 9.32 illustrates a transform fault between two ridge segments. In this
example, the offset is 200 km. The fault juxtaposes materials that differ in age
by 20 Ma. Note that, although the spreading rate is 1 cm yr
−
1
, the slip rate along
the fault (which is twice the spreading rate) is 2 cm yr
−
1
. This slip motion occurs
on the fault only between the two ridge segments. This is demonstrated by the
location of earthquake epicentres: outside the active transform faults, seismic
activity is negligible (Fig. 9.33).
Even though transform faults are active only between the two offset ridge
segments, they remain major features on bathymetric, gravity (Fig. 5.4) and
magnetic-anomaly maps outside the active zone because of the constant age
contrast across the fault. The majority of Atlantic fracture zones are bathymetric
and gravity features from America to Africa and start at the continental margin.
This observation was puzzling to Earth scientists before the development of plate
tectonics, and the problem of
transcurrent faults
(as they were then called) was
the subject of much debate. Before seafloor spreading was understood, the fault
shown in Fig. 9.32 would have been understood as a right lateral offset of the ridge
