Geoscience Reference
In-Depth Information
Table 9.5
Segmentation of mid-ocean ridges
First order
Second order
Third order
Fourth order
Fast-spreading
ridges
Transform fault
Overlapping spreading
centre
Overlapping
spreading centre
DEVAL
Segment length
600
±
300 km
140
±
90 km
50
±
30 km
14
±
8km
Offset
>
30 km
2-30 km
0.5-2 km
<
1km
Segment lifetime
>
5Ma
0.5-5 Ma
0.01-0.1 Ma
0.0001-0.01 Ma
Depth anomaly
300-600 m
100-300 m
30-100 m
0-50 m
Geochemical
anomaly
Ye s
Yes
Usually
Sometimes
Off-axis anomaly
Fracture zone
V-shaped discordant
zone
None
None
Slow-spreading
ridges
Transform fault
Oblique shear zone,
depressions
Inter-volcano gap
Intra-volcano gap
Segment length
400
±
200 km
50
±
30 km
15
±
10 km
7
±
5km
Offset
>
30 km
2-30 km
0.5-2 km
<
1km
Segment lifetime
>
5Ma
0.5-30 Ma
?
?
Depth anomaly
500-2000 m
300-1000 m
50-300 m
0-100 m
Geochemical
anomaly
Ye s
Yes
Usually
?
Off-axis anomaly
Fracture zone
Broad irregular valley
Faint or None
None
Source
: After Macdonald (1998).
and
overlapping spreading centre
(OSC) are also used. DEVAL refers to very
small offsets or deviations, which are not regular transform faults but sometimes
mark a petrological segmentation of the ridge; OSC refers to a phenomenon that
occurs on fast-spreading ridges when the transform fault does not mark the abrupt
termination of both ridge segments, but, instead, the segments overlap slightly
(Figs. 9.35(a) and 9.35(b)).
Discontinuities along the mid-ocean ridges divide the ridge up into a number of
discrete segments: a transform fault is a rigid plate boundary that offsets the ridge
axis normally by over 30 km and partitions the ridge into distinct tectonic units on
a length scale of hundreds of kilometres. However, the segmentation of the mid-
ocean ridges also occurs on shorter scales described as second, third and fourth
order. The main features of mid-ocean-ridge segmentation are summarized and
the terminology put into context in Table 9.5 and they are illustrated schematically
in Fig. 9.35(c).Transform faults represent the first-order scale of mid-ocean-ridge
discontinuity: they persist for many millions of years and are easily identified by
their large depth anomalies and by magnetic-anomaly patterns. Second-order dis-
continuities partition the ridge on a length scale of
50 km or more, with shorter
offsets of the ridge axis. The orthogonal linearity of the ridge/transform system
∼
