Page 150 - Global Tectonics

136 CHAPTER 6

Fast - spreading ridge
S4
D4
S4
Axial high S2
S2
D2
~10 - 50 km D4
-50 km ~ 5 -10 km S4
-1 km D3
S3
-20 km -1 km
S2
S4
S3 D4
S1 D1
Transform fault

Slow - spreading ridge

S3 S4
S2 D3
S3
S3 D4
S4
-10 km D2
-20 km
-5 km
D2
-20 km D4
Volcanoes S4
in floor of D3
-2 - 6 km
S2 rift valley S3
-1 km S4
S1 D4
D1 S4
Transform fault
Axial rift valley
Fig. 6.13 Summary of the hierarchy of segmentation on fast- and slow-spreading ridges. S 1 , S 2 , S 3 , and S 4 – first to

fourth order ridge segments. D 1 , D 2 , D 3 , and D 4 , – fi rst to fourth order discontinuities (redrawn from Macdonald et al.,
1991, Science 253, 986–94, with permission from the AAAS).
The different scales and hence “orders” of ridge seg- which does not even exhibit transform faulting, is in the

mentation were first recognized on the fast-spreading form of volcanic and tectonic, or magmatic and amag-
East Pacifi c Rise. Segmentation also exists on the slow- matic, segments (Michael et al., 2003) (Section 6.9).

spreading Mid-Atlantic Ridge but takes on somewhat The first order segment boundaries, transform
different forms, presumably because the ridge crest is faults, are marked by pronounced bathymetric depres-
cooler and hence more brittle (Sempéré et al., 1990; sions (Section 6.12). They are often underlain by thinner
Gente et al., 1995) (Fig. 6.13). First order segmentation crust than normal and anomalously low sub-Moho
is defined by transform faults, but overlapping spread- seismic velocities that may be due to partial serpentini-

ing centers are absent and second order segments are zation of the mantle as a result of seawater percolating
bounded by oblique offsets of the ridge axis associated down through the fractured crust. This thinning of the

with deep depressions in the sea floor. Third and fourth crust in the vicinity of fracture zones is particularly
order segmentation is in the form of geochemical vari- marked on the slow-spreading Mid-Atlantic Ridge
ations and breaks in volcanic activity in the inner valley (White et al., 1984; Detrick et al., 1993b). By contrast,

floor. The latter generate discrete linear volcanic ridges the central portions of segments are elevated, have
2–20 km long and 1–4 km wide (Smith & Cann, 1993). crust of normal thickness, and thinner lithosphere. This

Again first and second order segmentation is long-lived implies that the supply of magma from the mantle is
and third and fourth order segmentation is short-lived. focused at discrete points along the ridge axis at segment
Segmentation on the ultraslow-spreading Gakkel Ridge, centers. These regions of thicker crust and enhanced