Page 149 - Global Tectonics

OCEAN RIDGES 135

depth of the ridge axis. These features may migrate up
or down the ridge axis with time.
OSCs (MacDonald & Fox, 1983) are nonrigid discon-
tinuities where the spreading center of a ridge is offset
by a distance of 0.5–10 km, with the two ridge portions
overlapping each other by about three times the offset.
It has been proposed that OSCs originate on fast-spread-
ing ridges where lateral offsets are less than 15 km, and
true transform faults fail to develop because the litho-
sphere is too thin and weak. The OSC geometry is
obviously unstable, and its development has been
deduced from the behavior of slits in a solid wax fi lm
floating on molten wax, which appears to represent

a reasonable analogue (Fig. 6.12a). Tension applied
orthogonal to the slits (spreading centers) causes their
lateral propagation (Fig. 6.12b) until they overlap (Fig.
6.12c), and the enclosed zone is subjected to shear and
rotational deformation. The OSCs continue to advance
until one tip links with the other OSC (Fig. 6.12d). A
single spreading center then develops as one OSC
becomes inactive and is moved away as spreading
continues (Fig. 6.12e).
Fast-spreading ridges are segmented at several differ-
ent scales (Fig. 6.13). First order segmentation is defi ned
by fracture zones (Section 4.2) and propagating rifts
(Section 6.11), which divide the ridge at intervals of
300–500 km by large axial depth anomalies. Second
order segmentation at intervals of 50–300 km is caused
by nonrigid transform faults (which affect crust that is
still thin and hot) and large offset (3–10 km) OSCs that
cause axial depth anomalies of hundreds of meters.
Third order segmentation at intervals of 30–100 km is
defined by small offset (0.5–3 km) OSCs, where depth

anomalies are only a few tens of meters. Finally, fourth
order segmentation at intervals of 10–50 km is caused
Fig. 6.12 Possible evolutionary sequence in the
by very small lateral offsets (<0.5 km) of the axial rift
development of an overlapping spreading center
and small deviations from axial linearity of the ridge
axis (DEVALS). These are rarely associated with depth (redrawn from MacDonald & Fox, 1983, with permission
from Nature 302, 55–8. Copyright © 1983 Macmillan
anomalies and may be represented by gaps in the vol-
canic activity within the central rift or by geochemical Publishers Ltd).
variation. Clearly fourth order segmentation is on the
same along-axis length scale as the intervals between
pure melt pockets in the melt lens documented by meters. The scars do not follow small circle routes
Singh et al. (1998) (Section 6.6). about the spreading pole, but form V-shaped wakes at
Third and fourth order segmentations appear to be 60–80° to the ridge. This indicates that the OSCs
short-lived, as their effects can only be traced for a few responsible for the segmentation migrate along the
kilometers in the spreading direction. Second order seg- ridge at velocities of up to several hundred millimeters
mentations, however, create off axis scars on the spread- per year. Figure 6.14 summarizes the three general cases
ing crust consisting of cuspate ridges and elongate for the evolution of such ridge–axis discontinuities in
basins that cause differential relief of several hundred terms of the movement of magma pulses.