Page 161 - Global Tectonics

OCEAN RIDGES 147

highly asymmetric sea floor spreading. The propagation
model predicts abrupt boundaries between areas of
uniform magnetic anomaly and bathymetric trends of
different orientation. The rotation model predicts a con-

tinuous fanlike configuration of magnetic anomalies
whose direction changes from the old to new spreading
direction. Consequently, detailed bathymetric and mag-
netic surveys should be able to distinguish between the
two models.
Hey et al. (1988) reported the results of a detailed
investigation of the region where the direction of
spreading of the Pacific–Farallon boundary changed

direction at about 54 Ma, just north of the major bend
of the Surveyor Fracture Zone, using side-scan sonar,

magnetometry, and seismic reflection. They found that

the change in direction of sea floor fabric revealed by
sonar is abrupt, in accord with the propagating rift
model. Similar conclusions were reached by Caress
et al. (1988). Hey et al. (1980) described the results of a
survey of an area west of the Galapagos Islands at
96°W. They concluded that here a new ridge is progres-
sively breaking through the Cocos plate, and the mag-
netic data in particular (Fig. 6.21) provide convincing
evidence that the ridge propagation mechanism is oper-
ative. This interpretation was confirmed by detailed

mapping of the bathymetry in this area (Hey et al.,
1986). This clearly revealed the V-shaped pattern of the
pseudofaults, the active and failed rifts, and the oblique
tectonic fabric in the sheared zone of transferred litho-
sphere. The propagating rift model also elegantly
explains the way in which the change in orientation of
the Juan de Fuca Ridge (Fig. 4.1) has been achieved
within the past 10 Ma (Wilson et al., 1984).
Engeln et al. (1988) pointed out that the propagating
rift model described above assumes that the newly
formed rift immediately attains the full accretion rate
between the two plates, thereby rendering the pre- Fig. 6.21 (a) Predicted magnetic lineation pattern
existing rift redundant. However, if spreading on the resulting from ridge propagation; (b) observed magnetic
new rift is initiated at a slow rate, and only gradually anomalies near 96°W west of the Galapagos Islands
builds up to the full rate over a period of millions of (redrawn from Hey et al., 1980, by permission of the
years, the failing rift continues to spread, albeit at a American Geophysical Union. Copyright © 1980
slower and decreasing rate, in order to maintain the net American Geophysical Union).
accretion rate. In contrast to the original propagating
rift model, in this model the two rifts overlap and the
area of oceanic lithosphere between them increases the region between the points where the propagating
with time. In addition, as a result of the gradients in rift was initiated and the original rift started to fail. After
spreading rate along each rift, the block of intervening a few million years this transpression gives rise to an
lithosphere rotates. This rotation in turn produces com- additional propagating rift.
pression in the oceanic lithosphere adjacent to the tip This second propagating rift model was put forward
of the propagating rift and transtension (Section 8.2) in to explain the remarkable phenomenon of microplates