Page 162 - Global Tectonics

148 CHAPTER 6

tectonic elements of the Juan Fernandez microplate
PAC NAZ
SA GALAPAGOS (Fig. 6.23) clearly show the characteristic pseudofaults
MICROPLATE of the original propagating rift to the east, and the
EMP
subsequent propagating rift to the southwest of the
JFMP microplate. Microplates are thought to exist for no
more than 5–10 million years, by which time the initial
N rift succeeds in transferring the oceanic lithosphere of
ANT
the microplate from one plate to another, in the case of
the Juan Fernandez microplate, probably from the
PACIFIC Nazca to the Antarctic plate (Bird et al.,1998). Tebbens
10 S PLATE Wilkes Transform et al. (1997) have documented an analogous example in
the late Miocene when a newly formed rift, propagating
northwards from the Valdivia Fracture Zone on the
Garrett Transform Chile Ridge, ultimately transferred lithosphere from the
Nazca to the Antarctic plate. Brozena & White (1990)
have reported ridge propagation from the South Atlan-
NAZCA tic, so this phenomenon appears to be independent of
PLATE
spreading rate.
20 S The cause of the initiation of ridge propagation is
unknown but several researchers have noted that prop-
EASTER agating rifts tend to form in the vicinity of hot spots
MICROPLATE and on the hot spot side of the pre-existing ridge crest
(e.g. Bird et al., 1998; Brozena & White, 1990). An
important corollary of the mere existence of propagat-
ing rifts is that the ridge-push force at spreading centers
Easter Island
(Section 12.6) is not a primary driving mechanism as
30 S it appears to be quite easily overridden during ridge
propagation.
JUAN FERNANDEZ
MICROPLATE

ANTARCTIC 6.12 OCEANIC
115 W 105 W
PLATE
FRACTURE ZONES
Fig. 6.22 Map showing the location and extent of the
Galapagos, Easter and Juan Fernandez microplates in
the southeast Pacific Ocean. Arrows on ridge segments Transform faults in the oceans are well defined, in the

indicate active or previously active propagating rifts absence of sedimentary cover, by fracture zones. These

(modified from Bird et al., 1998, by permission of the are long, linear, bathymetric depressions that normally
American Geophysical Union. Copyright © 1998
American Geophysical Union). follow arcs of small circles on the Earth’s surface per-
pendicular to the offset ridge (Bonatti & Crane, 1984).
The apparent relative simplicity of oceanic fracture
zones is no doubt due in part to the fact that they are

in the southeast Pacific (Fig. 6.22). Detailed studies of commonly studied from the sea surface several kilome-
the Easter and Juan Fernandez microplates show that ters above the ocean floor. Direct observations of a

their bathymetric fabrics and structural evolution are fracture zone on the Mid-Atlantic Ridge (Choukroune

very similar, and fit well with the predictions of the et al., 1978) have shown that it consists of a complex
model of Engeln et al. (1988) (Searle et al., 1989; Rusby swarm of faults occupying a zone 300–1000 m in width.
& Searle, 1995; Larson et al., 1992; Bird et al., 1998). The Searle (1983) suggests that these multi-fault zones are