Page 163 - Global Tectonics

OCEAN RIDGES 149

30 S
PAC
J
JUAN PACIFIC NAZCA
FERNANDEZ 3
NAZ 2 2A
2 J 31
ANT
2A

32
3
ENDEAVOR
DEEP
WOPF ? J
WIPF 33
2A
2 JF EOPF
2A J EIPF
2
34
2A
FZ J
WOPF
2 J 2 2A 3
PT
2A
35
5 4A 4
CHILE TR
J
3A 3 2A 2 J J 2 2A 3
ANTARCTIC 3A 4
J
36 S
116 115 114 113 112 111 110 109 108 107 106 W
Fig. 6.23 The tectonic elements of the Juan Fernandez microplate, together with magnetic anomalies numbered
according to the timescale of Fig. 4.8. TR, transform; PT, paleo-transform; FZ, fracture zone; WIPF, WOPF, EIPF, EOPF:
western/eastern, inner/outer pseudofaults (redrawn from Larson et al., 1992, with permission from Nature 356, 571–6.
Copyright © 1992 Macmillan Publishers Ltd).

wider and more common on fast-spreading ridges such from the thick crustal sections exposed in the large
as the East Pacifi c Rise. equatorial Atlantic fracture zones, which were found

Fracture zones mark both the active transform to consist of ultramafic, gabbroic and basaltic rock
segment and its fossilized trace. It has been suggested types and their metamorphosed and tectonized equiv-
(Collette, 1979) that the fractures result from thermal alents. Serpentinite intrusion appears to be quite
contraction in the direction of the ridge axis. The inter- common within fracture zones, accompanied by alkali
nal stresses caused by contraction are much larger than basalt volcanism, hydrothermal activity, and metallo-
the breaking strength of the rocks, and it is possible that genesis. Investigations of the Vema Fraction Zone
fracture zones develop along the resulting lines of (Auzende et al., 1989) have indicated a sequence similar
weakness. to normal oceanic layering. St Peter and St Paul rocks,
Dredging of fracture zones has recovered both in the equatorial Atlantic, which lie on a ridge associ-
normal oceanic crustal rocks and rocks which show ated with the St Paul Fracture Zone, are composed of
much greater metamorphism and shearing. Very com- mantle peridotite.
monly large blocks of serpentinite lie at the bases of In the North Atlantic, fracture zone crust is very
the fracture zones. Bonatti & Honnorez (1976) and heterogeneous in thickness and internal structure
Fox et al. (1976) have examined specimens recovered (Detrick et al., 1993b). It is often thin (<3 km) with low