Page 152 - Global Tectonics

138 CHAPTER 6

magma supply are characterized by negative mantle that serpentinized rocks may be much more common
Bouguer anomalies (MBAs) and the areas of thinned in the oceanic crust than previously assumed, even in
crust between them by positive MBAs (Lin et al., 1990). areas distant from fracture zones. They dredged in the
These latter areas include second order discontinuities region of the North Atlantic Ridge at 22–24°N over
in addition to transform faults. Evidence for the focus- areas of positive gravity anomalies, indicative of rela-
ing of magma supply at segment centers is less obvious tively thin crust, and over areas with a normal gravity

on the fast-spreading East Pacific Rise, but discontinui- field. Over the former, which comprised some 23% of
ties and variations in the size and width of the magma the area surveyed, they encountered serpentinite with
chamber correlate with segmentation (Toomey et al., very few of the basaltic rocks which normally charac-
1990). These observations suggest that magma is terize oceanic layer 2. They suggested that as magmatic
emplaced at segment centers and migrates laterally centers grow, migrate along the ridge axis and decline,
along the ridge axis towards the segment ends. Increas- the normal oceanic crust would similarly migrate and
ingly it has been recognized that on cooler slow- would enclose those regions of serpentinitic crust that
spreading crust this can mean that segment ends are originate where magma was absent. This work is impor-
starved of magma and that parts of the crustal section tant as it implies that serpentinized peridotite is more
consist of serpentinized mantle. common in slow-spreading oceans than previously rec-
The extension of oceanic crust at ridge crests can ognized. There are wide ranging implications. Perido-
occur either by the intrusion of magma or by exten- tite is much more reactive with seawater than basalt and
sional faulting. If ridge crests in the vicinity of trans- on weathering would release magnesium, nickel, chro-
form faults are deprived of magma, amagmatic mium and noble metals. Sepentinite also contains far
extension becomes more important. Perhaps the most more water than altered basalt, which could account for
spectacular expression of this is the occurrence of major much of the water supplied to the mantle in subduction
low-angle detachment faults (Ranero & Reston, 1999; zones (Section 9.8), although at the present day the only
MacLeod et al., 2002) (Section 7.3) on the inside corners examples of oceanic crust formed at slow-spreading
of slow-spreading ridge–transform intersections that rates entering subduction zones are the Caribbean and
give rise to large corrugated and striated domes of ser- Scotia arcs.
pentinized peridotite and gabbro (Plate 6.1 between pp. Segmentation of ocean ridges appears to be con-
244 and 245). These corrugated domes are exposed fault trolled by the distribution of partial melts beneath
planes that deform the upper mantle and lower crust of them (Toomey et al., 1990; Gente et al., 1995; Singh
oceanic lithosphere. The corrugations parallel the et al., 1998), which feed magma chambers at discrete
spreading direction and indicate the direction of motion locations along them and create local depth anomalies.
on the fault. The offset on these faults is typically at least The ridge model of Sinton & Detrick (1992), described
10–15 km (Cann et al., 1997). These exposures are above, precludes extensive mixing within the small
thought to result from processes involving extension, axial magma chamber along the ridge, and could

detachment faulting, and crustal flexure that are similar explain the observed geochemical segmentation. With
to those that form metamorphic core complexes in time the magma may migrate away from its sources,
zones of continental extension (Sections 7.3, 7.6.2, creating a gradual increase in depth of the axis as the
7.7.3). For this reason the zones of exhumed peridotite pressure within it gradually wanes. This phenomenon
at ridge–transform intersections are referred to as may explain the noncoincidence of magma chamber
oceanic core complexes. Examples include the Atlantis and rise culmination noted by Mutter et al. (1988).
Massif at the Mid-Atlantic Ridge–Atlantis transform The brittle shell overlying the magma stretches and
intersection (Plate 6.1 between pp. 244 and 245) (Black- cracks and magma intrudes so that eruptions follow
man et al., 1998; Schroeder & John, 2004; Karson et al., the path of magma migration. After eruption the
2006) and along the Southeast Indian Ridge south of removal of supporting magma gives rise to the forma-
Australia (Baines et al., 2003; Okino et al., 2004). tion of an axial summit graben. Evidence for the pulse-
Figure 6.15 illustrates the along axis variation of like, episodic spreading of ridges, in which sea fl oor
oceanic crust for slow- and fast-spreading ridges as envi- spreading occurs by fracturing, dike injection and
sioned by Cannat et al. (1995) and Sinton & Detrick copious volcanism, has been provided by seismological
(1992) respectively. Indeed Cannat et al. (1995) suggested studies and direct observation (e.g. Dziak & Fox, 1999;