Page 158 - Global Tectonics

144 CHAPTER 6

km
4 3 2 1 0 1 2 3 4
Small volcano
0

1
2 New magma

km 3 body

4

5
Brittle – ductile
6 Plutons transition

Fig. 6.18 Model for the construction of oceanic crust at a slow-spreading ridge. Transient magma bodies rise to the
brittle–ductile transition within the crust and shoulder aside and depress older plutons. Part of the magma body erupts

through a fissure to produce a volcano or hummocky lava flow on the sea floor and the remainder solidifies to form

part of the main crustal layer (redrawn from Smith & Cann, 1993, with permission from Nature 365, 707–15. Copyright
© 1993 Macmillan Publishers Ltd).
overlying, hydrothermally altered dikes into the magma sient. In this case the alternative model derived from
chamber. early reinterpretations of ophiolites in terms of sea

The two gabbro units, isotropic and layered, are floor spreading may be more applicable. This invoked
often correlated with seismic layers 3A and 3B, respec- multiple small magma chambers within the main crustal

tively (Section 2.4.7). The ultramafic cumulates, rich in layer in the light of the multiple intrusive relationships
olivine and pyroxene, would then account for the sub- observed at all levels in the Troodos ophiolite of south-
Moho seismic velocities. Thus, the Moho occurs within ern Cyprus (Moores & Vine, 1971) (Section 2.5). Smith
the crystallized magma chamber at the base of the & Cann (1993) favor such a model for the creation of
mafic section. Off axis, however, in a lower temperature oceanic crust at slow-spreading ridge crests (Fig. 6.18).

environment, the uppermost ultramafics may become However away from segment centers, and particularly
partially hydrated (i.e. serpentinized) and as a result in the vicinity of transform faults, the magma supply
acquire lower seismic velocities more characteristic of may be greatly reduced, and serpentinized mantle peri-
layer 3B. The seismic Moho would then occur at a dotite appears to be a common constituent of the
somewhat greater depth, within the ultramafi c section. thinned oceanic crust. This type of crust becomes even
As a result of this uncertainty in defining the seismic more common on very slow-spreading ridges and ulti-

Moho, petrologists have tended to define the base of mately most of the crust is effectively exposed mantle
the crust as the base of the presumed magma chamber, with or without a thin carapace of basalts. On the ultra-
that is, the dunite/chromitite horizon. Hence, this level slow Gakkel Ridge the crust is essentially serpentinized
is termed the “petrologic Moho”. and highly tectonized mantle peridotite with volcanic
The model of Cann (1974) and Kidd (1977) has met centers at intervals of 100 × 50 km.
with considerable success in explaining the known An alternative approach to understanding the accre-
structure and petrology of oceanic crust created at fast- tionary processes at mid-ocean ridge crests is by way of
spreading ridge crests, where there is a steady state thermal modeling (Sleep, 1975; Kusznir & Bott, 1976;
magma chamber. At slow-spreading ridge crests, Chen & Morgan, 1990). Chen & Morgan (1990) made
however, the zone of crustal accretion is wider and it significant improvements to such models by including

seems probable that magma chambers are only tran- the effects of hydrothermal circulation at ridge crests