Page 157 - Global Tectonics

OCEAN RIDGES 143

modeled these processes of extrusion and intrusion and ~1 km
compared them with observations of ophiolite com- (a)
plexes. Layer 2C was found to consist entirely of sheeted
dikes, which were intruded through zones less than
50 m wide. The dikes show some 10% more chilled half-spreading rate
margins on one side than the other, showing that Rate
approximately 10% of the dikes are cut by later dikes,
such that the margins of the original dikes ended up on lava accumulation
opposite sides of the ridge crest. The symmetry of sea dike intrusion

floor spreading about the ridge axis is explained because
dike intrusion will proceed preferentially into the hot
central axis where existing dikes are weakest. It was
suggested that the lavas extruded above the dikes cool (b)

rapidly in contact with sea water and flow less than 2 km
before solidification. Lavas and dikes are predicted to

rotate towards the ridge crest as they move away from
the zone of extrusion as a result of isostatic adjustment
(Fig. 6.17). They also undergo metamorphism near the
ridge axis as they equilibrate at high temperatures in the
presence of seawater.
This model for the origin of layer 2 has received
striking confirmation from studies of sections through

the upper crust revealed by major fault scarps and drill
core from DSDP/ODP drill hole 504B, all in fast-spread- (c)

ing Pacific crust (Karson, 2002) (Section 6.9). Further-
more the model predicts that beneath the axial high the
extrusive layer should be very thin and the dikes cor-
respondingly closer to the sea floor (Fig. 6.17). This is

confirmed by seismic studies that reveal a narrow

central band of high seismic velocities beneath the axial
high (Toomey et al., 1990; Caress et al., 1992) and a thin
extrusive layer that thickens rapidly off axis within
1–2 km (Detrick et al., 1993b; Kent et al., 1994).
In the model of Cann (1974) the crust at lower levels
develops from the crystallization of the axial magma

chamber. The first minerals to crystallize in the magma Fig. 6.17 Geologic interpretation of the model of Kidd
chamber, olivine and chrome spinel, fall through the (1977) for the construction of Layer 2 at a fast-spreading
magma and form a basal layer of dunite with occasional ridge crest. Note the prediction of a rapid increase in the
accumulations of chromite. With further cooling pyrox- thickness of the extrusive layer away from the ridge axis
ene crystallizes and cumulate peridotitic layers (i.e. of and the presence of dikes at shallow depths near the
olivine and pyroxene) are produced, giving way upwards ridge axis (redrawn with permission from Karson, et al.,
to pyroxenites as the crystallization of pyroxene begins 2002, by permission of the American Geophysical Union.
to dominate. Ultimately, plagioclase also crystallizes Copyright © 2002 American Geophysical Union).
and layered olivine gabbros form. Much of the residual
liquid, still volumetrically quite large, then solidifi es
over a very small temperature range to form an upper, pockets of “plagiogranite” within the overlying sheeted
“isotropic” gabbro. A small volatile-rich residuum dike complex. The abundance of volatiles, notably
of this differentiation process, consisting essentially of water, in the uppermost part of the magma chamber
plagioclase and quartz, is the last fraction to crystallize, may be due, at least in part, to interaction with
sometimes intruding upwards to form veins and small seawater percolating downwards and/or stoping of the