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Figure 9.27. Reduced record sections for three expanding spread profiles (ESP) shot
along and parallel to the East Pacific Rise axis at 13 N. ESP9 was shot along the
ridge axis, ESP11 was shot 1.5 km away on 0.03-Ma crust and ESP13 was shot 3.6 km
away on 0.07-Ma crust. To the right of each record section is the velocity-depth
model determined from it by travel-time analysis. Note the low-velocity zone at the
ridge axis, which produces a shadow zone on ESP9. The slight discontinuity in first
arrivals at 8-10 km on ESP11 may indicate that a thin low-velocity zone is still
present there. Beneath ESP13, however, there is typical oceanic crust, and clearly no
low-velocity zone is present. (From Detrick et al .(1987).)
5 km. There is no evidence as to whether the magma chamber is literally that - a
chamber filled with liquid magma - or whether it is a chamber filled with partially
solidified crystal mush. There were no reflections from beneath the chamber to
enable estimates of interval velocities or attenuation to be made. A schematic
diagram of the magma chamber on the East Pacific Rise is shown in Fig. 9.28.
This model magma chamber is very much smaller than many that have been
proposed as results of studies of ophiolites. Figure 9.29 indicates where melt and
crystal mush are located beneath a fast-spreading ridge.
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