Page 142 - Global Tectonics

128 CHAPTER 6

its shallowest point beneath the ridge crest, but extend- of the Mid-Atlantic Ridge. In fact the region of
ing to 350 km to the west and 150 km to the east of the primary melt only underlies crust 2–3 Ma in age,
ridge crest (Fig. 6.8). Both the velocity anomalies and whereas the anomalous uplift of ridges extends out to
electrical conductivity are consistent with 1–2% partial crust of 70–80 Ma in age. Partial melt in the upper
melting (Evans et al., 1999). There is an indication of mantle may therefore account for some of the uplift
incipient melting to a depth of 180 km. The asymmetry of ridge crests but cannot account for the uplift of
of the region of partial melting is thought to be due to ridge fl anks.
a combination of two effects. Within the hot spot

framework the western flank of the ridge is moving at

more than twice the rate of the eastern flank (Fig. 6.8).
It is also close to the South Pacifi c superswell (Section 6.4 DEPTH–AGE

12.8.3). Enhanced upwelling and hence flow in the
asthenosphere from the superswell and viscous drag RELATIONSHIP

beneath the fast moving Pacific plate are thought to

produce higher rates of flow and hence higher tem-
peratures beneath the western flank of the ridge. These OF OCEANIC

elevated temperatures are reflected in shallower

bathymetry (Section 6.4) and a higher density of sea- LITHOSPHERE
mount volcanism on the western flank compared to the

eastern fl ank.
The width of the region of partial melt defi ned by The major factor contributing to the uplift of mid-
the MELT experiment seems to be quite wide. One ocean ridges is the expansion and contraction of the
must recall however that the spreading rate at this material of the upper mantle. As newly formed
point is very high, fi ve times higher than that on much oceanic lithosphere moves away from a mid-ocean

Distance from axis (km)
West 400 200 0 200 400 East
101 mm yr 1 Crust 45 mm yr 1
0

Primary Lithospheric
melting mantle
100 Incipient E
Depth (km) 200 melting

300

400 410 km Discontinuity

Fig. 6.8 Schematic cross-section beneath the East Pacific Rise at 17°S illustrating the extent of partial melting in the
mantle deduced from the results of the MELT experiment. Plate velocities are in the hot spot reference frame. The
region labeled E (embedded heterogeneity) indicates enhanced melting due to anomalously enriched mantle

or localized upwelling (modified from MELT seismic team, 1998, Science 280, 1215–18, with permission from the
AAAS).