Page 156 - Global Tectonics

142 CHAPTER 6

Axial Summit Graben-
10’s of m deep
Melt lens
10’s of m thick Basalts-
200–800 m thick

Basalts

Dikes
Partially Molten
Low-Velocity Zone
Gabbros

~4 km
Fig. 6.16 Schematic diagram of the upper crustal structure for a fast-spreading ridge (redrawn with permission from
Karson, et al., 2002 by permission of the American Geophysical Union. Copyright © 2002, American Geophysical Union).

segments on slow-spreading ridges. These have abun-
dant volcanoes, typically in the form of axial volcanic 6.10 ORIGIN OF THE
ridges. These are 15–25 km long, and rise 400–1500 m
from the axial valley floor. In the amagmatic sections OCEANIC CRUST

the rift valley is often deeper than on slow-spreading
ridges, up to 5000 m deep in places, and the rift valley
walls have up to 2000 m of relief. On the Gakkel Ridge A widely accepted model of the petrologic processes
the western section is magmatic, the central section occurring at ocean ridges was proposed by Cann (1970,
essentially amagmatic, less than 20% of the rift valley 1974). In this model hot asthenospheric material ascends
having a basaltic cover, and the ultraslow-spreading buoyantly (Nicolas et al., 1994) sufficiently rapidly up a

eastern section is very different again. It has six large narrow zone to pass through the basalt melting curve
volcanic centers on it that extend for 15–50 km along and provides an interstitial melt of basaltic composi-
axis and are 50–160 km apart. These volcanic edifi ces tion. The molten fraction increases in volume as the
are larger and more circular than those on other ridges. asthenosphere rises, and eventually departs the parental
The amplitudes of the magnetic anomalies recorded material to ascend independently and produce a magma
between the volcanic centers suggest that the basaltic chamber within the lower part of the oceanic crust at
cover is thin in these tectonized zones. These marked the level of layer 3. Part of this magma rises through
along-axis contrasts in the extent of magma supply, fissures in the crust and erupts onto the ocean fl oor to

which do not correlate with spreading rate, pose inter- produce pillowed lava flows. Beneath the flows is a zone

esting questions regarding the generation and/or of dikes formed by solidification of magma in the fi s-

migration of melts beneath the ridge (Section 6.8) sures that feed the flows. The lavas and dikes together

(Michael et al., 2003). make up layer 2 of the oceanic crust. Kidd (1977)