|Abstract or Summary
- Three seismic refraction profiles obtained between 42°N and
43°N along the median valley of the Gorda Ridge, an active spreading
center, allow the computation of the velocity structure underlying the
valley. Wide angle reflections which appear on the seismic records
suggest the existence of a velocity inversion underlying layer 3 and
were analyzed in combination with refraction arrivals. The resulting
velocity model has a low velocity zone with a directly-determined
velocity of 5.72 km/sec, between crust of velocity 6.48 km/sec and
Moho of velocity 7.54 km/sec. The velocity inversion is 0.7 km
thick and lies 3 km below acoustic basement.
Consideration of the velocity structure of the Gorda Ridge,
together with other information on processes involved in oceanic
crustal formation, suggests a model which is consistent with current
knowledge on oceanic spreading centers.
In the proposed model, the rise of asthenospheric material on
the ascending limb of a convection cell causes the generation of a
small percentage of partial melt. The molten fraction tends to
coalesce near the top of the ascending limb, forming a region of
significant partial melt under the ridge crest. This molten material
is the immediate source reservoir for mid-ocean ridge magmas.
The geophysical expression of the reservoir is a region of low
seismic velocity and low density. As the magma cools from the
upper surface, heavy minerals tend to work their way downward,
forming a layer of cumulate ultramafic rocks at the base of the
crust, while the lighter constituents work upward to form the
cumulate gabbros of oceanic layer 3. The injection and extrusion
of magmatic material upward leads to the formation of layer 2. The
crust under the median valley is in isostatic equilibrium with the
partial melt during formation, but as it is displaced laterally from
the magmatic center, the entire lithosphere becomes competent and
the isostatic depth of compensation moves downward into the mantle.
This is thought to cause the familiar ridge crest topography of a
median valley and adjacent axial mountains observed at slowly
The features of this general model in the specific case of the
northern Gorda Ridge between 42°N and 43°N have been tested by
the comparison of theoretical and observed gravity and magnetic
anomalies. The computation of the theoretical gravity anomaly for
this model gives values which match the observed anomaly. The
magnetic data show only the pattern of anomalies expected from sea
floor spreading and magnetic field reversals.