Graduate Thesis Or Dissertation
 

Median valley crustal structure and sea floor spreading at the Gorda Ridge, 42⁰ N latitude

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  • 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 spreading ridges. 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.
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