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. / Graduation date: 1978

Thesis (Ph. D.)--University of Wyoming, 2006. / Title from PDF title page (viewed on June 16, 2008). Includes bibliographical references.

Thesis (Doctoral)--University of Tokyo, 1973. / One folded plate in pocket. Includes bibliographical references (leaves 76-81).

Thesis--University of Connecticut. / Includes abstract (2 leaves). Includes bibliographical references (p. 86-94).

The Gorda Basin is a young oceanic plate which comes in direct contact with the convergent margin of western North America. Two long sonobuoy refraction profiles crossing the basin provide nearly continuous data for computing the velocity structure of the crust and adjacent continental slope. Time-term analysis utilizing multiple receivers and overlapping profiles revealed a thick transition layer which averages 2.3 km but displays considerable lateral variation. The seismic compressional velocity of this layer is 5.3 km/sec. Th average thickness of Layer 3 is 3.4 km with a velocity of 6.9 km/sec. The base of the crust is marked by the seismic Moho, the velocity below which is 8.1 km/sec. Refraction and reflection studies of sediment cover indicate a thickening of turbidite deposits to the southeast from less than 100 meters to over 2.5 km along the continental margin. Ophiolite studies indicate that the top of Layer 3 marks the upper extent of amphibolite facies metamorphism of basaltic sheeted dikes. Lateral depth variations of this seismic boundary in the Gorda Basin may suggest the occurrence of isograd relief along the spreading center. The Moho marks the boundary between mafic and ultramafic rocks near the ridge but may represent the maximum depth of serpentinization in the crust after it moves away from the spreading axis. Thin crust (4-5 km) and deep bathymetry in the central portion of the basin have resulted from crustal formation processes occurring at ridge crest offsets and are coincident with recent seismicity in the area. The Gorda ridge offsets and asymmetrical fan spreading of magnetic anomalies are features observed in response to a regional change in spreading directions and encroachment of the Pacific and North American plates. The Gorda plate as a whole does not respond rigidly to the resulting north-south compression. Complex structures of the continental slope, revealed by seismic reflection, limited the reduction of refraction data using plane layer methods. A simplified seismic section was computed consisting of three probable sediment layers with velocities of 1.8, 2.5 and 4.0 km/sec overlying oceanic crust. The crust is observed to dip about two degrees towards the continent at the base of the slope. A model of subduction unique to the northern California margin is one whereby young crust is subducted slowly and quickly reheated so that no brittle portion remains at normal Benioff depths. Rapid sedimentation rates balance the subduction of the crust at the margin, preventing the formation of a deep trench. / Graduation date: 1981

Seismic properties (P, S velocities and Poisson's ratio) of young (0.75 m.y.) and mature (110 m.y.) oceanic crust are obtained by studying explosive refraction data collected in the Pacific Ocean using ocean bottom and downhole seismometers. A comparison of the results for the two regions indicates that the upper crustal velocities increase with age due to the cementation of cracks and fractures, the upper mantle velocities increase with age due to cooling, and the crust (mainly the lower crust) thickens with age. The Poisson's ratios obtained in this study are too small to be consistent with the presence of any serpentinization of the lower crust or upper mantle which therefore precludes upper mantle serpentinization as the cause for the thickening of the crust with age. When comparing seismic structures of young and mature oceanic crust with ophiolite models, we find close similarities between the Samail ophiolite and young oceanic crust, and between the Bay of Islands ophiolite and old oceanic crust. The 110 m.y. old crust of the northwest Pacific Basin is characterized by high velocity gradients in the upper crust, low velocity gradients in the lower crust, a smooth 1 km-thick crust-mantle transition zone and the presence of a minimum 14% anisotropy in the upper mantle compressional wave velocities. Velocities are highest in an east-west direction. The 0.75 m.y. old crust at the intersection of the East Pacific Rise and the Orozco fracture zone is characterized by a steady increase in velocity with depth. A delay time analysis shows a trend to large Layer 3 delay times in the Orozco fracture zone indicating a thicker Layer 2 and/or low Layer 2 velocities. An investigation of different model parameterizations for the tau-zeta travel time inversion using a synthetic data set indicates that the best velocity gradient solutions, based on the least deviation of the solution from the true model, are obtained from models in which the velocities of the layer bounds take on the values of the observed velocities of the refracted waves. A trade-off curve obtained from varying the number of layers in the model shows that a model with as many layers as observed data points represents a satisfactory compromise between model resolution and solution variance. / Graduation date: 1984

The analysis and modelling of magnetic anomalies at the conjugate rifted continental margins of the southern Iberia Abyssal Plain (TAP) and Newfoundland Basin have revealed that the sources of magnetic anomalies are distinctly different across both each margin and between the two margins. Analyses of synthetic anomalies and gridded sea surface magnetic anomaly charts west of Iberia and east of Newfoundland were accomplished by the methods of Euler deconvolution, forward and inverse modelling of the power spectrum, reduction-to-the-pole, and forward and inverse indirect methods. In addition, three near-bottom magnetometer profiles were analysed by the same methods in addition to the application of componental magnetometry. The results have revealed that oceanic crust, transitional basement and thinned continental crust are defined by magnetic sources with different characteristics. Over oceanic crust, magnetic sources are present as lava-flow-like bodies whose depths coincide with the top of acoustic basement seen on MCS profiles. Top-basement source depths are consistent with those determined in two other regions of oceanic crust. In the southern IAP, oceanic crust, ~4 km thick with magnetizations up to +1.5 A/m, generated by organized seafloor spreading was first accreted -126 Ma at the position of a N-S oriented segmented basement peridotite ridge. To the west, seafloor spreading anomalies can be modelled at spreading rates of 10 mm/yr or more. Immediately to the east, in a zone -10-20 km in width, I identify seafloor spreading anomahes which can only be modelled assuming variable spreading rates. In the OCT, sources of magnetic anomalies are present at the top of basement and up to -6 km beneath. I interpret the uppermost source as serpentinized peridotite, and the lowermost source as intruded gabbroic bodies which were impeded, whilst rising upwards, by the lower density serpentinized peridotites. Intrusion was accompanied by tectonism and a gradual change in conditions from rifting to seafloor spreading as the North Atlantic rift propagated northwards in Early Cretaceous times. Within thinned continental crust, sources are poorly lineated, and distributed in depth. Scaling relationships of susceptibility are consistent with the sources of magnetic anomalies within continental crust. OCT-type intrusions may be present in the mantle beneath continental crust. At the conjugate Newfoundland margin, seafloor spreading anomalies can be modelled at rates of 8 and 10 mm/yr suggesting an onset age consistent with that of the IAP. In the OCT there, I propose that magnetic anomalies are sourced in near top-basement serpentinized peridotites. An absence of magmatic material and the differences in basement character (with the IAP) suggest that conjugate margin evolution may have been asymmetric.

Thesis (Ph.D.)--University of Wyoming, 2007. / Title from PDF title page (viewed on June 17, 2009). Includes bibliographical references.

Thesis (Ph. D.)--University of Hawaii at Manoa, 2005. / Includes bibliographical references (leaves 150-166).

Thesis (Ph. D.)--Stanford University, 1982. / Includes bibliographical references (leaves 200-211).