An airgun refraction line along the length of the caldera of Axial Seamount as recorded by three Ocean Bottom Seismometers has been analyzed using the tau-zeta inversion technique (Dorman and Jacobson, 1981). Five resulting velocity profiles show that the seismic velocities of the upper 1.4 km of the crust are low and similar to Mid-Atlantic Ridge upper crustal velocities. The low velocities and failure to observe shear waves are thought due to a combination of a thick section of pillow basalts and sheet flows, and a high porosity. The porosity is believed mostly due to a large fracture density. Hammond (pers. comm., 1986) has proposed the caldera is the site of overlapping spreading centers. A high fracture density in the upper crust of the caldera could be due to stresses generated in the area of overlap (Macdonald et al., 1984). No systematic variation in velocity structure along the length of the caldera could be resolved. Shallow water (about 1560 m), smooth bathymetry and the absence of sediments allowed direct measurement of the surface velocities of the caldera floor. An average surface velocity of 3.05 km/s is observed. Assuming a highly simplified crustal model, this velocity translates into a minimum porosity of 30% near the surface of the caldera floor. This minimum porosity is predicted to gradually decrease to about 0% at 1.4 km depth. Upper limits on the porosity can not be found with the available information. No magma chamber has been observed to a depth of 1.4 km. A compressional wave attenuation source in the northwest corner of the caldera below a depth of 1.4 km, however, might be a small magma body or alternatively an anomalously highly fractured area. / Graduation date: 1987

Thesis (Ph. D.)--University of Hawaii at Manoa, 1982. / Bibliography: leaves 182-202. / Microfiche. / xiii, 202 leaves, bound ill., maps, plates 29 cm

The Cocos Gap is a deeper portion, or saddle, of the Cocos Ridge and forms part of the western boundary of the Panama Basing It is probably typical of saddles within most submarine ridges, In order to determine the mechanisms controlling sediment dispersal, the nature and sources of the sediments at 23 core locations were defined by hydrodynamic size separation (> 63, 2- 63, <2 micron) and microscopic or X-ray diffraction analysis of the individual fractions. In addition, calcium carbonate, organic carbon, opal and quartz determinations were made for the total sediment. The silt sized fraction was resolved into eight textural modes, The coarse modes reflect the progressive breakage and winnowing of the corase fraction (foraminifera) under the influence of bottom currents and gravity. Above 2000 m mechanical breakdown, winnowing and relocation by bottom currents mask the effects of depth related dissolution of the carbonate fraction, Intermediate modes in general represent a transitional facies with both biogenic and terrigenous influences, while the finest modes characterize a distal regime of clay deposition, The clay fraction is amorphous material with very low percentages of well crystallized clays. Three main sources and transport paths were recognized, including one associated with the circulation of the Panama Basin. Sedimentation within the Gap is controlled by local processes, predominantly the interaction between tidally induced intensification of bottom water flow and directional (thermohaline) flow. The steep' ness of the sea floor slope is a major factor controlling the efficiency of winnowing of the sediment away from certain higher elevations (biogenic source areas) to the sheltered parts and flanks of the ridge. Superimposed upon this sediment dispersal is the influx of terrigenous material carried by directional bottom currents that operate as postulated upper and lower contour currents along the flanks of the ridge. The crest of the Cocos Gap acts as a catchment area for the biogenic components, while the adjacent more sloping region, the sub-plateau, acts as a source area. The extreme breakage of the foraminifera is most likely a function of the tidally induced intensification of the bottom water flow, characteristic of many shallow ridges, and is probably most significant in the subplateau. Hydrographic data indicates that there is no significant transport of bottom water across the Cocos Gap into the Panama Basin, but downslope transport of carbonate and siliceous fragments and minerals from the Gap into the basin is associated with cyclical tidal bottom water flow. / Graduation date: 1976

Thesis (Ph.D.)--University of Hawaii at Manoa, 1991 / Includes bibliographical references (leaves 177-195) / Microfiche. / xxii, 195 leaves, bound ill., maps 29 cm

Three folded maps in pocket. / Thesis (Ph. D.)--University of Hawaii at Manoa, 1991. / Includes bibliographical references. / Microfiche. / x, 173 leaves, bound ill. (some col.), maps 29 cm

We have determined the seismic crustal structure of the northern part of the Gorda Ridge using signals generated by explosive charges and recorded on Ocean Bottom Seismometers. The shot pattern forms two parallel lines, one on the east flank and the other along the median valley. Inversion of the travel time data and synthetic modelling of the signals resulted in two compressional velocity structures: the model for the flank indicates a 1.6 km thick upper crust characterized by high velocity gradients and 3.6 km thick lower crust characterized by a low gradient. A sharp mantle transition exists at 5.2 km depth with an upper mantle velocity of 7.6 km/sec. The median valley velocity model has a thicker high gradient upper crust of 3.0 km and a lower crust of at least 3.5 km thickness. No upper mantle velocities were detected beneath the median valley. We have also monitored the seismicity of the ridge during 15 days with two arrays of OBS and detected ~4 events/hour. Epicentral coordinates were determined for 140 earthquakes. Most of them lie within the median valley and show spatial clustering. Intraplate seismicity was also detected in the Gorda Basin with three of the earthquakes big enough to be reported by land stations. They suggest that the Gorda Plate is presently undergoing deformation. Good control over the focal depth was possible for ~80 earthquakes occurring on the ridge, and there the seismic activity appears to be pervasive throughout the upper 20 km suggesting that the the brittle lithosphere is at least this thick. From the earthquake shear-wave data we have obtained a value of 1.73 for the Vp/Vs ratio. Moments of the well constrained events derived from the spectra of the waveforms are of the order 10²⁰ dyne-cm and suggest an average fault width of 300 m. The refraction data is consistent with the earthquake results, and all the evidence suggests that a large magma chamber underlying the axis of spreading does not presently exist at shallow depths. / Graduation date:1985

Thesis (Ph. D.)--University of Hawaii at Manoa, 1995. / Includes bibliographical references (leaves 138-148). / Microfiche. / xiii, 148 leaves, bound ill., maps 29 cm

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Science, 1982. / Microfiche copy available in Archives and Science / Vita. / Bibliography: leaves 312-321. / by Anne Martine Tréhu. / Ph.D.