In order to determine whether the sediments found on the landward wall of the Peru Trench are accreted Nazca Plate sediments, the clay mineralogy and organic carbon contents of 52 surface samples were submitted to factor analysis. Q-mode factor analysis resolved the data from the Nazca Plate and Peru continental margin into three factors. The most important factor (oceanic assemblage) is strongly associated with Nazca Plate sediments and is comprised of smectite and aeolian illite. In contrast, upper continental margin sediments are dominated by either of the two continental factors (A or B). The principal difference between the continental factors is that mixed-layer smectite-chlorite clays are characteristic only of continental assemblage A. Lower continental margin sediments are characterized by either an oceanic or continental factor dominance. The boundary between sediments dominated by the oceanic factor and those dominated by the continental factor was as much as 100 km to the west of its present position earlier in the Quaternary. The seaward shift in the boundary is attributed to westward shoreline displacement in response to glacially-induced sea level changes, increased erosion rates on land during more humid times, and deposition of continental factor dominated sediments seaward of the present Peru Trench axis. Quaternary sediments from 27 cores reveal minor fluctuations with time in factor loadings in Nazca Plate and upper continental margin cores and significant variations in some areas near the trench axis and on the middle to lower continental slope. Displacement of oceanic sediments into areas with continental sediments is determined with respect to the factor dominance boundary. Using this method, continental accretion is indicated for five cores, located up to 3000 m above the trench floor. One core on the middle continental slope off Lima, Peru, contains diatom-rich Quaternary dolomite that probably originated as calcareous sediment on the Nazca Ridge. If this is true, left-lateral strike-slip motion of the Nazca Ridge along the Peru Trench axis is indicated. The bulk of the 28 cores recovered from the acoustically complex landward wall of the Peru Trench contain sand-silt turbidites of continental origin. Even though there is a distinct overprint of terrigenous sedimentation, accreted oceanic sediments can be recovered in a tectonically active convergent plate boundary. / Graduation date: 1974

Major (Fe, Mn, Al) and minor (Cr, Mo, Pb, Zn, Cd) element analyses of metalliferous sediments in cores from the East Pacific Rise and Bauer Deep indicate that the Bauer Deep sediments are similar in chemistry and origin to metalliferous sediments of the East Pacific Rises. Fe, Pb, and Zn are strongly associated in both cores and are probably related to the hydrothermal processes which are postulated to occur on the East Pacific Rise. Incomplete Cd data suggest that Cd may also be released during hydrothermal leaching of basalt Mn may originate by precipitation from hydrothermal solutions or by normal authigenic precipitation from sea water. The similar accumulation rates of Mo in both cores, and poor correlation of Mo with Fe imply that Mo is being extracted from sea water. The low abundances of Cr and Al in metalliferous sediments suggest that detritus is a minor component of the sediments. Poorly crystalline smectites constitute a major mineralogical phase of metalliferous sediments. Chemical and mineralogical studies indicate that the smectite is an iron-rich montmorillonite, similar in composition to the bulk sediment. Information on sedimentation rates in the two cores indicate that most elements are accumulating faster on the East Pacific Rise than in the Bauer Deep. Accumulation rates of metals in the Bauer Deep have decreased up to the present and are currently similar to those for normal pelagic sediments. It is inferred that the sedimentation rates in the Bauer Deep are influenced by the proximity of the East Pacific Rise, It appears that sediments in the Bauer Deep are largely the product of hydrothermal processes on the East Pacific Rise, and that the precipitates are transported in suspension to the Bauer Deep. Authigenic precipitation of elements from sea water also occurs, having a stronger influence on the sediments as the precipitation of rise-crest material decreases away from the ridge. / Graduation date: 1974

A total of 57 core top samples from the Panama Basin were used in a quantitative study of complete radiolarian thanatocoenoses to determine whether surface oceanographic conditions are reflected in the microplankton faunas deposited onto and buried in the sea floor. Information obtained from this study was used to resolve the sequence of temporal fluctuations in oceanographic and climatologic conditions in the eastern equatorial Pacific. Four piston cores from within the basin were sampled on the basis of carbonate stratigraphies to levels no older than 130,000 years B.P. Although the sample intervals are irregular and samples are not closely spaced, this procedure allowed reasonably good stratigraphic resolution. Because of high diversity at low latitudes, the 131 radiolarian taxa recognized in this study accounted for only 37-61% of the individuals encountered in counts of >1000 specimens. Q-mode factor analysis of the surface sediment samples yielded three end member samples, which allowed recognition of three different assemblages. The dominant assemblage can be identified with a tropical fauna and the least important assemblage is considered to be associated with the Peru Current fauna, The distribution of the tropical assemblage does not reflect surface oceanographic conditions whereas the distribution of the Peru Current assemblage shows a weak association with surface conditions, The other assemblage shows no coherent distribution pattern. The nature of this assemblage is best explained by considering it to be a residual of the tropical fauna resulting from chemical and mechanical modification in the water column and at the sea floor. All evidence suggests that solution, winnowing and lateral transport are major processes in determining the distribution of the faunal assemblages in the sediments of the Panama Basin. The signal of the Peru Current assemblage in the surface study was used for a quantitative analysis of the subsurface samples. This analysis yielded the following results: 1) During most of the last glacial, assemblages associated with the Peru Current.were deposited in the Panama Basin, indicating a northward shift of current boundaries, This has important climatological implications since such a shift must be associated with a shift of the major wind belts and of the South Pacific high pressure cell towards the equator. 2) At least once during, the last glacial, from approximately 50,000 to 40,000 years B.P., warmer conditions are indicated by a re-appearance of tropical assemblages. 3) The previous interglacial appears to have been slightly cooler than present interglacial conditions. 4) The warming trend towards present conditions began between 15,000 and 20,000 years B.P. This is earlier than observed in the North Atlantic. These results are correlative with records of oceanographic and climatic changes in the southeast Pacific, northwestern South America and the Galapagos Islands. / Graduation date: 1974

A study has been made of the vertical distributions and migrations of a large number of zooplankton species at Weather Station "P" in the Subarctic Pacific. Simultaneously towed horizontal opening-closing nets were used for the study. The distributions and migrations of 104 taxa have been subjectively grouped into seven basic patterns. A few taxa could not be so grouped. Examination of hydrographic features reveals correlations between animal distributions and strong hydrographic gradients. It is concluded that: 1. In boreal oceanic waters, few animals perform diurnal migrations, 2. Depth ranges for most zooplankton are on the order of hundreds of meters, and 3. Hydrographic features may influence the vertical distributions and migrations of zooplankton. / Graduation date: 1974

Moored instrument records, drogue displacements, and hydrographic observations are used in describing the coastal currents and upwelling off Peru. The data were obtained over the continental shelf near 15°S. during a two week study in late March and early April of 1969. First order statistics and graphical representations of current meter time series indicate that the longshore flow was poleward during most of the study period, interrupted by a three day 'event' of equatorward flow. The similarity of flow at all current meters indicates that the field of flow as quasi-barotropic. The depth, extent, and transport of poleward flow indicated by current meter time series and geostrophic sections were similar to those described in the literature for the Peru-Chile Undercurrent. The observations suggest that this flow moved further offshore as equatorward flow appeared over the shelf. Power spectral analyses performed on current meter records indicate the existence of semidiurnal tidal currents in the longshore direction. The magnitude of these currents is estimated at 10% to 15% of period mean speeds. Ten meter drogue displacements are compared with 25 m recorded currents and with winds. The observations indicate that: the drogues were affected by both the 25 m flow and the wind; the depth of the wind drift layer was between 10 m and 25 m; the drogue displacements were in the sense expected from the Ekman model. Vertical sections of sigma-t, oxygen, and nitrate indicate the existence of conditions consistent with upwelling. Surface maps of temperature, nitrate, and chlorophyll 'a' over the shelf are used to define the horizontal field of upwelling and its variations in time. The distributions suggest that upwelling existed throughout the period and underwent temporal and spatial modulations in intensity. The possibility of a causal mechanism between observed current and upwelling variations is examined. Vertical salinity sections indicated the presence of a weak salinity minimum between the surface and 100 m. It is suggested that this minimum manifests the remnants of a tongue of Subantarctic Water embedded in a much larger mass of Equatorial Subsurface Water. The occurrence of the minimum only in conjunction with poleward flow suggests that the water was advected or mixed coastward somewhere north of the area studied, was entrained in the Peru-Chile Undercurrent, and was carried south again. / Graduation date: 1970

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

A free-air gravity anomaly map of the continental margin of Peru between 12° and 18° S. Lat. shows a -110 to -220 mgl anomaly associated with the Peru-Chile Trench, a -60 mgl anomaly over the Pisco Basin on the continental shelf, and -120 mgl anomaly over the Mollendo (or Arequipa) Basin on the upper continental slope. Anomalies observed over the continental slope and shelf consist of slope and basin anomalies superposed on a very large, broad regional anomaly. The approximately zero mgl anomaly observed in the region of the Nazca Ridge indicates the ridge is isostatically compensated. A structural model constrained by the observed gravity anomalies and seismic refraction data indicates that compensation is due to a crust approximately 8 km thicker and about 0. 04 g/cm³ less dense than the oceanic crust on either side of the Nazca Ridge. Gravity anomalies are consistent with mass distributions expected at the Peru-Chi1e Trench as a consequence of subduction of the Nazca Ridge and the Nazca Plate. Crustal and subcrustal cross sections constrained by free-air gravity anomalies, seismic refraction data, and geologic information indicate approximately 2 km of crustal thinning seaward of the trench on the southeast side of the Nazca Ridge but no crustal thinning on the northwest side of the ridge. Crustal thickness increases from approximately 10 km near the trench to about 25 to 30 km under the southwestern flank of the Andes and to approximately 70 km under the Andes. The crust is inferred to be 33 km thick under the Amazon Basin. A cross section north of the Nazca Ridge suggests a rupture of the crust at depth under the coast mountains, and earthquake hypo centers projected onto this cross section indicate a relatively shallow, nearly horizontal Benioff zone under the Andes and the Amazon Basin. A cross section south of the Nazca Ridge does not show these features, hence a different subduction process on each side of the Nazca Ridge is indicated. Free-air gravity anomalies indicate a structural high extending northwest from 17° S. Lat, along the coast, the Paracas Peninsula and nearly 100 km offshore along the edge of the continental shelf. Computations based on gravity data suggest the Pisco Basin immediately east of this structural high contains approximately 2. 2 km of sediment. A similar computation for the Mollendo Basin yields a sediment thickness of approximately 1.4 km. Gravity anomaly patterns are consistent with uplift beneath the continental shelf edge and upper slope and suggest a continental margin composed of compacted, dewatered sediments of both continental and oceanic origin. / Graduation date: 1976

In April of 1974, Oregon State University conducted a geophysical survey of the Orozco fracture zone, a Left-lateral transform fault which offsets the East Pacific Rise off the coast of Mexico near 15°N, 105°W. Magnetic, gravity, bathymetric, and seismic reflection data were collected during a four day period. This survey is combined with previous surveys by Oregon State University and other institutions to provide a geophysical interpretation of the Orozco fracture zone and the surrounding area and to develop a tectonic history of the northern Cocos plate. The Orozco fracture zone is characterized by a typical zone of seismicity and an offset in the magnetic anomaly pattern. There is, however, a conspicuous absence of a well defined topographic trough. This appears to be a result of the small age offset of the ridge crest, a reorientation of the fracture zone trend, and a possible southward migration of the fracture zone down the ridge axis. Three crustal and subcrustal cross sections over the Orozco fracture zone are constructed from the gravity data. One, across the active portion between the ridge offset, shows the active troughs to be underlain by a broad, low-density root extending two kilometers into the mantle. Two gravity cross sections across the East Pacific Rise show a thinning of oceanic layer 3 of nearly 2 kilometers at the rise crest and a corresponding 0.5 kilometer thickening of layer 2. A large magnetic anomaly of over 1300 gammas is found at the intersection of the Orozco fracture zone and the East Pacific Rise. A comparison with a very similar observation at the intersection of the Juan de Fuca ridge and the Blanco fracture zone in the northeast Pacific suggests that the East Pacific Rise is "leaking" into the fracture zone in this area. Many features have been observed on the northern Cocos plate which cannot be accounted for by present Pacific-Cocos motion the northeast strike of the eastern extension of the Orozco fracture zone, an apparent fanning of magnetic anomalies, and the northeast strike, as well as the origin, of the Tehuantepec ridge. Several possible schemes are examined to explain these observations and all but one are completely eliminated. The proposed explanation supposes a reorientation of the spreading center after a large change in the Pacific- Cocos pole of rotation resulting in the Zed pattern described by Menard and Atwater (1968). / Graduation date: 1976

University deployed two arrays of three sonobuoys over the Blanco fracture zone and two arrays of four sonobuoys over the Gorda ridge to detect and locate microearthquakes. Microearthquake activity predicted by plate tectonic theory for the Blanco fracture zone and Gorda ridge was observed by these arrays to originate from these features. Microearthquake activity of one event per hour was observed along the southern flank of the ridge associated with the Blanco fracture zone near 128°10'W. Similar seismicity was observed in Cascadia Gap and associated with the fault scarp and basin of the gap. Cascadia Gap, which is located near the center of the Blanco fracture zone, exhibits ridge-like seismic activity based on a b-value of 1.5 determined from microearthquake observations. The seismic activity on the Gorda ridge, including swarm events, averaged 3.5 events per hour. The microearthquakes originated from the median valley floor, valley walls, and on top of the crestal hills. Other events, which could not be located, appeared to originate from the surrounding hills with a predominance of events from west of the Gorda-Blanco intersection. Focal depths at the intersection area are 6.5 to 9 km below a 3.5 km datum, while those farther to the south at 42°41'N range from 2.5 to 3 km below the datum. A composite fault plane solution for the events on the Gorda ridge indicates high angle faulting on the eastern valley wall with the eastern side down. Since microearthquake and earthquake activity are usually closely associated, the data obtained by this study suggests that the 20-30 km eastward offset of large earthquake epicenters from the topographic features is due to an incorrect use of excessively high lithospheric velocities for seismic stations lying predominantly to the east. / Graduation date: 1976

Graduation date: 1962