Previous experiments to record seismic data at wide angle on the continental shelf have generally been unsuccessful in determining velocity structure in the lower crust; either the lines were too short or shot-receiver density too sparse to identify lower crustal arrivals. In contrast, deep normal incidence profiles show good structural resolution in the crust and uppermost mantle. This dissertation describes a new sea-bottom multichannel instrument which was developed to record datasets containing closely spaced traces to improve the resolution of reversed wide-angle experiments on the continental shelf. The Pull-Up Multichannel Array (PUMA) is a 1200 m, 12 channel, hydrophone array for remotely recording seismic data on the sea-bed. It consists of 12 short hydrophone sections linked by 100 m long passive sections. A pressure case is attached at one end in which recording electronics, cassette tape recorders and a battery power supply are housed. The PUMA is deployed in less than 200 m of water from a research ship and moored to buoys for recovery. The instrument, which was successfully used in an experiment west of Lewis, Outer Hebrides, U.K., was specifically designed to determine a well constrained velocity structure for the crust and uppermost mantle over part of the BIRPS WINCH deep, normal incidence profile. Data recorded by the PUMA show a high signal to noise ratio and it is easy to correlate phases across the record section and to monitor changes in amplitude because traces are closely spaced. A velocity structure for the continental crust and uppermost mantle has been devised using amplitude modelling. The model is interpreted to show that: the upper crust consists of Lewisian gneiss metamorphosed in the amphibolite facies, and contains a low velocity zone which is probably granitic in composition; the middle crustal layer has virtually no velocity gradient; and the lower crust, represented by second arrivals, contains a high velocity gradient and probably consists of granulites. The Moho is at 27.0 ± 0.5 km depth. This is shown to be a layered boundary and it is suggested that this layering is caused by crustal underplating. There is little velocity gradient in the uppermost mantle. This model shows good agreement with the BIRPS WINCH section, although the Moho appears to coincide with the top rather than the base of a band of reflections at 8.3 s two-way travel time on unmigrated WINCH data.

No description available.

No description available.

Natural radionuclides may be indicators for dense mineral placers along marine shorelines. Relict beach and river deposits occur in continental shelf sediments. These deposits result from the reworking of beach sands by wave action during the Holocene Transgression. Some dense, resistant minerals associated with placer deposits are known to contain ²³⁸U and ²³²Th activities. Shelf sediments, enriched in heavy minerals, might be expected to be high in these natural radioactivities. The usefulness of natural radioactivity to locate and to map dense mineral deposits was therefore explored and relations between natural radioactivity in marine sediments and dense mineral content were established. High ⁴⁰K activity in sediments was positively correlated significantly with high mud content. High ²³⁰Th and ²³²Th activities were associated with fine, well-sorted sands and correlated very highly with the weight percent total heavy mineral sands in the sediment. These relations distinguish depositional environments as high in ⁴⁰K content and erosional environments as high in ²³⁰Th and ²³²Th content. Two areas of enrichment were found by radiometric mapping and by mineral analyses. Off Destruction Island, Washington, maximum ²³⁰Th and ²³²Th activities were found in 30 m water depth. Activity distribution and mineral enrichment were consistent with general northward transport of nearshore sediment. In 33 m water depth off Clatsop Spit, Oregon, the second enrichment area was found. The ²³²Th/²³⁰Th activity ratios are different for these two areas. Probably the heavy minerals in these two areas are from different sources. / Graduation date: 1975

Data collected by personnel of the Geophysics Group at Oregon State University and the Instituto Oceanografico of the Direccion General dé Oceanografia are used to construct a free-air anomaly map for the central part of the Pacific continental margin of Mexico, which shows the gravity expression of major structural features in the region. Specific features are the Middle America and Rivera trenches intersected by a possible extension of the Rivera Fracture Zone, and the Ulloa Trough on the western side of Baja California. Of particular importance are gravimetric minimums with no bathymetric expression observed east of the Tres Marias Islands, southeast and northwest of the Tamayo Fracture Zone, and as extensions of the bathymetrically mapped series of en echelon faults within the Gulf of California. A geophysical model cross section constructed along a profile southeast of the Tres Marias Islands indicates a continental type crust east of the islands with a Moho depth of 13 km at the base of the Tres Marias Scarp and dipping toward the continent. The structure at the transition zone between oceanic and continental crust suggests active subduction of the oceanic crust in the past followed by a period of shear or strike-slip motion. The thickness of the sediments along the cross section and east of the Tres Marias Islands reaches 1.5 km. / Graduation date: 1981

Gravity, magnetic and seismic data indicate that the oceanic crust is 9.7 km thick west of the Continental Borderland. The top of the mantle is about 12 km deep under the Borderland, and deepens to 27 km beneath the Peninsular Ranges of Baja California. The mantle is about 20 km below the surface of the Imperial Valley and deepens to 27 km under the area east of the Imperial Valley. The age of the youngest detectable remnant magnetic anomaly over the oceanic crust is about 16.5 million years at 21.3°N Lat. and decreases to the south. A magnetic anomaly expected along the continuation of the San Benito Fault Zone is not detected by this study. A gravity low along the base of the Patton Escarpment is at least partially the result of a buried trench-like depression. In the vicinity of 31.3°N Lat., 119.3°W Long. this depression is filled with 2 km of sediments. The geophysical and geological data are interpreted as indicating a 6 km thick section of Franciscan rocks that extends from the west edge of the Borderland to the Coronado Escarpment. Magnetic data suggest that an ophiolite may be present within or on top of the Franciscan rocks. Several of the ridges in the Borderland have cores of high density rocks which are interpreted as intrusives. The area just south of the San Clemente basin has an anomalously thin upper crust. The gross crustal structure of this region is comparable to the Imperial Valley region and may represent a former site of crustal rifting which occurred when the East Pacific Rise was subducted under this part of the North American plate. North of the Santo Tomas Fault Zone are several basins filled with more than 3 km of sediments, but south of this fault zone the sediment cover is discontinuous and generally less than 2 km thick. / Graduation date: 1978

Thesis (Ph. D.)--University of Washington, 2001. / Vita. Includes bibliographical references (leaves 117-134).

Diss--Giessen. / Lebenslauf. "Literaturverseichnis": p. 3-6.

Thesis (Ph. D.)--University of Texas at Austin, 2002. / Vita. Includes bibliographical references. Available also from UMI Company.

Thesis (M.S. in Physical Oceanography)--Naval Postgraduate School, December 2003. / Thesis advisor(s): Thomas H.C. Herbers, Edward B. Thornton. Includes bibliographical references (p. 71-73). Also available online.