Geophysical data collected in 1975 and 1976 reveal major crustal and tectonic elements of the continental margin of southern Baja California. Gravity, magnetic, seismic reflection and bathymetric data show seaward extension of the islands enclosing Magdalena and Almejas Bays. A seismic reflection profile, oriented approximately normal to the trend of the Baja peninsula, indicates normal faulting of the near surface sediment layers along the outer continental shelf. The reflection record also shows that sediment layers immediately above the acoustic basement dip toward the east at the base of the continental slope. A crustal and subcrustal cross section, oriented approximately parallel to the reflection profile and constrained by gravity, magnetic, bathymetric and seismic refraction data, indicates a maximum crustal thickness of approximately 21 km for Baja California, making it intermediate in thickness between normal continental and normal oceanic crusts. The section also indicates a low density zone in the mantle below the Gulf of California. Magnetic anomalies along the cross section require oceanic crust of the Pacific Plate to extend at least 50 km landward of the edge of the western continental shelf of Baja California. This suggests either a past period of oblique subduction of the Pacific Plate beneath Baja California or emplacement of Pacific Plate oceanic crust beneath the peninsula by descending spreading centers of the East Pacific Rise. / Graduation date: 1977

Seismic refraction, reflection and gravity data obtained across the Peru continental margin and Nazca Plate at 9° S. permit a detailed determination of crustal structure. Complex structures normal to the profile require the development of a ray trace technique to analyze first and later arrivals for eleven overlapping refraction lines. Other data integrated into the seismic model include velocities and depths from well data, near surf ac sediment structures from reflection profiles and velocities obtained from nearby common depth point reflection lines. Crustal and subcrustal densities and structures were further constrained by gravity modeling to produce a detailed physical model of a convergent margin. The western portion of the continental shelf basement consists of a faulted outer continental shelf high of Paleozoic or older rocks. It is divided into a deeper western section of velocity 5.0 km/sec and a shallower, denser eastern section of velocity 5.65 to 5.9 km/sec. The combined structure forms a basin of depth 2.5 to 3.0 km which contains Tertiary sediments of velocity 1.6 to 3.0 km/sec. In this area, near-surface sedimentary structure suggests truncated sinusoidal features caused by exposure to onshore-offshore bottom currents. The 3 km thick, 4.55 to 5.15 km/sec basement of the eastern shelf shoals shoreward. Together, this basement and the eastern section of the outer continental shelf high form a synclinal basin overlain by Tertiary sediments which have a maximum thickness of 1.8 km and a velocity range of 1.7 to 2.55 km/sec. The gravity model shows a large block of 3.0 g/cm³ lower crustal material emplaced within the upper crustal region beneath the eastern portion of the continental shelf. Refraction data indicates a continental slope basement of velocity 5.0 km/sec overlying a slope core material with n interface velocity of 5.6 km/sec. The sedimentary layers of the slope consist of an uppermost layer of slumped sediment with an assumed velocity of 1.7 to 2 km/ sec which overlies an acoustic basement of 2.25 to 3.6 km/ sec. The high velocities (and densities) of the slope basement suggest the presence of oceanic crustal material over lain by indurated oceanic and continental sediments. This slope melange may have formed during the initiation of subduction from imbricate thrusting of upper layers of oceanic crust. Once created, the melange forms a trap and forces the subduction of most of the sediments that enter the trench. A ridge-like structure within the trench advances the seismic arrival times of deeper refractions and supports the suggestion that it is thrust-faulted oceanic crust which has been uplifted relative to the trench floor. The model of the descending Nazca Plate consists of a 4 km thick upper layer of velocity 5.55 km/sec and a thinner (2.5 km) but faster 7.5 km/sec lower layer which overlies a Moho of velocity 8.2 km/sec. The gravity model indicates that the plate has a dip of 5° beneath the continental slope and shelf. West of the trench, the lower crustal layers shallow, which may represent upward flexure of the oceanic plate due to compressive forces resulting from the subduction process. The upper crustal layers of the 120 km long oceanic plate portion consist of a thin 1.7 km/sec sedimentary layer overlying a 5.0 to 5.2 km/sec upper layer. An underlying 5.6 to 5.7 km/sec lower layer becomes more shallow to the east within 60 km of the trench while a deeper 6.0 to 6.3 km/sec layer thickens to the east. The lower crustal model consists of a 7.4 to 7.5 km/sec high velocity layer which varies in thickness from 2.5 km to 4.0 km. The 8.2 km/sec Moho interface varies not more than ±0.5 km from a modeled depth of 10.5 km. / Graduation date: 1979 / Best scan available for figures.

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Seismic refraction measurements along two unreversed lines indicate that the earth's crust is 26 km thick in southeastern Alaska and 30 km thick along the Inside Passage of British Columbia. The crust in southeastern Alaska, north of Dixon Entrance, consists of a layer 9 km thick with a seismic velocity of 5.90 km/sec, a layer 7 km thick with a seismic velocity of 6.30 km/sec. and a layer 10 km thick with a seismic velocity of 6.96 km/sec. The crust along the Inside Passage of British Columbia, south of Dixon Entrance, consists of a layer 13 km thick with a seismic velocity of 6.03 km/sec, a layer 5 km thick with a seismic velocity of 6.41 km/sec, and a layer 12 km thick with a seismic velocity of 6.70 km/sec. The velocity of the mantle below the M discontinuity is 7.86 km/sec in southeastern Alaska and 8.11 km/sec in British Columbia. A compilation of Bouguer gravity data along the Inside Passage from northern Vancouver Island to northern southeastern Alaska indicates near-zero anomalies between steep gradients offshore and near the western margin of the Coast Mountains. A two-dimensional gravity model, constrained by seismic refraction measurements, suggests that the thickness of the crust is constant beneath the region of near-zero gravity anomalies and indicates a step-like transition between oceanic and continental structure. Seismic reflection, gravity, and magnetic measurements, obtained during a 1970 cruise of the R/V Yaquina, help to determine upper crustal structures in Dixon Entrance. Gravity models, constructed to agree with these data and the measurements of previous investigators, indicate sediment thicknesses of nearly 3 km east of Learmonth Bank and west of Celestial Reef. Magnetic models suggest large lateral changes in basement susceptibility. Either highly metamorphosed rock or basaltic intrusions can account for these changes in susceptibility. Folded sediments suggest post depositional distortion due either to regional compression or to major local intrusions. Several linear gravity features, observed in northern Dixon Entrance, disappear north of Graham Island. Either the structures responsible for the gravity features end or thick layers of basalt, extending northward from Graham Island, obscure the effect of the structures. A single-station survey detected microearthquakes at nine locations in western British Columbia and southeastern Alaska. The majority of the observed distant microearthquakes probably originated in the Queen Charlotte Islands fault zone. However, observed nearby microearthquakes indicate a microearthquake seismicity of several events per day along the mainland coast of British Columbia. Temporary seismic arrays located at a site along the central portion of Chatham Strait near the Chatham Strait fault and at a site in Glacier Bay recorded few nearby microearthquakes. Arrivals at the arrays permitted the location of distant microearthquakes, however, with epicenters in the vicinity of northern Lynn Canal and along the Fairweather fault. / Graduation date: 1972

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