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Seismic and potential field constraints on the shallow crustal structure of inner Bering shelf, offshore southwestern Alaska

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Thesis type
(Thesis) Ph.D.
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South-western Alaska comprises a collection of major fault bounded tectonostratigraphic terranes that were accreted during Mesozoic and early Tertiary time. To characterize the offshore extension of these terranes and the various major faults identified onshore, I have reprocessed three intersecting multichannel deep seismic reflection profiles totalling ~750 line-km that were shot by the R/V Ewing across part of the inner Bering continental shelf in 1994. Since the uppermost seismic section is often contaminated by high amplitude water layer multiples from the hard and shallow seafloor, I have supplemented the migrated reflection images with high-resolution P wave velocity models derived by both traveltime tomography and waveform tomography of the recorded first-arrivals to depths of 2000 m. In addition, I have also incorporated other geophysical datasets such as: well logs, ship-board gravity, ship-board magnetics, satellite-altimetry gravity and air-borne magnetics to produce an integrated interpretation. The results delineate the offshore extension of the major geological elements onshore including: the Togiak-Tikchik fault, East Kulukak fault, Chilchitna fault, Lake Clarke fault, Togiak terrane, Goodnews terrane, Peninsular terrane, Northern and Southern Kahiltna flysch deposits, and the Regional Suture Zone.This thesis also focuses on mitigation of guided wave contamination in the application of 2-D acoustic waveform tomography to two small sections of a seismic line across the shelf: one section with deep igneous basement overlain by a thick pile of sediments and the other section with shallow basement and a thin sedimentary cover. I discuss the appearance of dispersive guided waves in both datasets, and show that with appropriate data preconditioning, suitable forward modelling parameters and careful choice of objective function, it is possible to invert the data using 2-D acoustic waveform tomography. I highlight use of the gradient image as a quality control tool at each iteration of the inversion process to assess the corresponding model updates. This thesis also discusses the practical limitations of waveform tomography applied to seismic data contaminated with strong dispersive guided waves, and concludes that traveltime tomography is sufficient in cases where the objective is to image relatively large low-velocity anomalies.
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Supervisor or Senior Supervisor
Thesis advisor: Calvert, Andy
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