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The crustal structure of Winona Basin as determined by deep seismic sounding Lynch, Steven

Abstract

During August 1975, three reversed deep seismic sounding profiles were run over Winona Basin, a deep water sedimentary basin located west of the northern end of Vancouver Island. This thesis reports on the analysis of the data for reversed profiles 75-1 and 75-1R, which were recorded to distances of greater than 90 km along the axis of the basin. The seismic signals from explosive charges are detected by six individual hydrophones, amplified and recorded on a multi-channel digital acquisition system. The sub-critical reflection data and the refraction data are amplitude corrected for spherical spreading, amplifier gain, charge size and varying hydrophone sensitivity. Immediately prior to compilation into record sections, the data are bandpass filtered to improve their general appearance. Three methods of analysis were used to obtain velocity-depth information from the sub-critical reflection data; the ray parameter, T²-X² and the stripped T²-X² methods. A comparison of the results obtained from the three techniques shows that the ray parameter and T²-X² methods yield essentially the same result, whereas the stripped T²-X² method is of no use in analysing the data below the second sub-bottom layer. The analysis of profile 75-1H data at the northwest end of the basin gave a sedimentary structure divided into three prominent horizons, with velocities ranging from 1.7 to 2.1 km/s. The total depth to the basaltic layer was determined to be 1.8 km. This thickness is considerably less than that suggested by other authors on the basis of gravity studies and assumptions concerning continuous seismic profiles. Due to the presence of significant dips on the reflecting horizons in the southeast, no velocity-depth information could be obtained from the analysis of profile 75-1. Calculation of a preliminary velocity-depth model based on first arrival travel times was the first step in the analysis of the refraction data. These iso-velocity layered models provided an initial interpretive guide and a starting place for the calculation of synthetic seismograms. In order to utilize traveltimes of first and secondary arrivals and the relative amplitude characteristics of the seismograms, the final interpretation made use of synthetic seismogram sections for comparison with the real data. Based on such an analysis, the refraction data indicate that the crust underlying Winona Basin is separated into four sub-sediment layers, having significant velocity gradients. Average velocities and thicknesses for the layers are; 4.28 km/s, 1.6 km; 5.26 km/s, 2.75 km; 6.28 km/s, 4.13 km; 7.04 km/s, 3.76 km. The total sub-sediment thickness of this crustal section is thus 12 km. An unreversed mantle velocity of 7.8 km/s was interpreted from 75-1 results. On the basis of the velocity values, the layers have been identified with oceanic crustal layers 2a, 2b, 3a, 3b. The thick crust is postulated to be the result of complex plate interactions occurring in the region. A comparison of the data from this work with previous studies in the region of Explorer and Juan de Fuca Ridges has led to the speculative conclusion that Winona Basin has been created within the last 3 to 4 my, by the slow northward progression of the Pacific-North America-Explorer plate triple point.

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