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Structure of the Queen Charlotte Basin and underlying crust from modelling and inversion of three-dimensional seismic refraction data Hole, John A.

Abstract

A combined seismic reflection and refraction survey was carried out in 1988 to investigate the structure and tectonic evolution of the Queen Charlotte (QC) Basin and underlying crust off the northern coast of British Columbia. While the marine multichannel reflection data were being collected, refracted and wide-angle reflected energy from the large airgun array were recorded at surrounding land sites in both inline (2d) and broadside (3d) geometries. The broadside refraction data recorded on the QC Islands provide good 3d coverage beneath western Hecate Strait. In this study, these data are interpreted to determine the 3d structure of the basin and underlying crust. Modelling procedures are developed to interpret densely sampled 3d seismic travel time data. An inversion algorithm to determine the depth of a refracting interface and atomographic inversion algorithm to determine velocity structure are described. Travel times for 3d models are computed using a rapid finite difference algorithm that is extended to allow large velocity contrasts, the determination of rays, variable sampling of the model, and the computation of reflection times. The inversion in both algorithms is parameterized in a simple manner that eliminates the need to store or solve large systems of linear equations. Iterative nonlinear procedures allow arbitrarily large 3d anomalous velocities and interface structures to be determined. The advantages in computational speed of the procedures allow dense spatial sampling of the models, providing spatially well-resolved 3d images. Variations of the first arrival travel times from the broadside refraction data are inverted to determine the 3d basement depth structure of the QC Basin. The thickness of the basin varies rapidly between ,,,200 m and km in a complex sequence of 3d fault-bounded subbasins. The orientation of, and topography across, several major faults and the overall complexity of the subbasins support a distributed strike-slip extension evolutionary model for the basin. The first arrival travel times are then inverted to determine the 3d velocity structure of the upper (

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