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 a
tomographic 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 travel 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 ~6 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 (<12 km depth) crust
beneath western Hecate Strait. The average 1d velocity structure and the significant lateral
variations are interpreted in terms of regional geology. Wide angle reflection travel times
from
the Moho
constrain the thickness of the crust to be 29 km beneath the eastern coast of the QC Islands.
The Moho is deeper under the QC Islands than under Hecate Strait or QC Sound,
suggesting that crustal thinning during Tertiary extension was greatest beneath the surface
expression of the QC Basin. In an alternate or additional explanation, compression at the
plate margin during the last 4 Ma may have been taken up by thickening or underplating of
the continental crust adjacent to the margin beneath the QC Islands.
1993. Ph.D. thesis, Department of Geophysics and Astronomy, University of British Columbia, Vancouver.