Article,
Inversion of three-dimensional wide-angle seismic data from the southwestern Canadian Cordillera
Journal of Geophysical Research, 101 (B4): 8503--8529 (April 1996)
DOI: 10.1029/95JB02807
Abstract
Seismic refraction/wide-angle reflection data were recorded on a triangular
array in southwestern British Columbia centered on the boundary between
the Coast Belt to the southwest and the Intermontane Belt to the
northeast. The experiment, part of the Lithoprobe Southern Cordillera
transect, enabled determination of the three-dimensional (3-D) velocity
structure of the crust and upper mantie. An algorithm for the inversion
of wide-angle seismic data to determine 3- D velocity structure and
depth to reflecting interfaces is developed. The algorithm is based
on existing procedures for the inversion and forward modeling of
first arrival travel times and forward modeling of reflection travel
times, including (1) forward modeling using a 3-D finite difference
algorithm; and (2) a simple velocity model parameterization for the
inversion which eliminates the need to solve a large system of equations.
The existing procedure is extended to allow (1) the inversion of
reflection times to solve for depth to a reflecting interface and/or
velocity structure; (2) the inversion of first arrival travel times
to solve for depth to a refracting interface; and (3) layer stripping.
Application of the algorithm to southern Cordillera data uses Pg
to constrain upper crustal velocity structure, PmP to constrain lower
crustal velocity structure and depth to Moho, and Pn to constrain
upper mantle velocities and depth to Moho. The 3-D velocity model
for the southwestern Canadian Cordillera is characterized by (1)
significant lateral velocity variations at all depths that do not,
in general, correlate with surface geological features or gravity
data; (2) a relatively high velocity middle and lower crust in the
southwestern part of the study area which correlates with a strong
relative gravity high and outlines the eastern extent of lower Wrangellia,
an accreted terrane forming the Insular Belt to the west; (3) a narrow
zone of slower velocity in the lower crust and change in crustal
thickness associated with the Fraser Fault system, lending additional
support to the view that it is a crustal penetrating fault; (4) an
average upper mantle velocity of 7.85 km/s; and (5) a depth to Moho
of 33-36 km in the Intermontane Belt and 36-38 km throughout most
of the Coast Belt, decreasing in the west to 33 km near the Insular-Coast
contact. Horizontal velocity structure slices and an interpreted
cross section based on these and other results show the complexity
of crustal structure in the region.
