Inversion of three-dimensional wide-angle seismic data from the southwestern Canadian Codillera

B. C. Zelt, R. M. Ellis, R. M. Clowes, and J. A. Hole

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 mantle. 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 traveltimes and forward modeling of reflection traveltimes, including: (i) forward modeling using a 3-D finite-difference algorithm; and (ii) 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: (i) the inversion of reflection times to solve for depth to a reflecting interface and/or velocity structure; (ii) the inversion of first arrival traveltimes to solve for depth to a refracting interface; and (iii) 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 (i) significant lateral velocity variations at all depths that do not, in general, correlate with surface geological features or gravity data; (ii) 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; (iii) 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; (iv) an average upper mantle velocity of 7.85 km/s; and (v) 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.

1996. Journal of Geophysical Research, 101, 8503-8529.

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