CRUSTAL VELOCITY STRUCTURE IN THE SOUTHERN COAST BELT, BRITISH-COLUMBIA

We applied an iterative combination of two-dimensional traveltime inve rsion and amplitude forward modelling to seismic refraction data along a 350 km along-strike profile in the Coast Belt of the southern Canad ian Cordillera to determine crust and upper mantle P-wave velocity str ucture. The crustal model features a thin (0.5 - 3.0 km) near-surface layer with an average velocity of 4.4 km/s, and upper-, middle-, and l ower-crustal strata which are each approximately 10 km thick and have velocities ranging from 6.2 to 6.7 km/s. The Moho appears as a 2 km th ick transitional layer with an average depth of 35 km and overlies an upper mantle with a poorly constrained velocity of over 8 km/s. Other interpretations indicate that this profile lies within a collision zon e between the Insular superterrane and the ancient North American marg in and propose two collision-zone models: (i) crustal delamination, wh ereby the Insular superterrane was displaced along east-vergent faults over the terranes below; and (ii) crustal wedging, in which interfing ering of Insular rocks occurs throughout the crust. The latter model i nvolves thick layers of Insular material beneath the Coast Belt profil e, but crustal velocities indicate predominantly non-Insular material, thereby favoring the crustal delamination model. Comparisons of the v elocity model with data from the proximate reflection lines show that the top of the Moho transition zone corresponds with the reflection Mo ho. Comparisons with other studies suggest that likely sources for int racrustal wide-angle reflections observed in the refraction data are s tructural features, lithological contrasts, and transition zones surro unding a region of layered porosity in the crust.