Crustal velocity structure of the southern Nechako basin, British Columbia, from wide-angle seismic traveltime inversion

In the BATHOLITHSonland seismic project, a refraction wide-angle reflection survey was shot in 2009 across the Coast Mountains and Interior Plateau of central British Columbia. Part of the seismic profile crossed the Nechako Basin, a Jurassic-Cretaceous basin with potential for hydrocarbons within sedimentary strata that underlies widespread volcanic rocks. Along this 205 km-long line segment, eight large explosive shots were fired into 980 seismometers. Forward and inverse modelling of the traveltime data were conducted with two independent methods: ray-tracing based modelling of first and secondary arrivals, and a higher resolution wavefront-based first-arrival seismic tomography. Material with velocities less than 5.0 km/s is interpreted as sedimentary rocks of the Nechako Basin, while velocities from 5.0-6.0 km/s may correspond to interlayered sedimentary and volcanic rocks. The greatest thickness of sedimentary rocks in the basin is found in the central 110 km of the profile. Two sub-basins were identified in this region, with widths of 20-50 km and maximum sedimentary depths of 2.5 and 3.3 km. Such features are well-defined in the velocity model, since resolution tests indicate that features with widths greater than similar to 13 km are reliable. Beneath the sedimentary rocks, seismic velocities increase more slowly with depth - from 6.0 km/s just below the basin to 6.3 km/s at similar to 17 km in depth, and then to 6.8-7.0 km/s at the base of the crust. The Moho is found at a depth of 33.5-35 km beneath the profile, and mantle velocities are high at 8.05-8.10 km/s.