Crustal seismic velocity and density structure of the Intermontane and Coast belts, southwestern Cordillera

Abstract

Seismic refraction - wide-angle reflection data were recorded along a 450 km profile across the Intermontane, Coast, and Insular belts of the Canadian Cordillera. Crust and upper mantle structure was interpreted from traveltime inversion and forward-amplitude modelling, and the resultant seismic velocities were used to constrain modelling of the Bouguer gravity data along the profile. A high-velocity, high-density block in the upper 8 km of crust was interpreted as the subsurface extension of Harrison terrane; the Harrison fault at its eastern boundary may extend to at least 8 km depth and perhaps 20 km. Throughout the crust, both seismic velocities and densities are in general high beneath the Insular belt, low beneath the Coast and western Intermontane belts, and intermediate beneath the eastern Intermontane belt. However, densities are unusually low in the lower crust beneath the Coast belt (2800 kg/m3), relative to velocities (6.6-6.8 km/s). This indicates that Coast belt plutonic material is present throughout the crust; strong upper mantle reflectivity, previously interpreted on a Lithoprobe reflection line beneath the western Coast belt, may be high-density residue associated with the unusually low density plutonic material. Based on gravity data, Wrangellia must terminate sharply against the western edge of the Coast belt. In the lower crust, the lowest seismic velocities are found vertically beneath the surface trace of the Fraser fault, where velocities just above the Moho only reach 6.5 km/s, in contrast with 6.8 km/s beneath the western Coast belt and eastern Intermontane belt. This provides support for a subvertical geometry for the Fraser fault, perhaps with a broad zone of diffuse shearing in the lower crust. At this location, the Fraser fault does not appear to vertically offset the Moho, which is well-constrained at a uniform depth of km east of the Harrison fault.