Crustal velocity structure of the southern Nechako basin, British Columbia, from wide-angle seismic traveltime inversion¹This article is one of a series of papers published in this Special Issue on the theme of New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia.

Abstract

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 ∼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 ∼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.

Deep seismic reflection constraints on Paleogene crustal extension in the south-central Intermontane belt, British Columbia

“…In a velocity-depth function from the Batholiths profile (Fig. 2) representative of the Chilcotin plateau (Stephenson et al 2011), the velocity gradient increases at ϳ18 km depth where the velocity is 6.35 km/s, resulting in a velocity increase from 6.45 to 6.9 km/s through the reflective lower crust (Fig. 8).…”

“…In the southern Cordillera, although there is some variation in crustal thickness within the two belts, seismic reflection and refraction data show that the Moho increases in depth eastward from 32 to 37 km beneath the Omineca belt (Cook et al 1992;Clowes et al 1995). Just south of the 2008 Geoscience BC reflection survey, a seismic refraction survey indicates that the crustal thickness in the Chilcotin plateau is 34-36 km (Stephenson et al 2011). Thus, beneath Cartoon showing crustal extension accommodated by block faulting above a network of anastomosing shear zones in the middle crust that sole out into the top of the lower crust (after Allmendinger et al 1987).…”

“…8. Correlation between reflective lower crust on an unmigrated stack section from the west end of line 10 and an average onedimensional P-wave velocity function beneath the Chilcotin plateau from the Batholiths refraction survey (Stephenson et al 2011). The P-wave velocity increases from 6.45 km/s at 7.0 s to 6.9 km/s at the top of the Moho transition zone, which corresponds to a depth of 33.8 km below the seismic datum of 1.5 km above mean sea level.…”

Distribution of Paleogene and Cretaceous rocks around the Nazko River belt of central British Columbia from 3-D long-offset first-arrival seismic tomography

“…We have previously identified basement ridges from shoaling of the 5.0 and 5.5 km/s isovelocity contours, but recognize that the actual depth to the top of the igneous basement could be in error by up to ϳ500 m when using the 5.5 km/s contour due to the small velocity contrast between low porosity sedimentary rocks and volcanic rocks at depths of ϳ 2 km. In their interpretation of a crustal-scale refraction survey, Stephenson et al (2011) suggested that the 5.0 km/s isovelocity contour indicated the base of the sedimentary units, and that the deeper 5.6 km/s isovelocity contour marked the crystalline basement beneath interbedded sedimentary and volcanic rocks.…”

Mapping crustal structure of the Nechako–Chilcotin plateau using teleseismic receiver function analysis,

“…For example, the modelling of refraction data (Stephenson et al 2011) across the central portion of our array (near stations THMB and RAMB; see fig. 2 of Stephenson et al 2011) suggests that the thickest surface sediments along the refraction line are about 4-5 km thick ( fig. 11 of Stephenson et al 2011), in good agreement with our results at the nearby stations THMB and RAMB (4.2-4.9 km thick).…”