Seismic reflection constraints on upper crustal structures in the volcanic-covered central Nechako basin, British Columbia¹This 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 2008, a Vibroseis seismic reflection survey was acquired by Geoscience BC across the eastern part of the volcanic-covered Nechako basin in central British Columbia, where Cretaceous sedimentary rocks have been exhumed along a NNW trend. Good signal penetration through the volcanic cover is indicated by lower crustal reflections at 8–12 s, which were recorded by the entire seismic survey. Comparison of the 2008 seismic survey with data from a previous survey indicates that the lack of reflectivity in the earlier surveys is generally representative of the subsurface geology. The seismic data show that ∼1700 and ∼2900 m thick sub-basins are present at the northern and southern ends of this trend, but the intervening Cretaceous rocks are discontinuous and relatively thin. The creation of a passive-roof duplex by Campanian or later low-angle thrusting is inferred within the thickest Cretaceous strata, but elsewhere faulting is likely related to Eocene extension or transtension. Seismic reflections are also recorded from folded volcanic stratigraphy, the base of the surface volcanic rocks, an underlying volcaniclastic stratigraphy, and intrusions projecting into a Quaternary volcanic cone. Seismic interpretation is complemented by coincident audiofrequency magnetotelluric surveys, from which faulting is inferred at offsets in a regional conductor. No regionally extensive stratigraphy can be identified within the seismic data, and the central Nechako basin appears to be a complex network of small, deformed sub-basins, rather than a single large basin.

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

“…The datum for all the seismic displays is 1.5 km above mean sea level. Further details of the seismic acqui- sition and data processing, together with a comparison with the earlier survey shot by Canadian Hunter Ltd. can be found in Calvert et al (2011).…”

“…A few more steeply dipping reflections, e.g at CDP 2000 and CDP 2700 on line 5, extend from the surface down to ϳ2 s where they appear to sole out into subhorizontal reflections. On line 5 south of CDP 2000, the reflective stratigraphy corresponds to Cretaceous sedimentary rocks that have been identified by drilling (Cosgrove 1981;Calvert et al 2011).…”

“…The seismic reflection sections, which were increased in duration from 6 to 18 s using an extended correlation technique, reveal clear reflections from the lower crust. A preliminary interpretation of the upper crustal features in the 2008 Geoscience BC survey was presented by Calvert et al (2011), complementing an interpretation of the more closely spaced 4 s seismic lines recorded by Canadian Hunter Ltd. in the early 1980's . Using P-wave velocity models that extend to depths of 1-4 km derived from the long offset 2008 survey by first arrival tomography, Talinga and Calvert (this issue) were able to refine the interpretation of the shallow crust.…”

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

“…The maximum recording offset was 14 390 m, yielding a nominal stack fold of 240. Further details of the acquisition and processing of the reflection lines can be found in Calvert et al (2011). The recorded data are generally of good quality, allowing first arrivals to be picked to offsets greater than ϳ4 km.…”

“…Using straight seismic profiles with a far offset of 2.55 km that were shot by Canadian Hunter Ltd. in the 1980's, Hayward and Calvert (2009) estimated near-surface seismic P-wave velocities to ϳ0.5 km depth beneath the Nechako-Chilcotin plateau using two-dimensional (2-D) tomographic inversion of first-arrival travel times. The 2008 Geoscience BC survey was designed, as discussed in more detail by Calvert et al (2011), to record first arrivals to source-receiver offsets as great as 14 km so that tomographic velocity models could be derived to depths of 2-3 km or more. If first arrivals penetrate to the igneous basement, then the thickness of the sedimentary section below the surface volcanic rocks can be determined.…”