Crustal velocity structure of the Omineca Belt, southeastern Canadian Cordillera

Abstract

Travel time inversion and amplitude modeling of a 350‐km Lithoprobe seismic refraction/wide‐angle reflection profile determined the velocity structure of the crust and upper mantle along strike in the Omineca Belt of the Canadian Cordillera. The upper crust to 12–18 km depth has velocities from 5.6 to 6.2 km s−1, and two shear zones, the Monashee Décollement and Gwillim Creek Shear Zone, are imaged by the wide‐angle reflections and velocity trends. Minor velocity differences on either side of the Monashee Décollement may be related to separate rock origins. Prominent reflections define the boundaries of a low‐velocity midcrustal layer from 10–15 km to 20–25 km depth with velocities less than 6.1 km s−1. The low velocities of the midcrust, associated with high electrical conductivities and high heat flow, may be considered as support for the hypothesis of fluids in the Cordilleran crust, though other possibilities, such as the effect of high temperatures on rock velocities are possible. In the lower crust velocities range from 6.4–6.5 km s−1 at the top of the lower crust to 6.6–6.8 km s−1 at its base. The Moho is very clearly defined by the refraction/wide‐angle reflection data and has a gentle southerly dip. Crustal thicknesses are 35–37 km. A thin crust‐mantle transition zone of 1–2 km thickness in which velocities vary between 7.6 and 7.7 km s−1 is consistent with coincident reflection data. Upper mantle velocities range from 7.9 to 8.1 km s−1 with indications from the data of upper mantle layering. In comparison with neighboring regions, the Omineca Belt has an anomalously thin crust, low crustal velocities, and a low‐velocity upper mantle, similar only to the Basin and Range province. The velocity structure may partly mirror the temperature profile which has overprinted the geological signature of the region as measured by the seismic refraction method. The characteristics of a thin crust and lithosphere, along with low velocities from midcrust to mantle suggests that both the Basin and Range and the southern Canadian Cordillera are currently being heated from a source within the mantle.

Crustal structure and tectonics of the southeastern Canadian Cordillera

“…Also, crustal low-velocity layers have been interpreted for the Basin and Range in numerous studies [e.g.,Braile, 1977] and within the OB [e.g.,Kanasewich et al, 1994]. The corrected velocities are consistent, for example, with velocities measured for anorthosite, mafic granulite or amphibolite [from Holbrook et al, 1992).…”

“…Data were acquired along five profiles [Burianyk et al, 1992]. Kanasewich et al [1994] and Burianyk and Kanasewich [1995] present results for profiles along and across the strike of the OB, respectively. Kanasewich et al [1994] and Burianyk and Kanasewich [1995] present results for profiles along and across the strike of the OB, respectively.…”

Inversion of seismic refraction and wide-angle reflection traveltimes for three-dimensional layered crustal structure

“…They inverted for both interface geometry and layer velocity structure. Others who have used similar approaches to constrain 2-D multilayered crustal structure include Kanasewich et al (1994), Darbyshire et al (1998), Navin et al (1998 and Vogt et al (1998).…”

Seismic Traveltime Tomography of the Crust and Lithosphere

“…For example, numerous studies have been carried out in Canada (e,g, Hole et al, 1992;Kanasewich et al, 1994;Clowes et al, 1995;Zelt & White, 1995;Morozov et al, 1998;Zelt et al, 2001), the U.S. (e.g. Jarchow et al, 1994;Zhu & Ebel, 1994;Parsons et al, 1996;Wang & Braile, 1996;Lizarralde & Holbrook, 1997), Europe (e.g.…”

“…A number of authors have adopted the approach of Zelt & Smith (1992) to invert wide-angle data, including Kanasewich et al (1994), Riahi & Juhlin (1994), Staples et al (1997), Ye et al (1997), Darbyshire et al (1998), Kodaira et al (1998), Morozov et al (1998) and Navin et al (1998). Darbyshire et al (1998) applied it to data from the ICEMELT refraction line to image crustal Zelt & Smith (1992).…”