CRUSTAL VELOCITY STRUCTURE OF THE OMINECA BELT, SOUTHEASTERN CANADIAN CORDILLERA

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 Decollement and Gwillim Creek Shear Zone, are imaged by the wide-angle reflections and velocity trends. Minor velocity differences on either side of the Monashee Decollement 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.