Crustal velocity structure in the eastern Insular and southernmost Coast belts, Canadian Cordillera

Abstract

Seismic refraction data recorded along a 330 km cross-strike profile through the eastern Insular and southernmost Coast belts of the Canadian Cordillera are interpreted using an iterative combination of traveltime inversion and amplitude forward modelling. The resultant model is characterized by large lateral variations in velocity. The most significant of these variations is a decrease in upper and middle crustal velocities to the east of the surface trace of the Harrison fault, which likely represents the transition from crust of the Insular superterrane to that of the Intermontane superterrane. This interpretation is consistent with some present geological models that place the possible (probable) location of the suture between the two superterranes less than 20 km east of the Harrison fault. Velocities at the base of the upper crust average 6.4 and 6.2 km/s west and east of the fault, respectively. Mid-crustal velocities average 6.6–6.9 km/s to the west and 6.35–6.45 km/s to the east of the fault. Lower crustal velocities also decrease slightly to the east. Other features of the velocity model include (i) a thin near-surface layer with velocities between 2.5 and 6.1 km/s; (ii) upper crustal thickness of 12.5 km, thinning to 8 km at the eastern boundary of the Western Coast Belt (WCB); (iii) high velocity (6.6–6.9 km/s) mid-crustal layer west of the Harrison fault extending to 21 km depth; (iv) high-velocity (6.75–7.1 km/s) lower crustal layer; (v) low-velocity gradient upper mantle with depth to Moho at 34–37 km beneath most of the Coast Belt, decreasing to 30 km beneath the eastern Insular Belt, a depth much less than previous estimates. The inferred crustal velocity structure beneath the WCB is consistent with the three-layer electrical conductivity structure for this area derived from magnetotelluric surveys. The association of high resistivities with the upper crust suggests that the upper 8–12 km represents the massive cover of plutonic rocks which characterizes the WCB. Middle and lower crustal velocities beneath the WCB are consistent with Wrangellian velocities found beneath Vancouver Island, suggesting Wrangellia may extend at depth eastward as far as the Harrison fault.