CRUSTAL VELOCITY STRUCTURE IN THE EASTERN INSULAR AND SOUTHERNMOST COAST BELTS, CANADIAN CORDILLERA

Seismic refraction data recorded along a 330 km cross-strike profile t hrough the eastern Insular and southernmost Coast belts of the Canadia n Cordillera are interpreted using an iterative combination of travelt ime inversion and amplitude forward modelling. The resultant model is characterized by large lateral variations in velocity. The most signif icant of these variations is a decrease in upper and middle crustal ve locities to the east of the surface trace of the Harrison fault, which likely represents the transition from crust of the Insular superterra ne to that of the Intermontane superterrane. This interpretation is co nsistent with some present geological models that place the possible ( probable) location of the suture between the two superterranes less th an 20 km east of the Harrison fault. Velocities at the base of the upp er crust average 6.4 and 6.2 km/s west and east of the fault, respecti vely. 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 dec rease slightly to the east. Other features of the velocity model inclu de (i) a thin near-surface layer with velocities between 2.5 and 6.1 k m/s; (ii) upper crustal thickness of 12.5 km, thinning to 8 lan at the eastern boundary of the Western Coast Belt (WCB); (iii) high velocity (6.6-6.9 km/s) mid-crustal layer west of die Harrison fault extending to 21 km depth; (iv) high-velocity (6.75-7.1 km/s) lower crustal laye r; (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 eas tern Insular Belt, a depth much less than previous estimates. The infe rred crustal velocity structure beneath the WCB is consistent with the three-layer electrical conductivity structure for this area derived f rom magnetotelluric surveys. The association of high resistivities wit h the upper crust suggests that the upper 8-12 km represents the massi ve cover of plutonic rocks which characterizes the WCB. Middle and low er crustal velocities beneath the WCB are consistent with Wrangellian velocities found beneath Vancouver Island, suggesting Wrangellia may e xtend at depth eastward as far as the Harrison fault.