Crustal structure and tectonics of the southeastern Canadian cordillera

The crustal structure of the Omineca (OB) and Foreland (FB) belts of the southeastern Canadian Cordillera are interpreted from the inversion of seismic refraction/ reflection travel times and amplitudes, and modeling of the Bouguer gravity data from a 350-km east-west wide-angle profile. The main features of the resultant velocity and density models include (1) a low average crustal velocity of 6.2 km/s, (2) variable upper crustal velocities (5.6-6.3 km/s) within the FB and Purcell Anticlinorium as compared to farther west in the OB (6.1-6.2 km/s), (3) a midcrustal (similar to 20 km depth) 0.4-0.5 km/s velocity increase within the OB, (4) eastward crustal thickening from 35 to 42 km over 80 km distance beneath the OB-FB boundary, (5) the Slocan Lake fault (SLF) dipping east at 15-20 degrees to at least 35 km depth, (6) decreased lower crustal velocities (6.7-6.4 km/s) across the SLF from the OB into the FB, (7) increased uppermost mantle velocities (7.9-8.0 km/s) from the OB into the FB, and (8) an average crustal density increase of 40 kg/m(3) from the OB into the FB. Lower velocities, higher conductances, and increased densities within the upper crust (0-10 km depth) of the Purcell Anticlinorium appear to be associated with Middle and Upper Proterozoic rift-related metasediments and gabbroic dikes (35-55% by volume) in contrast to the mainly Mesozoic felsic intrusives and metamorphic rocks that dominate farther west in the OB. Elevated crustal temperatures within the OB (a high heat flow province) are responsible for low-velocity gradients to 20 km depth. A more mafic lower crust beneath the OB (inferred from higher velocities) and the westward decrease in crustal thickness are interpreted as resulting primarily from Middle and/or Late Proterozoic extension and rifting of the craton. Penetration of the SLF into the lower crust may be controlled by lateral strength contrasts at the edge of the craton. Farther west, a midcrustal strength contrast (a velocity boundary at 20-25 km depth) may have acted as a regional detachment zone during compressional and extensional tectonic episodes.