Depth migration of deep seismic reflection profiles: crustal thickness variations in Alberta

Abstract

Variations in crustal thickness provide important clues as to the formation of the crust, present-day isostatic equilibrium, and crustal stress. The map of the topography of the Mohorovicic discontinuity in Alberta is revised using 1900 km of reanalysed seismic reflection profiles acquired as part of the Lithoprobe Alberta Basement Transect. Time sections were depth migrated using a parallelized algorithm that accounts for steeply dipping structures. The migration process employed a geologically consistent velocity model of the metamorphic crust derived from earlier refraction experiments and constrained by compilations of high-pressure rock velocity measurements. We found that knowledge of the velocities in the sedimentary column strongly influenced the quality of the migration calculations. The Mohorovičić discontinuity is generally distinguished in these profiles, on the basis of sharp changes in reflectivity, at depths of 35–48 km. Sharp reflections from this boundary are rare. A number of geologic features are of note in these lines. A localized (~50 km extent) crustal thinning is observed in the Peace River region; this thinning is consistent with an adjacent oxygen isotopic anomaly indicative of crustal extension. In central Alberta, the Mohorovičić discontinuity topography is suggestive of a sharp jump of 10 km indicative of mantle faulting associated with the Snowbird tectonic zone. In southern Alberta, the crust thickens substantially across the Vulcan structure, with the greatest thickness correlating with the Vulcan structure itself indicating a collisional origin as noted by other authors.