Seismic structure of the crust and upper mantle in the Peace River Arch Region, Canada

Abstract

Four intersecting ∼300‐km‐long reversed refraction lines within northwestern Alberta and northeastern British Columbia have been interpreted for crustal and upper mantle seismic velocity structure in the Peace River Arch (PRA) region of the Western Canada Sedimentary Basin. The data have been modeled using a two‐dimensional ray trace forward modeling approach based on asymptotic ray theory to match travel times and amplitudes of first and coherent later arrivals. In addition, an inversion of first arrival travel times along a fan shot profile has been performed to constrain crustal thickness northwest of the arch in a region not sampled by the reversed profiles. The major features of the structural models are (1) weak to moderate lateral variations in crustal structure with no evidence of significant layering or thick low‐velocity zones within the crust, (2) an average sub‐basement RMS crustal velocity of 6.6 km/s, average upper mantle velocity of 8.25 km/s and average crustal thickness of 40 km, (3) regional variations in structure which appear related to the dominant N‐S trending cratonic structure, including crustal thickness, upper crustal and upper mantle velocities and PmP character, and (4) subtle variations in structure that may be associated with the E‐W trending Devonian axis of the PRA, including a shallowing of high lower crustal velocities, thickening of the crust, and an anisotropic PmP character beneath the arch and a weak trend in the dip of intracrustal reflectors away from the center of the PRA region. Evidence from the refraction and other geophysical data for the presence of a local crustal expression of the PRA is weak, but suggests a thermal as opposed to flexural origin for its anomalous vertical movements.