Crustal structure from an OBS survey of the Nootka fault zone off western Canada

Summary. The Nootka fault zone is the boundary between the small Explorer and Juan de Fuca plates which are situated between the America and Pacific plates off western Canada. To investigate the crustal structure in the region, three explosive/large airgun refraction lines were shot into three ocean bottom seismometers (OBSs) with three‐component geophone assemblies. In this phase of the study, P ‐wave velocity—depth models are interpreted by comparison of the travel time and amplitude characteristics of the observed data with theoretical seismograms computed using a WKBJ algorithm. The interpretation gives relatively consistent results for the upper crust. However, the structure of the lower crust is significantly different among the various profiles. Upper mantle velocities range from 7.5 to 8.3 kms −1 and the sub‐bottom crustal thickness vanes from 6.4 to 11 km. Nevertheless, these seismic models are consistent in general terms with oceanic crustal models represented by ophiolite complexes. Some aspects of the differences among profiles can be explained by consideration of a recent tectonic model for the development of the fault zone. This requires, within a 1 Myr time interval, variations in the process of crustal formation at the ridge, crustal ‘maturing’, or both. The abnormally thick crust near a spreading centre may result in part from the complex interaction of the Juan de Fuca and Explorer plates with the larger and older America and Pacific plates. Upper mantle velocity variations are consistent with the concept of velocity anisotropy. The different record sections show that seismic energy is attenuated for ray paths traversing the Nootka fault zone.