Application of bandpass and directional filtering to potential-field maps of western Canada has led to the discovery of regionally extensive anomalies crossing the northwest strike of Cordilleran structures. The most prominent of these, the Steamboat–Fraser trend, is a north–south-striking feature that projects from the foreland belt in northeastern British Columbia, where it becomes subparallel to anomalies east of the Mackenzie Mountains, southward to the northern limit of the Fraser River strike-slip fault, a distance of about 600 km. Within the hinterland of the Cordillera, the trend appears to spatially correlate with the margins of some Tertiary volcanics. The northeasternmost, and thus most cratonward, Tertiary volcanics are located on the northern projection of the trend. The trend may thus be interpreted as either late, post-orogenic intrusives (e.g., dikes and related volcanic rocks) that are only partially exposed, as pre-orogenic (pre-Mesozoic) features (e.g., faults, dikes) in the crust that were overridden by the Cordilleran thrust sheets, or a combination of these. Although it is not possible to determine which of these (pre- or post-orogenic) is appropriate for the trend, the eventual choice has important implications for the structure and evolution of this part of the Cordillera. If the source of the anomalies is pre-orogenic, a major implication is that Precambrian Shield rocks would be present at depth beneath the southern Canadian Cordillera as far west as the Fraser River fault. Alternatively, if the source is post-orogenic, it represents an anomalously linear feature that has no obvious expression on the surface other than a poorly defined spatial correlation with the margins of some Tertiary volcanics.