The boundary zone between the Churchill and Superior geological provinces was investigated by a 35-km-long seismic survey in an area where this zone is covered with 1.4 km of phanerozoic sedimentary rocks. The seismic reflection profile revealed a fault zone, which indicates that the boundary zone was tectonically active as late as Cretaceous time. Such late activity was not observed in areas where the boundary is exposed in the Precambrian shield. The present investigation shows that the central portion of this zone is associated with salt solution features in the sedimentary section and with topographic anomalies on the crystalline basement surface. The seismic section revealed that major intrusions and fracture zones formed the upper 13 km of the crystalline crust. A simple model study of the related gravity anomalies also exhibited anomalous bodies in the same zone.