A distinctive change in the character of acoustic basement has been mapped within the Mesozoic (Keathley) magnetic anomaly sequence in the western North Atlantic basin. This boundary, known as the rough-smooth basement boundary, separates relatively smooth acoustic basement on the older (western) side of the boundary from much rougher basement to the east. South of Bermuda, the boundary is well defined and nearly isochronous, corresponding to anomaly M-12 or M-13 (∼128 m.y. B.P.). North of Bermuda the boundary is diachronous and more transitional, with the change in basement relief occurring in a zone between anomaly M-21 and M-16–age crust (132 to 142 m.y. B.P.). Both the well-defined and transitional changes in basement relief found in the western basin are also identifiable in seismic-reflection records from the eastern (Canary) basin, and an analysis of magnetic-anomaly patterns in the central North Atlantic basin indicates that these changes occurred simultaneously on both sides of the ridge. These results strongly suggest a primary seafloor-spreading origin for the rough-smooth basement boundary, with the greater basement relief associated with slower spreading rates, changing directions of relative plate motion, and ridge jumps of isolated accretionary segments beginning in Late Jurassic–Early Cretaceous time.