Abyssal-hill topography in the central North Pacific, far removed from active sea-floor spreading centers, possesses a strong linear character that reflects the original grain imparted to the crust during its generation.
South of the Aleutian Trench, elongated abyssal hills closely parallel linear magnetic anomalies 25 to 32 (60 to 78 m.y. in age). In this area, isochronous bands of crust can be distinguished on the basis of subtle relief differences, as well as by magnetic anomalies. Topographic and magnetic evidence both indicate crustal extension within the south wall and floor of the trench.
A topographic grain related to sea-floor spreading also appears to be present throughout the magnetic “quiet zone” north of the Hawaiian Ridge in regions of the crust that may be as old as Early Cretaceous.