Paleocene to middle Eocene magnetic anomalies were mapped over oceanic crust that accreted at the Kula-Pacific spreading center and is now obliquely entering the western Aleutian Trench between 179°E and 168°E. The strike of anomalies and the pattern of abyssal hills and fracture zones changed abruptly during 56-55 Ma, when north-south spreading veered to northwest-southeast (310°-130°). Kula-Pacific spreading ceased in 43 Ma. A 75-km-long section of the fossil Kula Rift axis has avoided subduction, although it now intersects the trench axis (almost orthogonally) near 171.5°E. A narrow remnant of the former Kula plate, northwest of this fossil spreading center, is bounded by a fossil Kula-Pacific transform with a high transverse ridge alongside a sediment-filled transform valley. Anomalies on this remnant show that Eocene Kula-Pacific spreading was highly asymmetric (2:1).
The 56-55 Ma change in Kula plate rotation inferred from the change in spreading direction coincided with birth of the Aleutian subduction zone, and was probably a consequence of the resulting change in slab-pull stresses on the oceanic lithosphere. The change in direction of Kula-North American motion is a plausible explanation for the detachment of continental terranes from the Pacific Northwest and their migration around the Gulf of Alaska, and for the early Eocene demise of Alaska Range are volcanism. The cessation of Kula-Pacific spreading coincides with a major change in Pacific-plate rotation, and the subsequent direction of convergence of the Pacific plate with the Aleutian arc was similar to the 55-43 Ma direction of Kula-plate convergence.