sentences:  Seafloor spreading in the Pacific during the Cretaceous evolved from a more E-W 'Tethyan' orientation to the modern N-S trend.This occurred during mid-Cretaceous time, a ~35–40 Ma interval characterized by a lack of magnetic reversals known as the Cretaceous Superchron or Quiet Zone.Subsequently, the location of N-S trending spreading ridges relative to the Pacific Basin migrated progressively to the east throughout Cretaceous and Tertiary time, resulting in the present marked asymmetry of the Pacific, with very young seafloor in the Eastern Pacific and very old seafloor in the Western Pacific.