40Ar
39Ar age of seamounts in the western Pacific from 120 to 65 Ma ago. The seamounts are grouped into three linear zones as long as 5000 km. The seamounts become younger in the southeastern direction along the
strike of these zones. Correlation between age and location of seamounts allows division of the history of their
formation into three stages. The rate of seamount growth was relatively low (2–4 cm/yr) during the first and the
third stages within the intervals of 120–90 and 85–65 Ma, whereas during the second stage (90–85 Ma), the
seamounts were growing very fast (80–100 cm/yr). In the midst of this stage, at ~87 Ma ago, the magmatic
activity increased abruptly. The dynamics of seamount building is in good agreement with (1) pulses in the
development of the Ontong Java, Manihiki, and Caribbean–Colombian oceanic plateaus; (2) the age of spread-
ing acceleration in the mid-Cretaceous; and (3) the short period when the Izanagi Plate ceased to exist and the
Kula Plate was formed. The variation of the seamounts' age and location is in consistence with the hypothesis
of diffuse extension of the Pacific Plate in the course of its motion with formation of impaired zones of decom-
pression melting. The direction of extension (325°–340° NW) calculated from the strike of seamount zones is
consistent with the path of the Pacific Plate (330° NW) in the Late Cretaceous. The immense perioceanic vol-
canic belts were formed at that time along the margin of the Asian continent. The Okhotsk–Chukchi Peninsula
Belt extends at a right angle to the compression vector. Three stages of this belt’s evolution are synchronous
with the stages of seamount formation in the Pacific Plate. The delay in the origination of the East Sikhote-Alin
Volcanic Belt and its different orientation were caused by counterclockwise rotation of the vector of conver-
gence of oceanic and continental plates in the mid-Cretaceous. At the same time, i.e., 95–85 Ma ago, the vol-
canic activity embraced the entire continental margin and the tin granites were emplaced everywhere in eastern
Asia. This short episode (90
5 Ma) corresponds to the mid-Cretaceous maximum of compression of the con-tinental margin, and its age fits a culmination in extension of the Pacific Plate well.