Sea of Japan back-arc unit 

(Cenozoic) 

(Sea of Japan, unit QTb)

Consists chiefly of volcanic rocks ranging from basalt to trachyrhyolite as dredged from seamounts and 
ridges. Tholeiitic basalt forms about 80-90% of total dredged material, along with sparse clinopyroxene-
olivine ferrobasalt. The basalt commonly contains lehrzolite xenoliths with a predominant olivine-
plagioclase assemblage and subordinate chromium clinopyroxene and chrome spinel. The tholeiitic basalt 
is similar in geochemistry to basalt of oceanic spreading zones, differing with slightly higher Rb and Sr 
contents. The age of basalt is tentatively estimated as 0 to 15 Ma. The time of opening of the Japan Sea as 
determined by paleomagnetic data. Basalts are overlapped by a sedimentary cover up to 2 km thick. 
Seismic surveys indicate the sedimentary cover consists of two sequences, an upper, acoustically stratified 
sequence, and a lower, acoustically uniform sequence. P-wave velocity in the upper sequence ranges from 
1.6 to 2.1 km/s, and in the lower sequence ranges from 2.3 to 3.4 km/s. Deep drilling (IPOD) holes 
numbers 300 to 302 in the southern part of the Central Basin penetrated 531 m of Miocene-Pleistocene 
turbidite. REFERENCES: Bersenev and others, 1987; Ingle and others, 1975.