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During the cruise of IPOD Legs 56 ＆ 57, many cores were successfully recovered around the Japan Trench region off the Tohoku Arc.
Two different temporal patterns of the explosive volcanism were recorded at two different sites which represent the reference stratigraphic sections of landward and seaward of the trench.
The landward site is located on the Eurasian Plate and the seaward site on the Pacific Plate.
Both landward and seaward referencs sites, 438 & 439 and 436 show good continuous stratigraphy attaining as deep as Cretaceous.
Landward reference site shows two distinct maxima around 16-15 Ma and 5-2 Ma in the frequency versus age diagram of the volcanic ash layers, whereas seaward reference site represents gradual increase from 11 Ma toward present.
Sedimentation rates of each site are also relevant.
Landward site has two definite fast sedimentation periods between 16-15 Ma and 5-2 Ma, whereas seaward site shows the monotonously increasing sedimentation rate from 14 Ma to present.
Ash frequency patterns and sedimentation rates are identical with each other at the same site.
The disagreement between landward and seaward records of explosive volcanism may be explained as follows : landward pattern with two maxima is common to the one of the Tohoku Arc.
Earlier maximum corresponds to the acidic volcanic activity in Daijima and Nishikurosawa stages which may be related to the formation of the Kuroko deposits.
The later maximum corresponds to Funakawa and Kitaura stages when a large scale of the acidic volcanic activity including the intrusion of granites (the so-called Tertiary granites) related to the Dewa disturbance.
On the other hand, the apparent increase in the frequency of volcanic ash layer observed in the seaward reference site is governed by the movement of the Pacific Plate with time, approaching gradually towards the volcanic source, the Japanese Islands.