重力からみたフォッサマグナの構造とテクトニクス

抄録

The Itoigawa-Shizuoka Tectonic Line (ISTL) forms the western margin of the Fossa Magna separating the Northeast Japan from the Southwest Japan. The conventional idea insists that both the Northeast Japan and the Southwest Japan belong to the Eurasian plate (EUR). A hypothesis has recently proposed, however, that the Northeast Japan belongs to the North-American plate (NAM), while the Southwest Japan belongs to EUR. If we approve the hypothesis, it follows that ISTL is a plate boundary between these two plates. The purpose of this paper is to define ISTL as a kind of plate collision boundaries on the basis of gravity data.
ISTL runs across a positive Bouguer anomaly belt with the width of about 30km located on the Japan-Sea coast. The Bouguer anomaly there is about 30 mgal higher in the west side of ISTL than in the east side. This implies that, if we assume ISTL to be a plate subduction boundary, the eastern plate sinks down the western one along ISTL. On the other hand, the inland part of ISTL between Omachi and Matsumoto cities is characterized by a large NS-trending thrust called “East Matsumoto Basin Fault”, the west side of which subsides down the east side. The existence of this thrust is confirmed by both gravity and explosion seismic data. The movement of this thrust is evidently opposite to the plate sinking on the Japan-Sea coast.
ISTL turns its way to the NW-SE direction at the south end of the East Matsumoto Basin Fault. The left-lateral slip is predominant from Suwa to Kofu cities, and its horizontal displacement amounts to about 12 km long from geological considerations. The leftlateral movement there can be interpreted from a crustal compression in the EW direction which is caused by a plate collision. The graben lies along this part of ISTL. We find a gravity difference amounting to 20-30 mgal at its maximum between the inside and the outside of the graben. Assuming that a density contrast of basement with sediment buried in the graben is 0.5g/cm3, we estimate the vertical displacement of the graben wall as 1-1.4 km. Our estimation is consistent with the fact that a 800m deep drilling does not reach the basement in the Suwa basin.
If we approve the previously mentioned hypothesis, the Kofu basin is considered to be a tripple junction of three plates: EUR, NAM and the Philippine-Sea plate (PHS). Accordingly, the southernmost part of ISTL is a subduction boundary of PHS sinking down EUR, extending southward to the Suruga trough in the Pacific Ocean. Meanwhile, the NAM-PHS boundary runs southwestward from the Kofu basin to the Sagami trough. The highpass-filtered Bouguer anomaly on the inland part of PHS intensifies NE-SW trending short-wavelength undulations of gravity field, which may reflect the northeastward movement of PHS.
Geoscientific investigations can not define the Northeast Japan as a part of NAM, but find sufficient evidence for the fact that ISTL is a collision boundary between EUR and the other continental plate except for the PHS area. The gravity data also support this conclusion.