Crustal structure and tectonics of the northeast Honshu arc, Japan, revealed from detailed gravity anomalies and late Cenozoic caldera distribution

In order to discuss seismotectonics, geotectonics and the late Cenozoic caldera distribution beneath the northeast Honshu arc, Japan, crustal density structures deduced from precise gravity anomalies are examined. Although two large gravity databases in Japan, the Gravity CD-ROM of Japan published by the Geological Survey of Japan (2000) and the Gravity Database of Southwest Japan published by the Gravity Research Group in Southwest Japan (2001), have been successively issued, there had been poor station coverage in some areas over the northeast Honshu arc, Japan. In order to fill up the blank area of gravity measurements, we conducted an extensive gravity survey. By using our newest gravity database, we will present detailed gravity anomaly maps over the region. Bouguer gravity anomaly distributions over the northeast Honshu arc are characterized by NNE-SSW trend of steep horizontal gravity zones corresponding to the fault structures which were originated during Japan Sea opening. Short (less than 30km) wavelength components of gravity undulations, which are many gravity depressions surrounded by steep horizontal gravity cliffs, are also dominant in this region. Consequently, the northeast Honshu arc was very hard region to estimate the large-scale basement structure from the gravity data, which are disturbed by many gravity depressions due to near surface and/or local density contrasts. We here report that the distribution of the gravity depressions is, however, strongly corresponding to the distribution of the late Cenozoic large calderas. The caldera structure is a geographical feature with collapsed structure surrounded by a ring-fault. They are usually underlaid by plutonic body under cooling that often reaches 10 km or more in the diameter (Yoshida, 2001). Therefore the density structure of these calderas might be a major factor of the gravity undulations of this region. Although the number of the known caldera structures exceeds 80 in the northeast Honshu arc (Yoshida et al., 1999), Ohyagi (2000) was supposed that many calderas still exists under the overlying volcanic rocks of the Quaternary volcanoes. We have examined the detailed map of Bouguer anomaly of the northeast Honshu arc with special attention to caldera structures with related subsurface plutons. The results show that some unexposed calderas may located under the overlying young strata. On the other hand, by omitting gravity effects due to late Cenozoic caldera structures from the gravity anomalies, we will also present the large-scale crustal segments and their tectonic evolution in the northeast Honshu arc.