Recent paleomagnetic studies are reviewed in an effort to clarify the relationship between the intra-arc deformation of central Japan and the collision tectonics of the Izu-Bonin Arc. The cusp structure of the pre-Neogene terranes of central Japan, called the Kanto Syntaxis, suggests a collisional origin with the Izu-Bonin Arc. The paleomagnetic results and newly obtained radiometric ages of the Kanto Mountains revealed the Miocene rotational history of the east wing of the Kanto Syntaxis. More than 90" clockwise rotation of the Kanto Mountains took place after deposition of the Miocene Chichibu Basin (planktonic foraminifera1 zone of N.8: 16.6-15.2 Ma). After synthesizing the paleomagnetic data of the Japanese Islands and collision tectonics of central Japan, it appears that approximately a half rotation (40-50") probably occurred at ca 15Ma in association with the rapid rotation of Southwest Japan. The remainder (5040") continued until 6 Ma, resulting in the sharp bent structure of the pre-Neogene accretionary complexes (Kanto Syntaxis). The latter rotation seems t o have been caused by the collision of the Izu-Bonin Arc on the northwestward migrating Philippine Sea Plate.

[1] The need to quantitatively estimate future locations of volcanoes in the long-term is of increasing importance, partly as a result of the requirement of constructing certain types of installations in regions of low geologic risk. The complex geological factors and natural processes controlling the locations of volcanoes make it problematic to estimate future patterns deterministically. Instead, the probabilistic approach can be developed with quite high levels of confidence; however, for regions with few or no volcanoes, there is a need to include additional geological and geophysical data that may indicate the likelihood of future volcanism. We achieve this using Bayesian inference in the Tohoku volcanic arc, Japan, in order to combine one or more sets of geophysical information to a priori assumptions of volcano spatiotemporal distributions yielding modified a posteriori probabilities. The basic a priori assumption is that new volcanoes will not form far from existing ones and that such a distribution ranges from Gaussian (not so conservative) to Cauchy (conservative). Seismic tomographs are used as an indirect clue, and from this geophysical data a likelihood function is generated in the Bayesian context that updates or fine tunes the initial Gaussian or Cauchy kernels to better reflect the distribution of future volcanism. These models are evaluated using pre-100 ka volcanic events to forecast locations of subsequent events that actually formed from 100 kyr ago to present. Probabilities in Tohoku region range from 10 À10

Current models of Philippine Sea (PHS) Plate motion assume a general and large northward shift since the Eocene. In order to constrain better the age and amount of this northward shift, we have conducted a paleomagnetic study on drill and gravity cores, respectively, taken from the seafloor of the northern part of the PHS Plate. The core samples consist of sedimentary rocks or semi-consolidated sediments, and their ages, as estimated from microfossils and strontium isotope ratios, range from the Eocene to late Miocene. The results of stepwise alternating-field and thermal demagnetization experiments revealed that 19 sections at 17 sites out of 58 sections at the 29 sites examined yielded mean paleomagnetic directions with a 95% confidence limit (α 95) of <25 • , and 14 sections at 13 sites have α 95 < 15 •. An estimation of the amount of the northward shift at each site was obtained from the difference between the paleolatitude and the present latitude. This estimation revealed that the northern part of the PHS Plate was located near the equator at 50 Ma and that the majority of the northward shift took place between about 50 and 25 Ma. Very little northward movement occurred after 15 Ma. Based on our data, together with the available paleomagnetic information suggesting clockwise rotation of about 90 • since the Eocene and the requirements from geometry with the surrounding plates, we present a model in which the PHS Plate rotated 90 • clockwise between 50 and 15 Ma on the Euler pole near 23 • N, 162 • E, although it is impossible to specifically determine the Euler pole position.

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