(Table 1) Paleomagnetic of ODP Leg 126 sites

A paleomagnetic study was made on the deep-marine sediments and volcanic rocks drilled by Ocean Drilling Program Leg 126 in the Izu-Bonin forearc region (Sites 787, 792, and 793). This study evaluates the sense and amount of the tectonic drift and rotation associated with the evolution of the Philippine Sea Plate and the Izu-Bonin Arc. Alternating-field and thermal demagnetization experiments show that most of the samples have stable remanence and are suitable for paleomagnetic studies. Paleomagnetic declinations were recovered by two methods of core orientation, one of which uses a secondary viscous magnetization vector of each specimen as an orientation standard, and the other of which is based on the data of downhole microresistivity measurement obtained by using a formation microscanner. Oligocene to early Miocene samples show 10° to 14° shallower paleolatitudes than those of the present. Middle Miocene to early Oligocene samples show progressive clockwise deflections (up to ~80°) in declination with time. These results suggest large northward drift and clockwise rotation of the Izu-Bonin forearc region since early Oligocene time. Considering previous paleomagnetic results from the other regions in the Philippine Sea, this motion may reflect large clockwise rotation of the whole Philippine Sea Plate over the past 40 m.y.

(Table 1) K and Ar content, and age determination of ODP Holes 126-792E and 126-793B

Results of conventional K-Ar dating on five samples from two sites from the Izu-Bonin forearc are presented. Two samples recovered from a volcanic edifice and overlying sediments drilled on the western side of the forearc basin (Site 792) indicate a basement age of 34 Ma. This is consistent with early Oligocene biostratigraphic ages from the overlying sediments. Three samples from the basement of Hole 793B at the center of the basin are not analytically distinguishable, with a best age of 27.1 +/- 0.6 Ma. This is slightly younger than the 30-33 Ma biostratigraphic and magnetostratigraphic estimates from the overlying sediments, suggesting that alteration processes have lowered the apparent K-Ar ages. These ages suggest that syn-rift volcanism occurred in a forearc location during the middle Oligocene.