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Manned submersible studies have delineated a large and actively growing Kuroko-type volcanogenic massive sulfide deposit 400 kilometers south of Tokyo in Myojin Knoll submarine caldera. The sulfide body is located on the caldera floor at a depth of 1210 to 1360 meters, has an area of 400 by 400 by 30 meters, and is notably rich in gold and silver. The discovery of a large Kuroko-type polymetallic sulfide deposit in this arc-front caldera raises the possibility that the numerous unexplored submarine silicic calderas elsewhere might have similar deposits.

[1] We have conducted a geophysical survey of the northern Mariana Trough from 19°N to 24°N. The trough evolves southward from incipient rifting to seafloor spreading within this region. This study aims to clarify the location and time of the rifting-to-spreading transition, which was controversial previously, and processes of seafloor spreading after the transition. The new data set includes swath bathymetry with sidescan images and magnetic vector anomaly. The mantle Bouguer gravity anomaly (MBA) was calculated using the free-air gravity anomaly from satellite altimetry. The rifting-to-spreading transition occurs at about 22°N, which is proved by seafloor-spreading fabric in the bathymetry, clear magnetic lineations, and the bull's-eye pattern in MBA. Four ridge segments separated by three nontransform discontinuities are recognized between 19°N and 22°N. The northernmost segment has relatively abundant magma supply compared with the other segments, which is estimated from a larger segment length, shallower axial depths with no rift valley, and lower MBA. The next segment to the south is, on the other hand, a magma-starved segment with a prominent rift valley. Two anomalously deep grabens (called the Central Grabens) formed by amagmatic extension occur near the segment ends. The succession of magma-rich, magma-starved, and normal segments with increasing distance from the volcanic arc is the same as the observation in the Lau Basin reported by Martinez and Taylor spreading center have been significantly larger than the eastern counterpart in general. The asymmetry may have been caused by an interaction of mantle upwelling systems under the volcanic front and the backarc spreading center and would be a characteristic of backarc spreading.

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Sumisu volcano was the site of an eruption during 30-60 ka that introduced ∼48-50 km 3 of rhyolite tephra into the open-ocean environment at the front of the Izu-Bonin arc. The resulting caldera is 8 × 10 km in diameter, has steep inner walls 550-780 m high, and a floor averaging 900 m below sea level. In the course of five research cruises to the Sumisu area, a manned submersible, two ROVs, a Deep-Tow camera sled, and dredge samples were used to study the caldera and surrounding areas. These studies were augmented by newly acquired single-channel seismic profiles and multi-beam seafloor swath-mapping. Caldera-wall traverses show that pre-caldera eruptions built a complex of overlapping dacitic and basaltic edifices, that eventually grew above sea level to form an island about 200 m high. The caldera-forming eruption began on the island and probably produced a large eruption column. We interpret that prodigious rates of tephra fallback overwhelmed the Sumisu area, forming huge rafts of floating pumice, choking the nearby water column with hyperconcentrations of slowly settling tephra, and generating pyroclastic gravity currents of water-saturated pumice that traveled downslope along the sea floor. Thick, compositionally similar pumice deposits encountered in ODP Leg 126 cores 70 km to the south could have been deposited by these gravity currents. The caldera-rim, presently at ocean depths of 100-400 m, is mantled by an extensive layer of coarse dense lithic clasts, but syn-caldera pumice deposits are only thin and locally preserved. The paucity of syncaldera pumice could be due to the combined effects of proximal non-deposition and later erosion by strong ocean currents. Post-caldera edifice instability resulted in the collapse of a 15°sector of the eastern caldera rim and the formation of bathymetrically conspicuous wavy slump structures that disturb much of the volcano's surface.

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The bulk composition of