LEGS 125 and 126
Izu-Bonin-Mariana Region

The Izu-Bonin-Mariana region evolved in the early-middle Eocene with the onset of westward subduction of the Pacific lithosphere beneath the Philippine Sea Plate, followed in the early Oligocene by the formation of an intra-oceanic volcanic arc. Rifting split the arc and subsequent backarc spreading isolated the Kyushu-Palau Ridge remnant arc from the active Izu-Bonin-Mariana Arc. By 23 Ma, the systems rejoined, both basins sharing a common spreading axis until 17-15 Ma. The southern part of the arc split again in the late Miocene and subsequent spreading occurred in the Mariana backarc basin, isolating the active Mariana Arc from the West Mariana Ridge remnant arc. The Bonin Arc is still in the early rifting stage of backarc basin formation. Seamount chains and aseismic ridges collided only with the Mariana and southernmost Izu-Bonin arcs, accentuating the differences between the forearc basins. Nine sites were drilled during Leg 125. Site 778 to Site 781 are located on the Conical Seamount, east of the Marianas, and Site 782 to Site 786 were drilled in the Izu-Bonin system. During Leg 126, three sites were drilled in the Izu-Bonin forarc basin (Sites 787, 792, and 793), and four sites were drilled on the Sumisu backarc rift (Site 788 to Site 791).

Drilling during Leg 125 in the Mariana forearc basin recovered a thick, massive basalt of Pleistocene age; the first evidence for such recent magmatic activity in any extant intra-oceanic forearc terrane. The basalt is an island-arc tholeiite characterized by enrichment in large-lithophile elements relative to high-field-strength elements, similar to the submarine lavas of the southern arc seamounts. The basalt layer is probably a sill fed by magma intruded along a fault zone bounding the horst and graben in the forearc. The geochemistry is consistent with a magma source simnilar to that of the active island arc and from a mantle source above the subducting Pacific Plate.

The drilling results from Legs 125 and 126 indicate that the Izu-Bonin forearc basin formed in the mid-Oligocene by separation of the formerly contiguous frontal and outer arc highs. Eocene igneous basement beneath the center of the forearc basin included high-Mg series andesites, andesites with boninitic affinities, and low-Mg series lavas with tholeiitic affinities. The volcanic basement shows extensive hydraulic fracturing and precipitation of sulfide and other minerals from hydrothermal fluids. The upper basement at a site located on the eastern margin of the basin consists of Eocene intermediate-acid submarine volcanic rocks of island-arc tholeiite to calc- alkaline affinities. Following a minimum of volcanic output between 24 and 13 Ma, there has been a steady increase in explosive volcanic activity in the forearc with a dramatic increase in the late Quaternary. Paleomagnetic evidence shows that the forearc has been translated about 15 deg N since the middle Oligocene. Low-temperature alteration of the volcanogenic sediments has produced fluids extremely enriched in Ca and depleted in Mg, Si, and sulfate.

The present stage of rifting in the Sumisu Rift began between 3.5 and 1.1 Ma with both present- day and pre-rift volcanism along the volcanic front being dominated by rhyolitic pumice eruptions. The footwall of the rift has been uplifted 200-1700 m, and rift basement, prior to 1.1 Ma, exceeded 2 km. The basement is composed of early basaltic lavas and intrusives, as well as arc pyroclastics metamorphosed to zeolite or lower greenschist facies. Intra-rift basaltic eruptions and rhyolitic eruptions were common but explosive arc volcanic activity dramatically increased at 250 ka. Unlike the forearc region, fluids other than seawater are not circulating locally through the sediments in the Sumisu Rift.

Drill holes penetrated into the summits and flanks of the serpentinite seamounts. The ultramafic rocks are extremely uniform in composition between the two forearcs, suggesting a similar underlying mantle. Some of the mafic clasts from the Mariana Seamount had chemical signatures of basalts from normal oceanic crust and were metamorphosed in the lower greenschist or prehnite- pumpellyite facies, indicating moderate to low temperatures and pressures consistent with accretion of these clasts onto the forearc from subduction of the cold Pacific Plate or metamorphism of the clasts within the forearc and emplacement by diapirism of the serpentinite. Serpentinite muds recovered from the summit of Conical Seamount are fluid-rich and devoid of texture, whereas muds recovered from the flank of this seamount display plastic, convolute folding and repeated shearing, indicating that the low-density (buoyant) serpentinite mud and its entrained clasts rose to the surface at the center of the seamount. Deformation took place when this water-rich material became unstable and flowed down the flanks of the seamount. Larger-scale structures suggest that the serpentinite mud could have risen as a diapir along a relatively narrow channel or through exposure of a localized expansion of mantle over a broad region by large-scale normal faulting on the forearc.

The forearc fluids that convert peridotite to serpentinite at Conical Seamount may originate as circulating seawater or as water driven off the oceanic basement and its sedimentary cover as they are subducted; both sources are probably operating although deeper-source water dominate, carrying high concentrations of hydrocarbons, carbon dioxide, and sulfur. Serpentinization is actively taking place throughout the seamount. Serpentinite at the Torishima Forearc Seamount, located on the seaward part of the Izu-Bonin forearc, resulted from reaction between pore water and mantle minerals at low temperatures, forming solutions with low hydrocarbon concentrations. It is currently inactive in terms of flow generation and serpentinization is still taking place.