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doi:10.2204/iodp.proc.323.110.2011

Introduction

Bowers Ridge extends northward into the Bering Sea from the midpoint of the Aleutian Ridge and then sweeps westward in a recurving arc back toward the ridge. The dimensions of Bowers Ridge are large. Its curved length is ~800 km, and, where attached to the north side of the Aleutian arc massif, the ridge is nearly 200 km wide. At this junction, the ridge rises ~3500 m above the abyssal floor of the Bering Sea Basin to flatten at a wave-planed summit platform at ~600–1000 m water depth. The ridge's width and height decrease progressively with distance to the west.

Marine geophysical data establish that a ~20–25 km thick core of basement rock constructs the bulk of Bowers Ridge (Cooper et al., 1981, 1987, 1992). Its middle and lower flanks are draped by a mantle of sedimentary deposits as thick as 2 km. The ridge's magnetic, velocity, and gravity characteristics are typical of an arc massif. A sediment-buried trench at the base of the ridge's northern flank implies that the arc-generating subduction zone dipped southward beneath Bowers Ridge.

Confirming the ridge's arc origin, in 1970 the R/V Melville dredged altered volcaniclastic sediment of calc-alkaline affinity from the ridge's upper, northern flank (Scholl et al., 1975; Scholl, 2007). The dredge site is located near Integrated Ocean Drilling Program (IODP) Site U1342. In 2009, west of Site U1342, the R/V Sonne dredged similar material from exposed basement also along the ridge's northern flank.

At Site U1342, a total of 41.54 m of basement rock was recovered from the southern side of Ulm Plateau, a subsided, ~800 m deep wave-eroded platform. For the entire Aleutian-Bowers arc massif, no similar vertical sequence of its submerged arc framework exists. The recovered basement is evidently from a former ridge-cresting volcanic edifice demolished by subaerial and wave-base erosion in the late Miocene. Aboard the R/V JOIDES Resolution, the now vanished edifice was informally named Whiskey Mountain.

Two contrasting hypotheses have been proposed for the origin and evolution of the Bowers arc massif. The first is that, in the Late Cretaceous, Bowers Ridge formed in the Pacific Basin well to the south of its present location (Ben-Avraham and Cooper, 1981; Seliverstov, 1984). The second conjecture states that Bowers Ridge began to form within the Bering Sea region as an Eocene, in situ construct (Scholl et al., 1975; Scholl, 2007; Ben-Avraham and Cooper, 1981). The age, paleolatitude, geochemical, and petrologic analyses of the unprecedented recovery of a vertical section of basement rock at Site U1342, presumably from an erosionally decapitated and deeply submerged arc volcano, will determine which hypothesis is most correct or that neither is correct.

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