A geological and geophysical investigation of Baby Bare, locus of a ridge flank hydrothermal system in the Cascadia Basin

Baby Bare is one of three small basement outcrops on the eastern, sediment-buried Juan de Fuca Ridge flank that have localized heat loss and fluid movement within 3.5 Ma oceanic crust. Low-temperature (25 degrees C) hydrothermal vents near the summit of Baby Bare represent the highest-temperature occurrence of off-axis hydrothermal activity found in oceanic crust older than 1 million years. This site has been investigated with seismic reflection profiling, towed-camera surveys, and an Alvin dive series that included heat flow measurements to document the detailed geological setting of these off-axis vents. A new geologic map based on visual observations of Baby Bare shows that the distribution of rock, sediment, and biota appears to be controlled by seafloor slope and elevation, while specific vent locations are controlled by faulting and occur only in areas of thin or no sediment cover. Alvin heat flow data indicate that conductive heat loss fron the edifice is similar to 4.5 times greater than that from the sediment-blanketed area around the outcrops. Although the outcrop is generally conical, seismic reflection profiles reveal that the sediment-buried portions of the edifice have an asymmetric morphology, strongly suggesting that Baby Bare is a volcano built upon a preexisting, fault-generated abyssal hill. This evidence, combined with previously published petrologic data and results of Ocean Drilling Program Leg 168 drilling, is consistent with the hypothesis that Baby Bare formed by off-axis volcanism rather than at the adjacent ridge axis; sediment thickness and fossil assemblages indicate that it could be as young as 2.7 Ma. Off-axis volcanoes such as Baby Bare increase the overall roughness of basement topography and thus delay complete sediment burial during normal lithospheric aging, particularly in areas where near-axis sediment accumulation is rapid. Partially buried seamounts play an important role in focusing hydrothermal exchange between the oceans and young oceanic crust and, if Baby Bare is representative, may contribute as much as 85% of the heat flux from a sedimented ridge flank.