Deep-tow observations at the mounds abyssal hydrothermal field, Galapagos Rift

Abstract

Small 5–20 m high mounds of lithified sediment were discovered in 1972 during a deep-tow survey 20–30 km south of the spreading axis of the Galapagos Rift, at a depth of 2.7 km. They were reexamined with an improved deep-tow instrument in 1976. The features are 20–50 m wide, conical in section and generally elongate in plan, and arranged in parallel chains that are commonly 1–2 km long, and aligned parallel to the rise flank's fault-block relief. Their rough sides, sloping at 35–45° from sharp crests, are encrusted with thick deposits of manganese oxide. Manganese crusts also occur on the sea floor in 10–20 m wide stripes of high acoustic reflectivity which extend discontinuously for several kilometers, generally on the crests of low rises (sometimes linking the cones and ridges of a chain). Although they are structures of the 30 m thick sediment blanket rather than of the volcanic bedrock, many chains and stripes overlie minor bedrock faults, and their plan patterns mimic those of minor faults and fissures exposed and mapped at the present spreading axis. These landforms and bands of mineralization are interpreted as the result of precipitation around the discharge vents of an active hydrothermal circulation in the oceanic crust, though no plumes of modified bottom water were found above them. They seem to form only after the basaltic crust has been completely buried with sediment, even along the scarps at fault-block boundaries which are the likely sites of discharge on younger crust. Their existence demonstrates the ability of localized discharges to vent through a thin blanket of pelagic sediment, and their shapes and distribution patterns emphasize the importance of zones of fracturing for controlling the site and scale of circulation in the bedrock.