Submarine lava flow emplacement at the East Pacific Rise 9° 50′N:: Implications for uppermost ocean crust stratigraphy and hydrothermal fluid circulation

Meter scale seafloor topography and sidescan backscatter imagery of volcanic terrain along the axis of the fast-spreading northern East Pacific Rise (EPR) near 9degrees 50'N, coupled with visual and photographic observations provide data that constrain spatial relationships between hydrothermal vents and primary volcanic features and processes along the EPR axis. High-temperature (greater than or equal to 350degreesC) hydrothermal vents are present in several areas within the EPR axial trough where recent eruptions have been focused and where drainback of lava into the primary eruptive fissure occurred. Chaotic collapse crusts and draped sheet lava surfaces along the margin of eruptive fissures typify sites where drainback primarily occurred. These areas are also coincident with similar to10-30 m-wide channels that serve to transport lava across the crestal plateau. Regions of diffuse hydrothermal flow at low temperatures (< 35degreesC) and vent animal communities are concentrated along the primary eruptive fissure that fed the 1991 eruption at the 9degrees 50'N EPR area. The relationship between seafloor eruption processes and hydrothermal vent locations within the EPR axial trough is linked to the formation of high permeability zones created by: focusing of eruptions along discrete portions of fissures; volcanic episodicity during eruptive phases lasting hours to days; and drainback of lava into the primary fissure at these same locations during waning stages of seafloor eruptions.