AN EXAMPLE OF A RECENT ACCRETION ON THE MID-ATLANTIC RIDGE - THE SNAKE PIT NEOVOLCANIC RIDGE (MARK AREA, 23-DEGREES-22'N)

The inner rift valley of the MARK area (Mid-Atlantic Ridge at the Kane fracture zone) was investigated with the submersible Nautile at the latitude of the Snake Pit hydrothermal field (23-degrees 22'N). A transect across the axial valley shows a stepwise increase in the sediment thickness away from the axis. These steps probably reflect major magmatic and tectonic events in the evolution of the inner floor. Dives on the summit of the Snake Pit neovolcanic ridge constrain a generalized geological map, and document relations between hydrothermal activity, magmatism and faulting. The neovolcanic ridge was constructed by the build up of several hundred metres of pillow-lava flows. Its summit is intensely fissured and cut by an axial graben about 100 m wide. Three hydrothermal sites were recognized within the axial graben. The eastern site, the most active, is structurally related to the eastern fault scarp; the central site is located within the graben; the western site, associated with the western fault scarp, is more complex and was constructed by two successive hydrothermal events. The new data suggest a model of evolution for the inner floor of the rift valley, involving alternating stages of magmatism and volcanism which produce ridges and troughs preserved outside of the axial domain. These volcano-tectonic cycles have an approximate duration of 100,000 years. We discuss variations in the morphology of the MAR along-strike, in the light of this model of evolution.