In-situ study of the eastern ridge-transform intersection of the Vema Fracture Zone

Abstract

Fourteen dives of the submersible Nautile have been carried out at the eastern intersection of the Vema Fracture Zone with the Mid-Atlantic Ridge. Direct observations allow us to locate precisely the surficial expression of the transform fault motion in the southern trough of the fracture zone valley. The present-day displacement zone, corresponding to an extremely narrow (less than 300 m) transform fault zone, is confined to a belt of disturbed terrain situated at the foot of the southern wall and characterized by transform-parallel 30–100 m high ridges of basaltic fragments and pelagic sediments. Active splays of the transform fault zone lie generally between these ridges, creating steep V-shaped furrows. Despite a rather sinuous trend one of the fault strands appears to be laterally continuous and other minor active splays branch out from it or run parallel to it. The surface manifestations of the tectonic activity along the present-day displacement zone include (R) Riedel shears, low-angle shears, P shears, “en échelon” structures, open fissures, small pull-apart structures and funnel alignments in the sedimentary cover. The transform fault motion reactivates large normal faults on the adjacent flanks of the southern wall and the median ridge. The transform-parallel topography defines a 10 km wide zone with dip-slip motion. The ridge-transform intersection area is entirely floored by fresh basalts contrasting with other intersections of major fracture zones such as the Kane and Oceanographer Fracture Zones. Recent volcanic activity is not restricted to the central volcanic ridge but is widespread across the whole nodal basin area. The present-day displacement zone cross-cuts the neovolcanic zone, and the disconnected northernmost extension of the volcanic ridge is attached to the median ridge and so belongs to the African Plate. The extensive volcanism is suggestive of a magmatic production phase in the intersection area. Extremely fresh pillowed lava forms have been observed at the RTI inside-corner high, indicating possible off-axis volcanism.