A GEOLOGICAL CROSS-SECTION OF THE VEMA FRACTURE-ZONE TRANSVERSE RIDGE, ATLANTIC-OCEAN

This paper presents a detailed geological account of five Nautile dives along the north flank of the Vema fracture zone's transverse ridge near 42-degrees-42'W. It shows the existence of three EW trending geological domains: (i) the lower slopes of the transverse ridge's flank, with outcrops of serpentinized mantle periodotites and amphibolites, and talus covered intervals which yielded blocks of amphibolite facies metadolerites and gabbros. EW-trending alignments of live clams in this lower domain are interpreted as marking the trace of active faults; (ii) an intermediate domain of more moderate slopes, heavily covered with pelagic ooze, with rare outcrops of gabbro and of consolidated limestones; (iii) an upper domain of steeper slopes, which extends up to the transverse ridge's crest and exposes a sheeted dike complex and its transition to the uppermost crustal unit of lava flows, pillow lavas and basalt breccia. The dike margins are vertical, and parallel with the Mid-Atlantic Ridge. There is no indication for recent tectonic activity in the intermediate and upper domains. We propose that the apparent crustal thickness below the north wall of the transverse ridge, defined by seismic and gravity studies, may be a direct function of the extent of tectonic slicing and serpentinization of the upper mantle and lower crustal rocks which outcrop along this wall.