HORST AND GRABEN STRUCTURES ON THE FLANKS OF THE MID-ATLANTIC RIDGE IN THE MARK AREA (23-DEGREES-22'N) - SUBMERSIBLE OBSERVATIONS

South of the Kane fracture zone (23 degrees 40'N), the Mid-Atlantic ri dge is segmented into 20-100-km-long spreading cells in which morpholo gy, seismic and gravity signatures change along and across the ridge-a xis. The rift valley, considered as a typical structure along slow-spr eading ridges, can be symmetric or asymmetric, and segment centers exp erience strong magmatic activity whereas low magma supply results in t hinning of the crust at segment tips. Along the segment centered on th e Snake Pit neovolcanic ridge (23 degrees 22'N), the rift valley is cl early asymmetric, and mantle and lower crustal rocks are commonly expo sed on the steepest and highest ridge flanking wall. In this area, we analyzed submersible observations (Gravinaute cruise) collected up to 50 km on both sides of the axis to discuss the processes responsible f or the axial valley shape, its possible relationship with ''deep'' roc k exposures, and to determine the origin of off-axis structures. Our d ata reveal that the structure and the geology vary on both walls of th e axial valley and is different from the volcanic and tectonic organiz ation observed on the neovolcanic ridge itself. Furthermore, the struc ture of the off-axis domain is different from the axial valley and rif t mountains are organized in a set of regularly spaced relief (about 1 0 km) on both flanks of the ridge, Scarps which built these topographi c highs face either toward the axis or away. Thus, the off-axis morpho logy appears as a set of horsts and grabens which could partly result from off-axis tectonic processes and/or from a shifting of the volcano -tectonic activity out of the axial valley during the last 3 Ma. Off-a xis data show that serpentinized peridotites, more commonly observed o n the western rift valley wall, crop out on the western flank up to 30 km away from the axis. Models proposed to account for such rock expos ures are usually related to the formation of the deep axial valley alo ng slow-spreading ridges. Here, we propose that mantle rocks are expos ed only in a very particular structural context which is directly cont roled by the ridge segmentation and is independent of the shape of the axial valley (symmetric or not). Serpentinites crop out within a V-sh aped structure corresponding with the boundary between two adjacent se gments, where seismic and gravity data reveal that the crust is thin a nd the lithosphere is thick. Here, the crust is fed by a lower magma s upply than at the segment center (Snake Pit area). Thus, we interpret this segment end to be a region of extremely thin crust, where modest normal fault offsets can bring mantle rocks to the surface.