Introduction

The Vema Fracture Zone (VFZ) is the northernmost of a set of major transform faults that displace the Mid Atlantic Ridge (MAR) in the equatorial region (Fig. 1). These fracture zones are characterized by long offsets, slow slip rates and rugged bathymetry, with deep transform valleys flanked by prominent transverse ridges. The Vema VFZ is located at a latitude of 10:50' N and offsets by 320 km the MAR. The transform valley, 15 to 22 km wide, trends mostly E-W and displays a flat floor on the north-facing wall of the VTR at longitude of 42:42' W, AUZENDE1989.

This section includes from bottom to top: 1) upper mantle peridotites ($\sim$ 1km thick), 2) lower crustal gabbros ($\sim$ 500m thick), 3) sheeted dike complex ($\sim$1 km thick), and 4) upper basalt layer (200-800 m thick). Geophysical experiments ROBB1975, LUDWIG1980, LOUDEN1986, POTTS1986 and morphobathymetric data KASTENS1998 suggest that the exposed sliver of oceanic lithosphere extends continuously on the VTR for over 270 km along a seafloor spreading flow-line. Assuming an average half spreading rate of 16 mm/y for the last 30 My CANDE1988 this uplifted section corresponds to over 16.8 My. Therefore, the VTR gives us the opportunity to explore temporal variations of processes of accretion of the oceanic lithosphere such as composition, thermal properties and processes of melt extraction and injection in a slow spreading ridge. Determining temporal variations of ridge activity, as recorded in the structure and composition of the upper mantle rocks of different ages, was the main target of cruise S19. Accordingly, we carried out close-spaced rock sampling along the base of the VTR north-facing wall, where ultramafic rocks were expected. We also carried out seismic reflection profiles in order to detect variations of the thickness of the basaltic layer. These variations might be related to alternating cycles of magmatic and amagmatic accretion at the MAR. Seismic reflection profiles were collected to analyze transform-related tectonic deformations across the principal displacement zone of the Vema transform valley. We also obtained magnetometric data in order to determine spreading rates and ages of the oceanic crust along the Vema FZ.

Figure: .