Atlantic Plate Production Mechanisms over the last 120Ma: A Geological map of the Vema Fracture Zone Region

Seafloor formed at mid-ocean ridges carries a clear bathymetric signature of the processes (volcanic, tectonic) which created and later modified it. The activity of axis-parallel volcanic ridges, normal faults, migrating and stationary axial offsets and core-complex-forming detachment faults all produce unique bathymetric signatures, although these signatures were not generally expected to be visible on older crust due to sediment cover. During a 6-week deployment of the new German research vessel Sonne (complemented by transit data from other ships) we mapped and sampled a 50-km-wide corridor along the Vema Fracture Zone and Transform at ca. 11°N in the Atlantic, from ca. 120Ma-old crust south of the Cape Verde islands to the presently active spreading axis. Contrary to expectations, seafloor structure is widely visible along the whole transect, with successful dredge sampling of basement rocks even at 120Ma suggesting that extensive basement outcrop are present. Over the last 120Ma the spreading axis near Vema appears to have produced mainly magmatic crust, with only one detachment surface recognized - exhumation of deep crustal and mantle rocks on an inside-corner high, sometimes seen at ridge-transform intersections, was evidently not an important process at Vema. The spacing of the axis-parallel volcanic ridges shows strong periodicity in some parts of the transect - wavelengths of around 10km (corresponding to ≈ 0.5 mill. year cyclicity at ca. 1.7cm/yr half-rate) and 50km (≈ 3 mill. year) are particularly common. These may reflect characteristic periodicity in crustal accretion or be related to the mechanical strength of the crust and/or lithosphere.