Time constraints on the emplacement of an uplifted sliver of oceanic lithosphere at the Vema transverse ridge (Central Atlantic)

The Vema fracture zone is the northernmost of a set of major transform faults which displace the Mid Atlantic Ridge (MAR) at 10°50' N by over 320 km. It is paralleled on its southern flank by a prominent transverse ridge, the summit of which shoals up to 450 m below the sea level near the western ridge transform intersection. The north facing wall of the Vema transverse ridge exposes a relatively complete and undisturbed sliver of upper oceanic lithosphere. Consolidated pelagic limestones locally encrust the exposed igneous rocks, while shallow water carbonate platforms, up to 600 m thick, cap the summit of the transverse ridge. The Vema transverse ridge originated by vertical movements that have uplifted a flexured sliver of oceanic lithosphere. Multichannel seismic reflection profiles taken across the transverse ridge and the transform valley give evidence of recent transpression in the valley, and of extension across the transverse ridge and in the area south of it. Vertical movements might be due to these transpressive and transtensional stresses, related to small changes in spreading direction. Pelagic, as well as shallow water limestones recovered from the transverse ridge, will provide age constraints on the emplacement history of the lithospheric sliver exposed on the transverse ridge.