Timing of transverse ridge uplift along the Vema transform (Central Atlantic)

Transverse ridges are large topographic anomalies running adjacent to slow-slip oceanic transforms. They form due to different processes, including thermal stresses, hydration-dehydration of peridotites, non-linear viscoelastic rheology of the oceanic crust and vertical tectonic motions of lithospheric slivers induced by changes in ridge/transform geometry, causing transpression and/or transtension along the transform boundary. A prominent transverse ridge on the southern side of the Vema transform (Central Atlantic) rose probably between 12 and 10 Ma along the entire length (≈ 320 km) of the transform, exposing a relatively undisturbed section of oceanic lithosphere. We used pelagic limestones encrusting serpentinized peridotites sampled from the lower slopes of the uplifted lithospheric section to date this uplift and define mechanisms of its emplacement. Ages were obtained both by micropaleontology (foraminifera and nannofossils) and by 87Sr/86Sr isotope ratios. No ages older than ≈ 12 Ma were obtained, even in samples recovered at sites with crustal ages (determined by magnetic anomalies) well over 12 Ma; on the other side, ages as young as 5.6–8.3 Ma were found in clusters of samples collected from the eastern part of the transverse ridge, probably due to mass-wasting episodes that rejuvenated the substratum. These results support the hypothesis that the Vema Transverse Ridge rose between 12 and 10 Ma due to flexural uplift related to transtension along the transform, in line with a general model whereby transverse ridges rise during discrete events as a consequence of changes in ridge-transform geometry.