Morphology and tectonics of the Mid‐Atlantic Ridge, 7°–12°S

Abstract

We present swath bathymetric, gravity, and magnetic data from the Mid‐Atlantic Ridge between the Ascension and the Bode Verde fracture zones, where significant ridge–hot spot interaction has been inferred. The ridge axis in this region may be divided into four segments. The central two segments exhibit rifted axial highs, while the northernmost and southernmost segments have deep rift valleys typical of slow‐spreading mid‐ocean ridges. Bathymetric and magnetic data indicate that both central segments have experienced ridge jumps since ∼1 Ma. Mantle Bouguer anomalies (MBAs) derived from shipboard free air gravity and swath bathymetric data show deep subcircular lows centered on the new ridge axes, suggesting that mantle flow has been established beneath the new spreading centers for at least ∼1 Myr. Inversion of gravity data indicates that crustal thicknesses vary by ∼4 km along axis, with the thickest crust occurring beneath a large axial volcanic edifice. Once the effects of lithospheric aging have been removed, a model in which gravity variations are attributed entirely to crustal thickness variations is more consistent with data from an axis‐parallel seismic line than a model that includes additional along‐axis variations in mantle temperature. Both geophysical and geochemical data from the region may be explained by the melting of small (<200 km) mantle chemical heterogeneities rather than elevated temperatures. Therefore, there may be no Ascension/Circe plume.