Crustal Structure and Evolution in the last 5 Ma of the Mid-Atlantic Ridge near the Fifteen-Twenty Fracture Zone

The Mid-Atlantic Ridge around the Fifteen-Twenty Fracture Zone is a unique portion of slow spreading ridge where igneous crust is locally absent and rocks of the lower crust and upper mantle are distributed on the seafloor over 100 km along both sides of the axis. The study of this area can provide important insights into the mechanical structure of oceanic lithosphere and magmatic processes at mid-ocean ridges. Well lineated, high-amplitude, and symmetrical magnetic reversals are present to 5 m.y. off-axis in a spreading segment south of 14° 30'N. This crust is also characterized by well developed, linear abyssal hills and low residual mantle Bouguer anomaly (RMBA). These characteristics indicate magmatically robust ridge development. In our survey closer to the FZ, magnetic reversal patterns are low-amplitude and partly disorganized, abyssal hills are shorter and irregular, and RMBA is regionally elevated. On-axis near 14° 45'N and 15° 40'N, normal magnetization highs are conspicuously missing, indicating that basaltic crust is very thin or absent. This is confirmed by high RMBA, which is elevated by ~15 mGal compared to adjacent crust. An extensive along-axis zone (>100 km) of limited magma supply is demonstrated by these features, consistent with observed widespread exposure of sampled ultramafic rocks. RMBA typically is asymmetric across the ridge axis. The general pattern of more positive RMBA over inside corner (IC) crust than over outside corner (OC) crust appears across most of the area. The IC/OC gravity patterns indicate that tectonic thinning of crust may be caused by detachment faults dipping toward the spreading axis from IC corners near segment ends, as has been proposed elsewhere in slow-spreading crust. The gravity data also suggest that detachment faulting is a relatively continuous and long-lived process. But OC crust also has abnormally high RBMA. Seafloor exposures of ultramafic rocks in both the footwall and hanging wall of detachment faults, are possibly attributed to this anomalously low magma supply and/or a relatively amagmatic phase in this region. This could result of lateral transport of melt toward the magmatically robust segments at 14° N and 16° N.