Seismic Evidence for Velocity Heterogeneity Along ∼40 Ma Old Oceanic Crustal Segment Formed at the Slow‐Spreading Equatorial Mid‐Atlantic Ridge From Full Waveform Inversion of Ocean Bottom Seismometer Data.

In slow spreading environments, oceanic crust is formed by a combination of magmatic and tectonic processes. Using full waveform inversion applied to active‐source ocean bottom seismometer data, we reveal the presence of a strong lateral variability in the 40–48 Ma old oceanic crust formed at the slow‐spreading Mid‐Atlantic Ridge in the equatorial Atlantic Ocean. Over a 120 km‐long section between the St Paul fracture zone (FZ) and the Romanche transform fault (TF), we observe four distinct 20–30 km long crustal segments. The segment affected by the St Paul FZ consists of three layers, an ∼2 km thick layer with a P‐wave velocity <6 km/s, a 1.5 km thick middle crust with a velocity of 6–6.5 km/s, and an underlying layer where velocity is ∼7 km/s, representing the lower crust. The segment associated with an abyssal hill morphology contains a high velocity of ∼7 km/s at 2–2.5 km below the basement, indicating the presence of primitive gabbro or serpenized peridotite. The segment associated with a low basement morphology seems to have 5.5–6.5 km/s velocity starting near the basement extending down to ∼4 km depth, indicating chemically distinct crust. The segment close to the Romanche TF, a velocity 4.5–5 km/s near the seafloor increasing to 7 km/s at 4 km depth indicates a magmatic origin. The four distinct crustal segments have a good correlation with the overlying seafloor morphology. These observed strong crustal heterogeneities could result from alternate tectonic and magmatic processes along the ridge axis, possibly modulated by thermal and/or chemical variations in the mantle during their formation along the ridge segment. Plain Language Summary: Generally magmatically accreted oceanic crust has a nearly uniform crustal structure. The speed of seismic waves, which is widely used as a proxy for the physical properties of the Earth, is relatively low in the upper crust but increases rapidly with depth, while in the lower crust the velocity is high but increases more slowly. Nevertheless, oceanic crust formed at slow‐spreading mid‐ocean ridges can be very heterogeneous. However, it is challenging to quantify the nature of this heterogeneity using conventional travel time based analyses. Here we apply a cutting‐edge full waveform inversion technique to an active‐source ocean bottom seismometer data set from the equatorial Atlantic Ocean. The aim is to create seismic velocity models of oceanic crust with enhanced details. The waveform inversion reveals four distinct zones within a 120 km long crustal section, with boundaries consistent with the seafloor morphology. The section contains two segments of normal oceanic crust, a segment with high velocities at shallow depth, possibly containing primitive gabbro, and another chemically distinct segment with nearly constant velocity in the whole crust. The observed strong crustal heterogeneities seem to result from a combination of magmatic and tectonic accretion processes possibly induced by thermally or chemically heterogeneous mantle. Key Points: We apply full waveform inversion to the crustal turning waves recorded by ocean bottom seismometers in the equatorial Atlantic OceanThe velocity model exhibits strong crustal velocity heterogeneity, containing four distinct segments correlating well with seafloor morphologyThe strong crustal heterogeneity seems to be caused by a combination of magmatic and tectonic processes [ABSTRACT FROM AUTHOR]