Oceanic crust at fast-spreading ridges is formed by melt percolating through the Mohorovičić Transition Zone (MTZ), the boundary between crust and mantle. However, the relationship between the crustal structures and MTZ remains elusive. Applying full waveform inversion to wide-angle seismic data acquired near the 9oN East Pacific Rise, we show that the variations in crustal MTZ thicknesses are inversely correlated along the segment, although their total cumulative thickness shows little variations. These variations could be attributed to different melt migration efficiency through MTZ or variation in mantle thermal structures. Thin MTZ could be due to rapid percolation of melt from mantle to crust whereas the thick MTZ results from the crystallization of melt within the transition zone. On the other hand, for relatively hot segments, melt will accumulate at shallower depth within the lower crust. In contrast, melt could freeze at Moho depth for relatively cold segments thickening the MTZ.

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 at the slow-spreading Mid-Atlantic Ridge in the equatorial Atlantic Ocean. Over a 120 km-long section between the St Paul fracture zone and the Romanche transform fault, we observe four distinct 20-30 km long crustal segments. The segment affected by the St Paul FZ consists of three layers, 2-km thick layer with velocity <6 km/s, 1.5 km thick middle crust with velocity 6-6.5 km/s, and an underlying layer with velocity ~7 km/s in the lower crust. The segment associated with an abyssal hill morphology contains high velocity ~7 km/s from a shallow depth of 2 – 2.5 km below basement, indicating the presence of primitive gabbro. 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 transform fault, a normal oceanic crust with 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. The observed strong crustal heterogeneities could result from alternate tectonic and magmatic processes along the ridge axis, possibly modulated by chemical variations in the mantle.

The magmatically accreted oceanic crust contains two distinct layers, the upper and the lower crust, whereas the tectonically controlled crust may have gabbros and serpentinite close to the seafloor. Using full waveform inversion applied to ocean bottom seismometer data, we reveal the presence of a strong lateral variability in the 40 – 48 Ma old oceanic crust in the slow-spreading equatorial Atlantic. Over a 120 km-long section we observe four distinct 20-30 km long crustal segments. The segment affected by the St Paul FZ consists of three layers, 2 km thick layer with velocity <6 km/s, 1.5 km thick middle crust with velocity 6-6.5 km/s, and an underlying layer with velocity ~7 km/s in the lower crust. The segment associated with an abyssal hill morphology contains high velocity ~7 km/s from a shallow depth of 2 – 2.5 km below the basement, indicating the presence of either serpentinized peridotite or primitive gabbro close to the seafloor. The segment associated with a low basement morphology has 5.5 – 6 km/s velocity starting near the basement extending down to a depth of 4 km, indicating chemically distinct crust. The segment close to the Romanche transform fault, a normal oceanic crust with velocity 4.5-5 km/s near the seafloor indicates a magmatic origin. The four distinct crustal segments have a good correlation with the overlying seafloor morphology features. These observed strong crustal heterogeneities could result from alternate tectonic and magmatic processes along the ridge axis, possibly modulated by chemical variations in the mantle.

Plate tectonics characterize transform faults as conservative plate boundaries where the lithosphere is neither created nor destroyed. In the Atlantic, both transform faults and their inactive traces, fracture zones, are interpreted to be structurally heterogeneous, representing thin, intensely fractured, and hydrothermally altered basaltic crust overlying serpentinized mantle. This view, however, has recently been challenged. Instead, transform zone crust might be magmatically augmented at ridge-transform intersections before becoming a fracture zone. Here, we present constraints on the structure of oceanic crust from seismic refraction and wide-angle data obtained along and across the St. Paul fracture zone near 18°W in the equatorial Atlantic Ocean. Most notably, both crust along the fracture zone and away from it shows an almost uniform thickness of 5-6 km, closely resembling normal oceanic crust. Further, a well-defined upper mantle refraction branch supports a normal mantle velocity of 8 km/s along the fracture zone valley. Therefore, the St. Paul fracture zone reflects magmatically accreted crust instead of the anomalous hydrated lithosphere. Little variation in crustal thickness and velocity structure along a 200 km long section across the fracture zone suggests that distance to a transform fault had negligible impact on crustal accretion. Alternatively, it could also indicate that a second phase of magmatic accretion at the proximal ridge-transform intersection overprinted features of starved magma supply occurring along the St. Paul transform fault.