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Very long oceanic transform faults do not just offset mid-oceanic ridges by hundreds of kilometers, but they also induce variations in the lithospheric structure and affect mantle melting, and hence the crustal accretion at the ridge-transform intersection (RTI). However, the deep structure of large offset RTIs remains unknown. Here, we present three-dimensional seismic tomography results (P- and S-wave velocities, Vp, Vs) in the vicinity of the eastern Romanche transform and the Mid-Atlantic Ridge in the equatorial Atlantic Ocean, using arrival times from microearthquakes recorded by a recent temporary array of seafloor seismometers. We find that normal to low Vp/Vs ratios (∼1.7) are observed at the Mid-Atlantic Ridge, representing weak serpentinization and the dominant magmatic processes, whereas high Vp/Vs ratios (∼1.8-1.95) are observed west of the RTI, at the eastern termination of the 2016 M w 7.1 earthquake. However, unusually low Vp/Vs ratios (∼1.6-1.7) are observed at 0-15 km depth and in the region of the 2016 M w 7.1 earthquake rupture zone. We suggest that the unusually low Vp/Vs ratios result from the presence of thick cracks at shallow depths (0-8 km) and/or the enrichment of orthopyroxene in the deep mantle (8-15 km). • 3-D Vp and Vs models for the Romanche eastern ridge-transform intersection. • Low Vp/Vs ratios (∼1.6-1.7) at depths of <15 km along the transform are observed. • The low ratios may be due to thick cracks and/or orthopyroxene enrichment. [ABSTRACT FROM AUTHOR]

Deep-sea sediments record the geological and climate history of the Earth. The seismic properties of these sediments, namely the P- and S-wave velocities, depend on their chemical compositions, grain size, age and depth of burial, which, in turn, influence the porosity. We use multichannel seismic reflection data to estimate the sediment thickness and P-wave velocity over 4-74 Ma old oceanic crust on the African Plate and 3-27 Ma old crust on the South American Plate in the equatorial Atlantic Region. We then use the P-to-S converted waves from the basement-sediment interface recorded by 69 multi-component ocean bottom seismometers to determine the P- and S-wave velocity ratios ( V P V S ) over 3 to 74 Ma. We find that the V P V S decreases from 8.9 ± 2.0 at 3 Ma to 5.0 ± 1.0 at 17 Ma but abruptly increases to 19.5 ± 2.8 at 37.5 Ma. Then, it gradually decreases for crustal ages >37 Ma, reaching 7.0 ± 0.3 at 74 Ma. These variations are almost symmetric across the Mid-Atlantic Ridge over 4-27 Ma, suggesting a relation to climate change rather than regional effects. We estimate the average sediment porosity using the V P V S and find that the average porosity decreases from 89.2 ± 1.5% at 37.5 Ma to 69.9 ± 1.2% at 74 Ma, possibly associated with accumulation of terrigenous input from Africa. Between 4 and 37 Ma, the porosity fluctuates between 70% and 85%. Such a fluctuation correlates well with the decrease in the global atmospheric CO 2 from 800 ppm to 400 ppm during the Oligocene and with carbonate compensation depth, which are possibly related to fauna/flora diversification and calcareous sediment accumulation. The variation in porosity at 23 and 17 Ma also appears to be linked with major climate shifts, suggesting that sediment porosity could be used to shed light on the past climate and fauna/flora characterization. • We use seismic data to characterize sediment porosity in the equatorial Atlantic. • Porosity has anomalous and symmetric trend across the MidAtlantic Ridge. • We show that in deep-sea environments, climate affects the sediment porosity. • Our porosity results correlate with the climate history of the Oligocene and Miocene. [ABSTRACT FROM AUTHOR]

Extension at slow- and intermediate-spreading mid-ocean ridges is commonly accommodated through slip on long-lived faults called oceanic detachments. These curved, convex-upward faults consist of a steeply-dipping section thought to be rooted in the lower crust or upper mantle which rotates to progressively shallower dip-angles at shallower depths. The commonly-observed result is a domed, sub-horizontal oceanic core complex at the seabed. Although it is accepted that detachment faults can accumulate kilometre-scale offsets over millions of years, the mechanism of slip, and their capacity to sustain the shear stresses necessary to produce large earthquakes, remains subject to debate. Here we present a comprehensive seismological study of an active oceanic detachment fault system on the Mid-Atlantic Ridge near 13°20′N, combining the results from a local ocean-bottom seismograph deployment with waveform inversion of a series of larger teleseismically-observed earthquakes. The unique coincidence of these two datasets provides a comprehensive definition of rupture on the fault, from the uppermost mantle to the seabed. Our results demonstrate that although slip on the deep, steeply-dipping portion of detachment faults is accommodated by failure in numerous microearthquakes, the shallow, gently-dipping section of the fault within the upper few kilometres is relatively strong, and is capable of producing large-magnitude earthquakes. This result brings into question the current paradigm that the shallow sections of oceanic detachment faults are dominated by low-friction mineralogies and therefore slip aseismically, but is consistent with observations from continental detachment faults. Slip on the shallow portion of active detachment faults at relatively low angles may therefore account for many more large-magnitude earthquakes at mid-ocean ridges than previously thought, and suggests that the lithospheric strength at slow-spreading mid-ocean ridges may be concentrated at shallow depths. [ABSTRACT FROM AUTHOR]

• S-to-P images the lithosphere–asthenosphere boundary near the Mid-Atlantic Ridge. • The lithosphere–asthenosphere boundary deepens with age, not always monotonically. • We infer melt in punctuated upwellings and in channels, 2 stages of melt migration. • Melt generation and migration dictate a dynamic lithosphere–asthenosphere boundary. • Rising melt may push the plates from below, and melt channels facilitate decoupling. In plate tectonic theory a weak asthenosphere is required to facilitate the motions of the rigid plates. Partial melt could weaken the mantle, in turn impacting convection, but to date the existence of persistent melt has remained controversial. A wide range of scenarios has been reported in terms of the location, amount and pathways of melt. Here we use data collected by 39 ocean bottom seismometers deployed near the equatorial Mid-Atlantic Ridge on 0 to 80 Myr old seafloor. We calculate S-to-P (Sp) receiver functions and perform waveform modeling. We jointly interpret with shear-wave velocity tomography from surface waves and magnetotelluric (MT) imaging to take advantage of a range of resolutions and sensitivities and illuminate the structure of the oceanic lithosphere and the underlying asthenosphere. We image a tectonic plate thickness that increases with age in one location but undulates in another location. We infer thin and slightly thicker melt channels and punctuated regions of ascending partial melt several hundred kilometers off the ridge axis. This suggests melt persists over geologic timescales, although its character is dynamic, with implications for the lithosphere–asthenosphere boundary (LAB) and the driving forces of the plates. Ascending melt intermittently feeds melt channels at the base of the plate. The associated melt-enhanced buoyancy increases the influence of ridge-push in driving plate motions, whereas the channelized melt reduces the resistance of the plates to motion. Therefore, melt dynamics may play a larger role in controlling plate tectonics than previously thought. [ABSTRACT FROM AUTHOR]