Seismic evidence for uniform crustal accretion along slow-spreading ridges in the equatorial Atlantic Ocean

<jats:title>Abstract</jats:title>         <jats:p>The crustal accretion along mid-ocean ridges is known to be spreading-rate dependent. Along fast-spreading ridges, two-dimensional sheet-like mantle upwelling creates relatively uniform crust. In contrast, the crust formed along slow-spreading ridges shows large along-axis thickness variations with thicker crust at segment centres, which is hypothesised to be due a three-dimensional plume-like mantle upwelling with melt focusing to segment centres. Using wide-angle seismic data acquired from the equatorial Atlantic Ocean, we show that the crustal thickness is nearly uniform (~5.5 km) across five segments for crust formed at the slow-spreading Mid-Atlantic Ridge with age varying from 8 to 70 Ma. The crustal velocities indicate that this crust is predominantly of magmatic origin. We suggest that this uniform magmatic crustal accretion is due to a two-dimensional sheet-like mantle upwelling, contradicting previous hypothesis. Our results also indicate that the lateral extend of magmatic accretion could be used to define the sheet-like mantle upwelling regions beneath the global ridge system.</jats:p>

section_content: The oceanic crust covers ~60% of the Earth's surface and is continuously generated along the ~65,000 km-long divergent plate boundaries at the Mid-Oceanic Ridges (MORs) 1. 

The MORs are partitioned into tens to hundreds kilometre-long first-order segments by oceanic transform faults (TFs) 1. Between two oceanic TFs, these ridge segments are further divided into smaller ridge segments (second-order segments) by non-transform offsets (NTOs) and overlapping spreading centres (OSCs) 1. 

The crustal accretion along MORs and the resulting along-axis crustal thickness variation are known to be spreading-rate dependent. At the fast-spreading ridges, such as the East-Pacific Rise (EPR), the oceanic crust exhibits relatively uniform thickness with a modest crustal thinning (≤1. 6 km; orange triangles in Fig. 1 ) at oceanic TFs, NTOs and OSCs [2] [3] [4] [5] [6], interpreted to result from a two-dimensional (2-D) sheet-like mantle upwelling beneath these ridges 3 and/or rapid ductile flow of hot lower crust along these ridges 7. In contrast, the studies using gravity data 3, [8] [9] [10] [11] collected along the slow-spreading Mid-Atlantic Ridge (MAR) reveal systematically significant reduction (up to 50%) in crustal thickness from segment centres to the associated distal ends at TFs and NTOs. Crustal thickness estimation using active-source seismic data also shows large and systematic along-axis crustal thickness variations at MAR, where the crust at distal ends could be up to ~3. 2-5. 7 km thinner than that at the segment centres 9, [12] [13] [14] [15] [16] [17] [18] (orange dots in Fig. 1 ). All these observations demonstrate focused magmatic crustal accretions along the slow-spreading ridges, which are interpreted to result from either a three-dimensional (3-D) plume-like mantle upwelling 3, 8 or by a 3-D melt migration to the segment centres at the base of the lithosphere beneath slow-spreading ridges 19, 20. However, these previous studies on MAR are generally concentrated within or close to the spreading centres, sampling only young (≤2 Ma) oceanic crust. In contrast, old oceanic crusts formed along MAR show different patterns of segment-scale crustal thickness variation. For example, although large segment-scale crustal thickness variations are observed for ~5 and ~65 Ma old crust in the North Atlantic Ocean 17, 21, 22, the crustal thinning is only ~2. 8-3. 4 km (blue dots in Fig. 1 ), smaller than those observed along or near ridge axis. For much older crust (~130-150 Ma) in the North Atlantic Ocean, the crustal thinning is very modest 23, 24. A recent study degree of alteration 34. Therefore, the Vp/Vs ratio is a useful physical parameter for constraining the crustal composition 37, hence the thickness of the crust. 

Here, we estimate the Vp and Vp/Vs ratio of five segments of the slow-spreading crust covering ~8-70 Ma lithosphere in the equatorial Atlantic Ocean (Figs. 2a) using an active-source seismic refraction dataset to characterize the nature of the oceanic crust and to shed light on the crustal accretion and mantle upwelling processes along slowspreading ridges. 

The equatorial Atlantic Ocean started opening ∼100-140 Ma 38, 39. The average halfspreading rate varied several times in the past between 15 and 25 mm/yr, and the current half-spreading rate is ~16 mm/yr 40 (Fig. 2b ), hence the equatorial MAR can be classified as a slow-spreading ridge 41. In this region, three east-west striking mega-transforms (St. 

Paul, Romanche and Chain TFs) offset the MAR by a total of ~1,800 km (Fig. 2a ). The St. Paul TF system encompasses four TFs interrupted by three intra-transform ridge segments, generating a total offset of ~600 km 42. Away from its eastern ridge-transform intersections (RTI), the traces of the four TFs persist up to ~20 Ma of seafloor and then only two traces of the TFs can be identified on the older seafloor 43. Our seismic profile transects the St. Paul fracture zone (FZ) where the crustal age is ~70 Ma in the north and ~40 Ma in the south. The Romanche TF offsets the MAR by ~880 km 44. The portion of the Romanche TF crossed by our seismic profile is characterized by a ~6 km-deep and ~40 km-wide valley with bounding walls shallowing to <3 km depth below sea level. The age contrast along our profile is ~32 Ma across the Romanche TF, with crustal age of ~40 Ma in the north and ~8 Ma in the south. Our seismic profile crosses the Chain FZ at ~160 km west of the western Chain RTI, where the crustal age is ~10 Ma in the north and ~24 Ma in the south. Along our profile, the Chain FZ is characterized by a ~10 km-wide sedimentary basin, which is bounded by a transverse ridge to the south and shows gradual seafloor shallowing to the north 45. An oblique pseudofault is observed between the Chain and Charcot FZs, intersecting the Chain FZ at ~46 Ma to the west and NTO within the ~17 Ma 45, showing a typical half V-shaped feature on the seafloor. 

The seismic data used in this study was acquired in 2018 during the ILAB-SPARC experiment 42, 44, 45. The ~855 km-long seismic profile is nearly parallel to the ridge axis in an approximately N-S direction, starting just north of the Charcot FZ in the south and extending to ~230 km distance north of the St. Paul FZ (Fig. 2a ). Fifty OBSs were deployed along the ~700 km part of the profile at average 14. 2 km interval. An airgunarray of 16 guns with a total volume of 4990 cubic inches was towed at 10 m depth below sea level and was fired every 300 m. Both P and S-wave arrivals were hand-picked on the pressure data recorded by hydrophones (Supplementary Figs. 1 and 2 ) and were inverted using a 2-D ray-based travel time tomographic method 2, 46 to obtain Vp and Vs models (Figs. 3a, b ; See Methods). Then the crustal and upper mantle Vp/Vs ratios (Fig. 3c ) were calculated using the inverted Vp and Vs models. Using these results, we first determine the predominant mechanism of crustal formation for each crustal segment and then analyse the segment-scale crustal thickness variations to understand the crustal accretion process along the slow-spreading MAR in the equatorial region.

section_name: Results

section_content: To determine the dominant mode of crustal accretion, we use a Vp/Vs ratio of 1. 9 as an indicator 37 to discriminate between the magmatic crust (Vp/Vs<1. 9) and the serpentinised peridotite (Vp/Vs>1. 9) at crustal depth. The tomographic results show two different types of crustal Vp structures for the five ridge segments characterized by distinct crustal Vp/Vs ratios (Fig 3 ), which can be interpreted resulting from two different modes of crustal accretion: magmatic accretion and tectonic accretion along detachment fault. 

The oceanic crust within the Segments 1, 2, 3-S, 4 and 5 exhibits a two-layered Vp structure (Figs. 3a and 4a ) characterized by distinct vertical Vp gradients. Using the vertical Vp gradient of 0. 5 s -1 to define the boundary between the upper and lower crust 47, the upper crust is ~1. 9-2. 3 km thick showing high vertical Vp gradients of ~0. 66-0. 80 s -1 (Supplementary Table 1 ). Beneath the upper crust, the Vp in the lower crust, which is ~3. 1-3. 5 km thick, increases at a much-reduced velocity gradient of ~0. 13-0. 17 s -1 (Supplementary Table 1 ). The maximum crustal Vp within these segments is generally <7. 2 km/s (Figs. 3a and 4a ). These crustal Vp structures fall in the velocity range typical for older slow-spreading magmatic oceanic crust 27, 28 (Fig. 4a ). These crustal segments are generally characterized by crustal Vp/Vs ratios less than 1. 9 (Fig. 3c ), indicative of basaltic and gabbroic rocks in the crust 37. The Moho, the crust/mantle boundary, is well constrained using Moho wide-angle reflections. A rapid increase from crustal velocities to mantle velocities across the Moho is observed throughout these ridge segments, where the Vp increases abruptly by ~0. 7-0. 9 km/s across the Moho (Figs. 3a and 4a ). Both the Vp and Vp/Vs ratio models support that the observed two-layered crustal velocity structure can be explained by the 'Penrose' model 1 indicating a predominantly magmatic accretion of crust throughout these ridge segments. 

In contrast, the Segment 3-N between 340 and 400 km profile distance immediately south of the Romanche TF shows a rapid increase in the crustal Vp of ~7. 7 km/s at ~2. 2 km sub-basement depth (Figs. 3a and 4b ). This anomalously high Vp exceeds the typical velocity for the normal oceanic crust (<7. 2 km/s 27, 28 ). Conversely, the upper crustal Vs within Segment 3-N shows a similar velocity-depth variation as that within Segment 3-S. 

The crustal Vp/Vs ratios are larger than 1. 9 in the upper crust (Fig. 3c ), suggesting the presence of serpentinised peridotite, indicative of predominantly tectonic extensions 37. 

The rapid increase in the crustal Vp with depth can be interpreted as a gradual decrease in mantle serpentinisation. Assuming a Vp of 8. 0 km/s and 5. 0 km/s for 0% and 100% serpentinised peridotite 34, 35, respectively, the observed ~7. 7 km/s velocities represent ~10% of mantle serpentinisation at ~2. 2 km sub-basement depth. A recent petrological study of dredged rocks from the eastern Romanche RTI region identified fragments of old OCCs on the south flank of the Romanche TF 48. This is consistent with our interpretation that immediately south of the Romanche TF, the plate separation is mainly accommodated by tectonic slip along detachment faults.
section_name: Magmatically accreted crust versus tectonically controlled crust

section_content: The thickness of the igneous crust is defined as the thickness between the top basement and the seismically constrained Moho. Based on the crustal Vp and Vp/Vs structures, Segments 1, 2, 3-S, 4 and 5 (Fig. 3 ) are interpreted as consisting of magmatic crust. The average crustal thicknesses within these magmatic segments are ~5. 4-5. 6 km (Fig. 5 ), consistent with some previous estimates in this region 42, 44, 45. The average crustal thicknesses of the five segments are ~500-700 m thinner than the global average of the old (>7. 5 Ma) slow-spreading crust but still fall into the standard deviation (6. 1±1. 0 km 27 ). The oceanic crust shows a slight thinning at FZ, TF and pseudofault regions, which is confined in a narrow zone in the vicinity of the trough of these tectonic discontinuities (Fig. 5 and blue squares in Fig. 1 ). At the southern end of Segment 1, the crust thins by ~600 m at the centre of the northern St. Paul FZ valley over <5 km distance and the average crustal thickness within the St. Paul FZ is only ~200-400 m thinner than the average thickness of the adjacent segments 42 (Figs. 5a, b ). The crust within the ~20 km-wide Romanche transform valley is ~5. 1 km thick on average, which is only ~500 m thinner than the average of Segment 2 to the north (Fig. 5b ). Though the crustal Vp/Vs ratio is not constrained, Gregory et al. 44 argued the thick crust at the Romanche transform zone is likely composed of fractured mafic rocks, and hence can support the supershear rupture during the 2016 M w 7. 1 earthquake 49. The crust beneath the ~10 km-wide Chain FZ trough has an average thickness of ~4. 7 km, which is ~700-800 m thinner than the averages of the adjacent Segments 3-S and 4 (Figs. 5c, d ). The thinnest crust within the Chain FZ trough is ~4. 1 km, which is ~500 m thinner than that measured in ref. (45). The average crustal thickness within the pseudo-fault trough is ~5. 2 km with the thinnest crust of ~4. 8 km thick, which is ~200-400 m thinner than the adjacent segments (Fig. 5e ). The crustal Vp and Vs (Figs. 3a, b ) are lower at the FZ, TF and pseudofault regions relative to the surrounding crust, which could be due to the presence of faults and fluids. 

We take the standard deviation of mean crustal thickness as a measure of the intrasegment crustal thickness variation 13, 14, 25, 50, 51. The standard deviations of the mean crustal thickness within these five segments are 0. 1-0. 4 km (Fig. 5 ), which is much smaller than those (≥1. 0 km 13, 14 ) for other MAR segments where large along-axis crustal thinning is observed. However, they are very close to those (≤0. 3 km 50, 51 ) observed along the fast-spreading EPR, suggesting relatively uniform crustal thickness within these segments. Though the locations of NTOs are difficult to identify over the old crust in this region, there is no evidence supporting thicker crust at the segment centre and >2. 8 km gradual crustal thinning towards the tectonic discontinuities within any of the five segments. The relatively low crustal Vp at the FZ, TF and pseudofault regions (Fig. 3a ) indicates the thick crust at these tectonic discontinuities is not due to the inherent positive velocity-Moho depth trade-off in the travel time tomography 46. A Monte-Carlo based uncertainty analysis yields the variance in the Moho depth of <330 m (see Methods; Supplementary Fig. 5b ), demonstrating that the observed uniform crust does not result from a particular starting Vp model used in tomography. The checkerboard tests (see Methods; Supplementary Figs. 6 and 7 ) demonstrate that the used tomography method can recover ~1. 5-2. 0 km crustal thickness variations when the real variations in crustal thickness are ~2. 5-3. 0 km over ~70 km lateral distance. These tests demonstrate that the observed uniform crust within these five ridge segments is real.
section_name: Intra-segment crustal thickness variation within magmatic segments

section_content: Most of previous studies using gravity and seismic data collected on the slow-spreading MAR reveal systematic and significant along-axis reduction in crustal thickness from the centre of segments towards the oceanic TFs, FZs and NTOs 3, 8-18, 21, 22, suggesting a 3-D plume-like mantle upwelling 3, 8 and/or a focused melt concentration to segment centres 19, 20 beneath the slow-spreading ridges. However, the five segments with predominantly magmatically accreted crust in the equatorial Atlantic Ocean show slightly thin crust with little intra-segment crustal thickness variation throughout each segment (Figs. 1 and 5 ), which is remarkably different from the most previous observations in the Atlantic Ocean but similar to some recent observations on 6. 6-61. 2 Ma oceanic crust at 31 o S in the South Atlantic Ocean 25. The crust within these segments is either (1) originally formed uniformly at the ridge axis, or (2) has been modified after crustal formation by tectonic extension/stretching during the amagmatic period, or (3) by a second-stage crustal accretion at RTIs. Here, we analyse all these hypotheses and propose the best mechanism that explains the formation of uniformly thick crust in the equatorial Atlantic Ocean. 

The tectonic extension and stretching account for ~10% of plate separation at the slowspreading ridges 52, 53, which could thin the oceanic crust. The fault spacing and heave are generally larger at segment ends than at segment centres of slow-spreading ridges, indicating more tectonic extension should occur at segment ends due to the decreasing magma supply 54. If the focused mantle upwelling and melt supply did occur at the time of crustal formation, the tectonic extension and stretching would increase the magnitude of the intra-segment crustal thinning 10, 54 within these five ridge segments, rather than diminish this trend. Furthermore, numerical modelling shows that the gravity anomaly caused by tectonic faulting increases with increasing fault throw and spacing 55, further supporting the idea that the tectonic extension and stretching will enhance the along-axis variation in crustal thickness. All the evidence demonstrates that the tectonic extension and stretching cannot explain the uniform crust observed along our profile. Some recent studies indicate that the crust at oceanic TF can get augmented by a second stage of magmatic accretion at RTI 45, 56. This second-stage crustal accretion at RTIs could laterally emplace dykes at the segment ends and into the transform valley 45, forming Jshaped structures on the seafloor and thickening the crust. However, the gravity data reveal a ~5 km-thick crust within the Chain transform zone 57, which is comparable with the average ~5. 1 km-thick crust within the Chain FZ trough along our profile (Fig. 5c ). 

Although we do not have any information about the crustal thickness for the Romanche FZ, the crust within the Romanche TF has similar thickness to those within St. Paul and Chain FZs along our profile (Fig 5 ). These observations suggest the second-stage magmatic accretion may have limited augmentation to the crust at RTIs. Furthermore, the J-shaped structures have very short extensions over the old ocean floor 56, suggesting the second-stage crustal augmentation occurs within a limited region in the vicinity of the RTI. Thus, the second-stage magmatic accretion at RTIs is not likely to change the segment-scale variations in the crustal thickness. 

Another possibility is that the uniform crust might reflect the patterns of crustal and mantle flows and the melt migration along the ridge axis at the time of crustal formation. 

The slow-spreading (~10 mm/yr half-spreading rate) Reykjanes Ridge in the North Atlantic Ocean produces 8-10 km-thick crust showing an absence of significant alongaxis variation in thickness 58. This is interpreted to be due to a rapid ductile flow within the hot lower crust because of the influence of the Iceland hotspot 7. Though the ridge between 5 o N and the St. Paul TF is suggested to be influenced by the Sierra Leone plume 59, 60, the crustal thicknesses north and south of the St. Paul FZ are similar, ruling out any significant influence of the thermal anomaly to crustal accretion in our study area. Furthermore, the average crustal thickness of 5. 4-5. 6 km is not thick enough, and hence not hot enough to enable rapid ductile flow in the lower crust. On the other hand, a 3-D diapiric mantle upwelling would produce a large along-axis variation in crustal thickness 3, 8, which is inconsistent with our observations of the five crustal segments studied here. 

Finally, the relatively uniform crustal thickness observed in the equatorial Atlantic Ocean suggests a relatively uniform (nearly 2-D) mantle upwelling beneath the ridge axis at the time of the crustal accretion (Fig. 6 ), contradicting the previously proposed 3-D plumelike mantle upwelling model for the slow-spreading ridges 8 based on the observations from the North and South Atlantic Ocean, instead it is more similar to the uniform mantle flow pattern occurring at the fast-spreading ridges 3. The petrological studies of basaltic glasses dredged along the equatorial MAR between the St. Paul and Charcot TFs show a long-wavelength trend (~600 km) in the mantle potential temperature, the mean degree of partial melting and the maximum depth of the decompression melting. This longwavelength trend is independent of the offset and the location of TFs 59, 60. While north of the St. Paul TF to ~5o N, the mantle potential temperature, the initial depth and the mean degree of mantle melting have little variations over ~400 km lateral distance due to the influence of the Sierra Leone plume 59, 60. These observations suggest that the extent of mantle melting and mantle isotherms are not significantly suppressed approaching TFs and NTOs as required in the 3-D diapiric mantle upwelling 3, 8, supporting a relatively uniform mantle upwelling in the study region. 

The lithosphere-asthenosphere boundary beneath the slow-spreading ridges is expected to progressively deepen from the segment centre towards the oceanic TFs 61 due to the juxtaposition of ridge axis with cold lithosphere. The lithospheric thinning away from the TFs towards the centre of segments would shallow the top surface of the melting region, and subsequently enhance the melting at the centre of the ridge segment and produce a thick crust. Magde et al. 19, 20 argued that the segment-scale crustal thickness variations observed at the slow-spreading MAR between 33 o N and 35 o N are primarily produced by the focused melt migration to the centre of segments along the slope of the top surface of the melting region, though the mantle upwelling is roughly uniform. In our study region, however, the basaltic glasses dredged from the vicinities of the western RTI of the St. 

Paul TF, the eastern RTIs of the Romanche and Chain TFs show large local geochemical and isotopic dispersions, suggesting the melt did not pool and mix in the magma chamber but segregated and erupted rapidly from the mantle 59. This implies that the melt would migrate vertically and rapidly beneath the ridge axis in the equatorial Atlantic Ocean (Fig. 6 ), rather than focusing to the segment centres before eruption. The nearly 2-D sheet-like mantle upwelling together with nearly vertical melt migration and rapid eruption would facilitate the formation of relatively uniform crust along the ridge axis at the time these five crustal segments were formed. Although a possible existence of a relatively uniform mantle upwelling can be made on the petrological observations along the present-day ridge axis in the equatorial Atlantic Ocean, our results indicate that this process has persisted over the last 70 Myr. 

The observed uniformity in crustal thickness within a second-order ridge segment between the Rio Grande and Cox FZs in the South Atlantic Ocean is suggested to be related to a long first-order ridge segment (~320-350 km) facilitating large-scale mantle upwelling 25. To investigate the influence of the first-order ridge segment length on the crustal accretion, we compiled the maximum crustal thickness variation between the centre and ends of second-order ridge segments and the length of the corresponding firstorder ridge segments in the Atlantic Ocean (Fig. 7 ). Our compilation demonstrates that there is no positive or negative correlation between the first-order segment lengths and the crustal thickness variations. For example, the ~220 km-long first-order segment between the Hayes and Oceanographer TFs shows a maximum of 2. 8-4. 3 km along-axis crustal thickness variations over the 0-5 Ma crust 13, 14, 17 while the ~800 km-long segment between the Kane and Atlantis TFs shows a maximum crustal thickness variation of 3. 5 km along the ridge axis 8. The lengths of the first-order ridge segments in the equatorial Atlantic Ocean are similar to that of the segment between the Hayes and Oceanographer TFs, but relatively uniform crust is observed. Lin et al. 8 and Detrick et al. 10 argued that at the slow-spreading ridges, the magnitude of the along-axis crustal thickness variation within a second-order ridge segment is positively correlated with the length of the second-order segment. We cannot identify the locations of the fossil NTOs (except the pseudofault between the Chain and Charcot FZs) along the seismic profile 43, but the present-day spreading centres in the equatorial Atlantic Ocean have similar second-order segment length (for example ref. 45) as those observed in other regions in the Atlantic Ocean. We therefore argue that the lengths of first-and second-order ridge segments have no influence on the crustal accretion process. 

We envisage that the formation of the uniform crust might be associated with the brittle mantle weakening and hydrous mantle melting at the transform zone and the enhanced hydrothermal circulations on the spreading centres. By considering a more realistic brittle mantle weakening, Behn et al. 62 argued the thermal structure of the transform zone is much warmer than predicted from the half-space cooling model, which can better fit the depth of seismicity on the oceanic TFs. Their modelling also demonstrates an enhanced mantle upwelling along the transform zone and a much thinner lithosphere at the transform zone, especially at the centre of the transform, than estimated in previous studies using simplified rheologic laws. The base of the lithosphere at the transform zone can be further uplifted by the low-temperature hydrous mantle melting due to the presence of water (Wang et al., 2022). Meanwhile, the lithosphere beneath the ridge axis in the equatorial Atlantic Ocean is imaged to be thickened due to the enhanced hydrothermal cooling 63, 64. The combined effect of all these processes could significantly decrease the cold edge effect of large oceanic TF, leading to a relatively flat isotherm (lithospheric base) beneath the ridge axis with only a slight deepening of the lithospheric base in the vicinity of the transform zone. Using the crustal thickness inferred from gravity data, Detrick et al. 10 argued that the magnitude of the along-axis crustal thinning is approximately proportional to the offset of the adjacent TF. However, only three oceanic TFs of length <120 km are investigated in their studies, which are much shorter than the three mega-transforms (~300-880 km) in the equatorial Atlantic Ocean. In contrast, the compilation of crustal thickness variations (Fig. 8 ) inferred from activesource seismic data demonstrate that there is no positive correlation between the magnitude of the along-axis crustal thickness variation and the length of the adjacent TF or NTO. The magnitude of the crustal thickness variation is more scattered and larger when the adjacent TF or NTO is short but is smaller for longer TFs. This suggests the mega-transform could facilitate a stable 2-D sheet-like mantle upwelling and a relatively uniform crustal accretion along the slow-spreading ridges. 

The tectonic fabric of the ocean basin also reflects the different magmatic accretion patterns (focused or uniform) along the MAR. Though NTOs are observed on the ridge axis in the equatorial Atlantic Ocean 43, 45, the seafloor fabric 43 indicates that these NTOs are more transient features with nearly no imprint away from the ridge axis, suggesting that the base of the lithosphere beneath these short-lived NTOs does not deepen much and remains nearly flat, hindering the melt focusing towards segment centres and leading to a relatively uniform crustal accretion. 

The equatorial Atlantic Ocean has been suggested to be a cold spot 60, 65, where the mantle temperature is at least 150 o C lower than that beneath other parts of the MAR 65. 

Our results suggest that the dominantly uniform magmatic accretion can occur not only at fast-spreading ridges 3 and at slow-spreading ridges influenced by hotspots 7, but can also occur at slow-spreading ridges influenced by cold spots over a long geological period. 

The average crustal thicknesses of the five magmatic segments in the equatorial Atlantic Ocean are thinner than the global average 27, which can be attributed to a low degree of mantle melting due to the cold mantle 59, 60. Studies using bathymetry and seismic refraction datasets found that the detachment fault accommodating plate separation occurs along ~50% of the ridge axis in the North Atlantic Ocean 22, 66. However, along our seismic profile, the magmatically accreted crustal segments comprise >90% of the total profile length, which is much higher than the previous estimates for the slow-spreading ridges, suggesting that the mode of accretion plays a more important role in defining the OBSs as secondary arrivals after the P-wave first arrivals and codas (Supplementary Fig. 2 ). In this study, we use Sg, SmS and Sn to represent the S-wave crustal refractions, Moho reflections and mantle refractions, respectively. We first pick the S-wave arrivals that have high signal-to-noise ratios using the trace-to-trace coherency of phases and then we invert these picks to obtain a smooth velocity model for the crust and uppermost mantle. Synthetic travel times of S-wave phases are modelled using this smooth velocity model, which is then used as a guide to identify and pick more S-wave arrivals that have low signal-to-noise ratios 25, 67. The Sg arrivals are identified and picked between 5 and 35 km offsets (Supplementary Fig. 2, orange dots) with picking uncertainty ranging from 30 to 60 ms. The SmS phases are picked at offset ranges similar to the PmP arrivals (Supplementary Fig. 2, cyan dots), with picking uncertainty of 60-80 ms. The Sn arrivals are picked on 38 OBSs (Supplementary Fig. 2, blue dots) with picking uncertainty of 50-100 ms. 

Starting velocity models for travel time tomography. The seafloor depth and sediment thickness along the seismic profile are determined using coincident high-resolution bathymetry data and seismic reflection data 44, 45. The velocities of water and sediment are set to 1. 5 km/s and 1. 86 km/s in tomography 44, 45, respectively. The initial crustal Vp model is constructed using a simplified one-dimensional (1-D) velocity profile 44, 45 hanging from the basement. We introduce a smooth Moho interface into the starting Vp Ray-based travel time modelling and tomography. We model and invert all the picked travel times using a two-dimensional ray-based travel time tomography method 2, 46. The ray paths and travel times of the crustal and mantle arrivals are calculated using the shortest path method 68. We take a top-down inversion strategy in the tomography of Pwave arrivals. The Pg arrivals are inverted first to constrain the velocity of the upper crust, followed by a joint inversion of Pg and PmP arrivals simultaneously to constrain the crustal velocity and the Moho depth. After the joint inversion of Pg and PmP arrivals, we fix the crustal velocity and the Moho depth, and invert the travel times of Pn arrivals only to update the mantle velocity. First-and second-order velocity regularizations are imposed to obtain a smooth velocity model 46. The regularization parameters are tested and selected in each iteration step to avoid the introduction of artefacts. We use the standard 2 value 46 to measure the mismatch between the modelled and manually picked travel times. We stop the inversion when the 2 value approaches 1. 0 or when nonphysical artefacts appear when further decreasing the 2 value. The final 2 value is 1. 6 for Pg and PmP arrivals (Supplementary Fig. 3a ) and is 2. 2 for Pn arrivals (Supplementary Fig. 3b ). The final RMS misfits 46 The same inversion method and strategies are used in the S-wave tomography, except the Moho depth constrained by the PmP arrivals is fixed in the joint inversion of Sg and SmS arrivals. Doing so, we have assumed that P-and S-waves share the same boundary between crust and upper mantle. The final 2 value of combined Sg and SmS arrivals is 1. 2 (Supplementary Fig. 3c ) and that of Sn arrivals is 1. 5 (Supplementary Fig. 3d ). The final RMS misfits are 51, 63 and 98 ms for Sg, SmS and Sn arrivals, respectively. Ray coverage. We use the derivative weight sum (DWS) 69 to represent the density of ray coverage through the final tomographic models. The DWS of the P-wave arrivals throughout the crust and upper mantle is shown in Supplementary Fig. 4a. The Vp of the upper crust is constrained by dense rays of Pg arrivals between 60 and 760 horizontal distances along the profile, while the lower crust has relatively sparse PmP ray coverage. 

In the mantle, the Pn arrivals sample the upper mantle down to ~60 km depth below sea level beneath the Romanche transform valley and the maximum sampled depth gradually decreases to south and north (Wang et al., 2022), but here we only show the model down to 12. 5 km depth (Supplementary Fig. 4a ). Supplementary Fig. 4b shows the DWS of S-wave arrivals through the crust and mantle. 

There is good ray coverage throughout the crust between the horizontal distance ~50 to 385 km and ~460 to 760 km. The crustal Vs within the Romanche transform zone is not constrained by any S-wave rays. 

Monte-Carlo analysis. To assess the accuracy of the crustal and mantle velocities and the Moho depth, we perform Monte-Carlo analyses 70 starting from different initial models to produce different inverted models and estimate the standard deviation of these models from the mean model to measure the model variance. Here we only describe the details of the Monte-Carlo analysis for crustal Vp and Moho depth. We create 50 starting crustal models by randomly perturbing the minimum and maximum velocity of the onedimensional crustal velocity profile by ±5% and the initial crustal thickness by ±750 m. 

These starting crustal models are then inverted using the same tomography algorithm and inversion parameters as described before. These 50 models show similar crustal Vp structure, suggesting the inversion of the picked Pg and PmP travel times is robust. The variance in the final crustal Vp model is less than 0. 06 km/s in the upper crust and is less than 0. 18 km/s in the lower crust (Supplementary Fig. 5a ). The maximum standard deviation of the Moho depth is ~330 m (Supplementary Fig. 5b ). The preferred Moho (Fig. 3a ) falls in the standard deviation of the average Moho depth from the Monte-Carlo analysis (see red curve in Supplementary Fig. 5b ), and the maximum depth difference between the preferred Moho and the averaged Moho from Monte Carlo analysis is ~158 m (Supplementary Fig. 5b ). 

Similar Monte-Carlo analyses are performed to assess the variance in the crustal Vs. Supplementary Fig. 5c shows the variance of the crustal Vs calculated using 50 final inverted models. For most portion of the crust, the crustal Vs has a variance < 0. 06 km/s, and large variances up to ~0. 18 km/s are observed around the TF and FZs and at the southern and northern extremity of the model. The variance in the crustal Vp/Vs ratio is estimated using the preferred crustal Vp model (Fig. 3a ) and the 50 crustal Vs models from the Monte-Carlo analysis. These models are used because they share the same Moho. We calculate the average of the 50 Vp/Vs models and take the standard deviation as the variance (Supplementary Fig. 5d ). The variance in crustal Vp/Vs ratio is <0. 025 for most portion of the crust, and large variances up to ~0. 1 only occur at local regions where the crustal Vs shows large variance.
section_name: Discussions

section_name: Acknowledgements

section_content: We are grateful to the officers, crew, and scientific technicians of the 2018 ILAB-SPARC cruise for their hard work. The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ ERC Advance Grant no. 339442 TransAtlanticILAB. 

Zhikai appreciates Milena Marjanović, Emma P. M. Gregory, Mathilde Cannat, Venkata Vaddineni, Kevin Growe and Jie Chen for their help and useful discussions. Some results presented in this paper are performed on the S-CAPAD platform of Institut de Physique du Globe de Paris, France.
section_name: Acknowledgements

section_name: Data availability

section_content: The high-resolution bathymetry data, the multichannel seismic reflection data and the
section_name: Data availability

section_name: Author contributions

section_content: Checkerboard tests. We also use the checkerboard test described in ref. (4) to assess the resolution of the final crustal Vp and Vs models. We first perform the checkerboard test to assess the crustal Vp. The checkerboard input models are designed by adding 2-D sinusoidal velocity perturbation of ±8% into the starting velocity model after slight smoothing. The anomaly size in the checkerboard models is 20×3 (Supplementary Fig. 6a ) and 40×2 km (Supplementary Fig. 7a ), respectively. A sinusoidal perturbation with a half-wavelength of 70 km and the vertical variation of 1. 25 km was added to the initial Moho to examine the resolvability in the Moho depth 4, 45. The checkerboard models show ~2. 5-3. 0 km crustal thickness variations over ~70 km distance (red curves in Supplementary Figs. 6c and 7c ). Synthetic seismic travel times are calculated using the checkerboard models and the same receiver-source geometry as the Pg and PmP picks. Some random noise is added to these synthetic picks to represent the picking uncertainty. 

We invert these synthetic travel times using the same workflow as that for the OBS dataset starting from the same initial model. The results show that velocity anomalies of sizes 20×3 and 40×2 km are almost completely resolved (Supplementary Figs. 6b and 7b ) with better recovery for anomaly with 20×3 km size. The crustal thickness within the recovered model shows ~1. 5-2. 0 km variations over ~70 km distance (magenta curve in Supplementary Figs. 6c and 7c ), suggesting the tomography method can partially recover the large along-axis variations in crustal thickness. A similar checkerboard test is applied to assess the resolution of crustal Vs model. The checkerboard test shows a higher resolution of the crustal Vs model than the crustal Vp model, where the 8% velocity anomaly with the 15×3 km size can be well recovered (Supplementary Fig. 8 ). Higher resolution is obtained in crustal Vs model because no velocity-Moho depth trade-off occurs in the tomography of S-wave arrivals.

section_content: Z. W. processed and analysed the OBS data and wrote the paper. S. C. S. developed the project, led the data acquisition and supervised the data processing, interpretation and writing.
section_name: Author contributions

section_content: The authors declare no competing interests.
section_name: Competing interests

section_name: OBS

section_content: This is a list of supplementary les associated with this preprint. Click to download. 

Supplementaryinformation. pdf
section_name: Supplementary Files