Summary Full waveform inversion (FWI) is a data-fitting technique capable of generating high-resolution velocity models with a resolution down to half the seismic wavelength. FWI is applied typically to densely sampled seismic data. In this study, we applied FWI to 3D wide-angle seismic data acquired using sparsely spaced ocean bottom seismometers (OBSs) from the Deep Galicia Margin west of Iberia. Our dataset samples the S-reflector, a low-angle detachment present in this area. Here we highlight differences between 2D, 2.5D and 3D-FWI performances using a real sparsely spaced dataset. We performed 3D FWI in the time domain and compared the results with 2D and 2.5D FWI results from a profile through the 3D model. When overlaid on multichannel seismic images, the 3D FWI results constrain better the complex faulting within the pre- and syn-rift sediments and crystalline crust compared to the 2D result. Furthermore, we estimate variable serpentinisation of the upper mantle below the S-reflector along the profile using 3D FWI, reaching a maximum of 45 per cent. Differences in the data residuals of the 2D, 2.5D and 3D inversions suggest that 2D inversion can be prone to overfitting when using a sparse dataset. To validate our results, we performed tests to recover the anomalies introduced by the inversions in the final models using synthetic datasets. Based on our comparison of the velocity models, we conclude that the use of 3D data can partially mitigate the problem of receiver sparsity in FWI.

Accretion of the lower crust at mid‐ocean ridges is a debated topic, with modern seismic observations pointing to a complex magmatic system that includes an axial multisill system of middle‐ and lower‐crustal melt lenses and near‐ and off‐axis melt bodies. Here we revisit the hot spot‐influenced section of the western Galápagos Spreading Centre and reprocess multichannel seismic reflection data using a wide‐angle seismic tomography model. Our new images show that the magma reservoir in the lower crust at this ridge section is intruded with partially molten melt lenses. The images also show evidence for off‐axis melt lenses, magmatic‐hydrothermal interactions and Moho reflections in this region. We conclude that the similarities between the axial crustal structure of this hot spot‐influenced mid‐ocean ridge and the multisill magmatic structure imaged at the East Pacific Rise indicate that these features are common along the global mid‐ocean ridge system where seafloor spreading is dominated by magmatic accretion.

High-resolution velocity models developed using full waveform inversion (FWI) are capable of imaging fine details of the nature and structure of the subsurface. Using a 3D FWI velocity model of hyper-thinned crust at the Deep Galicia Margin (DGM), we constrain the nature of the crust at this margin by comparing its velocity structure with those in other similar tectonic settings. Velocities representative of both the upper and lower continental crust are present in this hyper-thinned crust. However, unlike in many other rifted margin settings, there is no clear evidence for distinct upper and lower crustal layers within the hyperextended crust. Our velocity model also shows evidence for exhumation of the lower crust under the footwalls of fault blocks to accommodate the extension. We used our model to generate a serpentinization map for the uppermost mantle at the DGM, at a depth of 100 ms (~340m) below the S-reflector, a low-angle detachment that marks the base of the crust at this margin. Based on this map, we propose that serpentinization began during rifting and continued into a postrift phase until the faults were sealed. We find a weak correlation between the fault heaves and the degree of serpentinization beneath the hanging-wall blocks, indicating that serpentinization was controlled by a complex crosscutting and unrecognized faulting during and after rifting. A good match between topographic highs of S and local highly serpentinized areas of mantle suggests that the serpentinization process resulted in variable uplift of the S-surface.

High-resolution velocity models developed using full waveform inversion (FWI) are capable of imaging fine details of the nature and structure of the subsurface. Using a 3D FWI velocity model of hyper-thinned crust at the Deep Galicia Margin (DGM), we constrain the nature of the crust at this margin by comparing its velocity structure with those in other similar tectonic settings. Velocities representative of both the upper and lower continental crust are present in this hyper-thinned crust. However, unlike in many other rifted margin settings, there is no clear evidence for distinct upper and lower crustal layers within the hyperextended crust. Our velocity model also shows evidence for exhumation of the lower crust under the footwalls of fault blocks to accommodate the extension. We used our model to generate a serpentinization map for the uppermost mantle at the DGM, at a depth of 100 ms (~340m) below the S-reflector, a low-angle detachment that marks the base of the crust at this margin. Based on this map, we propose that serpentinization began during rifting and continued into a postrift phase until the faults were sealed. We find a weak correlation between the fault heaves and the degree of serpentinization beneath the hanging-wall blocks, indicating that serpentinization was controlled by a complex crosscutting and unrecognized faulting during and after rifting. A good match between topographic highs of S and local highly serpentinized areas of mantle suggests that the serpentinization process resulted in variable uplift of the S-surface.