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 post-rift 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.