A seismic refraction transect across the Galicia Bank continental margin shows that the original continental crust thins westward from 17 to 2 km immediately east of a margin-parallel peridotite ridge (PR). Immediately west of the PR, oceanic crust is only 2.5--3.5 km thick, but farther west (oceanward) it thickens to 7 km. The PR caps a ~60-km-wide lens shaped serpentinized peridotite body underlying both thinned continental and thin oceanic crust. When superimposed on a reflection time version of the velocity model, the S reflector is clearly intracrustal at its east end. Westward, S cuts down to lower crustal levels, eventually coinciding with the top of the serpentinized peridotite lens (original crust-mantle boundary). These observations render almost impossible the seafloor exposure of the PR by S acting as a top-to-the-west detachment fault. Numerical models of melting and borehole subsidence information constrain our rifting model. The easternmost continental crust experienced a total stretching factor of 4.3 (most likely in two stages); it probably occurred over ~25 m.y., with the highest rate of stretching at the beginning of the main earlier rift phase (Valanginian; 141--135 Ma). The 3 (4.7) km thick continental crust (depending on whether serpentinized peridotite is assigned to crust or mantle), which may include melt products, requires stretching factors of more than 11 (7) and a rift duration of more than 25 (13) m.y. The thin oceanic crust immediately west of the PR is explained by conductive cooling of the mantle during the long prebreakup stretching phase, which temporarily caused reduced melting immediately after breakup. ¿ American Geophysical Union 1996