THE OCEAN-CONTINENT BOUNDARY OFF THE WESTERN CONTINENTAL-MARGIN OF IBERIA - CRUSTAL STRUCTURE WEST OF GALICIA BANK

A seismic refraction transect across the Galicia Bank continental marg in shows that the original continental crust thins westward from 17 to 2 km immediately east of a margin-parallel peridotite ridge (PR). imm ediately west of the PR, oceanic crust is only 2.5-3.5 km thick, but f arther west (oceanward) it thickens to 7 km. The PR caps a similar to 60-km-wide lens-shaped serpentinized peridotite body underlying both t hinned continental and thin oceanic crust. When superimposed on a refl ection time version of the velocity model, the S reflector is clearly intracrustal at its east end. Westward, S cuts down to lower crustal l evels, eventually coinciding with the top of the serpentinized peridot ite lens (original crust-mantle boundary). These observations render a lmost 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 c ontinental crust experienced a total stretching factor of 4.3 (most li kely in two stages); it probably occurred over similar to 25 m.y., wit h the highest Fate 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 c rust or mantle), which may include melt products, requires stretching factors 6 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 c onductive cooling of the mantle during the long prebreakup stretching phase, which temporarily caused reduced melting immediately after brea kup.