We have studied a nonvolcanic margin, the West Iberia margin, to understand how the mechanisms of thinning evolve with increasing extension. We present a coincident prestack depth-migrated seismic section and a wide-angle profile across a Mesozoic abandoned rift, the Galicia Interior Basin (GIB). The data show that the basin is asymmetric, with major faults dipping to the east. The velocity structure at both basin flanks is different, suggesting that the basin formed along a Paleozoic terrain boundary. The ratios of upper to lower crustal thickness and tectonic structure are used to infer the mechanisms of extension. At the rift flanks (stretching factor, ¿ ≤ 2) the ratio is fairly constant, indicating that stretching of upper and lower crust was uniform. Toward the center of the basin (¿ ~ 3.5--5.5), fault-block size decreases as the crust thins and faults reach progressively deeper crustal levels, indicating a switch from ductile to brittle behavior of the lower crust. At ¿ ≥ 3.5, faults exhume lower crustal rocks to shallow levels, creating an excess of lower crust within their footwalls. We infer that initially, extension occurred by large-scale uniform pure shear but as extension increased, it switched to simple shear along deep penetrating faults as most of the crust was brittle. The predominant brittle deformation might have driven small-scale flow (≤40 km) of the deepest crust to accommodate fault offsets, resulting in a smooth Moho topography. The GIB might provide a type example of nonvolcanic rifting of cold and thin crust.