A comparison of the reconstructed southeastern margin of the Tethys ocean with the present-day Galicia margin shows that although both margins are of different age and had a different fate, their architectures and tectonic evolutions are very similar. Along both non-volcanic margins the site of rifting shifted from a broad area in the future proximal margins to a localized area in its distal parts, accompanied by a change in the mode of extension. During the initial phase of rifting, extension was accommodated by symmetrically arranged listric faults which soled at midcrustal levels, indicating that deformation in the upper crust was decoupled from deformation in the upper mantle along a hot and weak lower crust. During advanced rifting, extension was dominated by simple shear along low-angle detachment faults with a top-to-the-ocean sense of movement. These shallow crustal structures formed a series of breakaways in the continental crust and cut into mantle rocks, indicating that now deformation in the upper crust and in the upper mantle was no longer decoupled. Cooling and strengthening of the lower crust during an initial stage of rifting apparently led to localization of deformation and a different style of deformation, documenting that the tectonic evolution of nonvolcanic margins is largely controlled by the thermal state of the lithosphere. Seafloor spreading initiated only after exhumation and exposure of the subcontinental mantle on the ocean floor and may have been accompanied by a loss of the yield strength of the upper mantle, due to a combination of simple shear extension, asthenospheric uplift, and increased melt production. ¿ 1999 American Geophysical Union