Architecture and tectonic evolution of nonvolcanic margins: Present-day Galicia and ancient Adria

A comparison of the reconstructed southeastern margin of the Tethys ocean w ith the present-day Galicia margin shows that although both margins are of different age and had a different fate, their architectures and tectonic ev olutions are very similar. Along both non-volcanic margins the site of rift ing 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 symmet rically arranged listric faults which soled at midcrustal levels, indicatin g that deformation in the upper crust was decoupled from deformation in the upper mantle along a hot and weak lower crust. During advanced rifting, ex tension was dominated by simple shear along low-angle detachment faults wit h a top-to-the-ocean sense of movement. These shallow crustal structures fo rmed a series of breakaways in the continental crust and cut into mantle ro cks, indicating that now deformation in the upper crust and in the upper ma ntle was no longer decoupled. Cooling and strengthening of the lower crust during an initial stage of rifting apparently led to localization of deform ation and a different style of deformation, documenting that the tectonic e volution 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 c ombination of simple shear extension, asthenospheric uplift, and increased melt production.