Metamorphic core complexes of the Aegean region have revealed midcrustal, shallow-dipping extensional shear zones. These shear zones display constant kinematic indicators over large regions (100--200 km). We analyze the example of the northern Tyrrhenian Sea and then compare it to the Aegean region. We first summarize our observations on ductile extension and metamorphic evolution in the northern Tyrrhenian Sea from Alpine Corsica to Tuscany. (1) Extension migrated from west to east from the early Miocene in Corsica to the Recent in the Apennines; (2) Extension is accommodated by shallow east dipping extensional shear zones at the depth of the brittle-ductile transition, from the early Miocene to the Pliocene. (3) West dipping normal faults accommodate extension on the eastern side of the volcanic arc. (4) Extension is preceded along the convergence front by the formation of a thrust wedge, where high-pressure and low-temperature conditions are recorded; maximum PT conditions decrease toward the east, and PT paths are systematically very cold, suggesting that a large part of the exhumation occurred during synorogenic extension. We discuss the possible mechanisms that account for constant shear sense over large domains. The model involves retreat of the slab and migration of the volcanic arc. Partially molten lower crust acts as a low strength zone where extensional strain is localized. Eastward motion of the upper mantle as a consequence of the migration of the slab induced a component of shear toward the volcanic arc at the base of the stronger upper crust. In the weak upper mantle and lower crust, to the west of the volcanic arc, extensional stresses are not transmitted; this produces a top-to-the-east sense of shear at the base of the upper crust that migrates eastward, following arc migration. ¿ 1998 American Geophysical Union