MIDCRUSTAL SHEAR ZONES IN POSTOROGENIC EXTENSION - EXAMPLE FROM THE NORTHERN TYRRHENIAN SEA

Metamorphic core complexes of the Aegean region have revealed midcrust al, shallow-dipping extensional shear zones. These shear zones display constant kinematic indicators over large regions (100-200 km), We ana lyze the example of the northern Tyrrhenian Sea and then compare it to the Aegean region. We first summarize our observations on ductile ext ension and metamorphic evolution in the northern Tyrrhenian Sea from A lpine Corsica to Tuscany. (1) Extension migrated from west to east fro m the early Miocene in Corsica to the Recent in the Apennines; (2) Ext ension is accommodated by shallow east dipping extensional shear zones at the depth of the brittle-ductile transition, from the early Miocen e to the Pliocene. (3) West dipping normal faults accommodate extensio n on the eastern side of the volcanic are. (4) Extension is preceded a long the convergence front by the formation of a thrust wedge, where h igh-pressure and low-temperature conditions are recorded; maximum PT c onditions decrease toward the east, and PT paths are systematically ve ry cold, suggesting that a large part of the exhumation occurred durin g synorogenic extension. We discuss the possible mechanisms that accou nt for constant shear sense over large domains. The model involves ret reat of the slab and migration of the volcanic are. Partially molten l ower crust acts as a low strength zone where extensional strain is loc alized. Eastward motion of the upper mantle as a consequence of the mi gration of the slab induced a component of shear toward the volcanic a re at the base df the stronger upper crust. In the weak upper mantle a nd lower crust, to the west of the volcanic are, 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 are mig ration.