Migration of compression and extension in the Tyrrhenian Sea, insights from 40Ar/39Ar ages on micas along a transect from Corsica to Tuscany

Opening of the Tyrrhenian Sea is the consequence of the eastward retreat of the Calabria-Apennines subduction from the Oligocene to the Present. Structural and petrological studies suggest a migration of extension from the Gulf of Lion to Alpine Corsica and to the present-day Apennines. During the same period the thrust front of the Apennines migrated eastward. Oceanic crust was first formed in the Liguro-Provencal Basin, then in the Southern Tyrrhenian Sea, while thinning of the continental crust took place in the Northern Tyrrhenian Sea. Syn-rift deposits and frontal thrust show eastward migration. Metamorphic rocks were exhumed from Alpine Corsica to the Tuscan archipelago and the western coast of Tuscany. High pressure and low temperature parageneses are found along the transect and published stratigraphic and isotopic dates also suggest an eastward migration. We conducted a series of 40Ar/39Ar age determinations on metamorphic micas along this transect on mineral populations and/or single grains. The results show: (1) the Late Oligocene-Early Miocene age of the top-to-the-east sense of shear in Alpine Corsica; (2) a transition from compression to extension around 32 Ma; (3) the eastward migration of the HP/LT event related to compression from 45 to 17 Ma, and of the LP event, related to extension from 32 Ma to recent. These results show a faster migration rate of extension than compression. In the Quaternary, extension has almost caught up with compression and the onshore northern Apennines are presently extending. This faster migration of extension is interpreted as the consequence of a progressive steepening of the slab. (C) 2000 Elsevier Science B.V. All rights reserved.