Migration of compression and extension in the Tyrrhenian Sea, insights from Ar-40/Ar-39 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. Struc tural and petrological studies suggest a migration of extension from the Gu lf of Lion to Alpine Corsica and to the present-day Apennines. During the s ame period the thrust front of the Apennines migrated eastward. Oceanic cru st was first formed in the Liguro-Provencal Basin, then in the Southern Tyr rhenian Sea, while thinning of the continental crust took place in the Nort hern Tyrrhenian Sea. Syn-rift deposits and frontal thrust show eastward mig ration. Metamorphic rocks were exhumed from Alpine Corsica to the Tuscan ar chipelago and the western coast of Tuscany. High pressure and low temperatu re parageneses are found along the transect and published stratigraphic and isotopic dates also suggest an eastward migration. We conducted a series o f Ar-40/Ar-39 age determinations on metamorphic micas along this transect o n 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. The se results show a faster migration rate of extension than compression. In t he Quaternary, extension has almost caught up with compression and the onsh ore northern Apennines are presently extending. This faster migration of ex tension is interpreted as the consequence of a progressive steepening of th e slab. (C) 2000 Elsevier Science B.V. All rights reserved.