Syn- versus post-orogenic extension: the case study of Giglio Island (Northern Tyrrhenian Sea, Italy)

Abstract

Combined structural, petrographic and fluid inclusion data from Giglio Island (Northern Tyrrhenian Sea, Italy) are used here to constrain kinematics, timing and thermal regimes associated with the post-thickening tectonic evolution of the innermost sector of the Northern Apennine belt. Giglio Island is composed of Pliocene intrusives in tectonic contact with a poorly retrograded HP/LT, Fe-carpholite bearing nappe pile. Peak pressure estimates for the carpholite bearing Verrucano metasediments range between 10 and 14 kbar for temperatures lower than 350°C. Data from this study indicate a nearly isothermal exhumation of the Giglio HP metamorphic complex, which occurred during the Early–Middle Miocene as a consequence of a top-to-the-east syn-orogenic extensional shear. This resulted in the transposition of the HP fabric while the lithostatic pressure was decreasing, with a final re-equilibration in the greenschist facies along a cool geothermal-gradient regime. This relatively cold regime is referred to the continuous underthrusting of the cool continental Adria plate during exhumation. Afterwards, during the Late Miocene–Early Pliocene, the onset of the post-orogenic crustal thinning also showed an eastward asymmetry in an HT/LP back-arc environment, leading to partial melting of previously exhumed deep crustal levels and genesis and emplacement of the Giglio intrusives. A comparison with other peri-Tyrrhenian regions allows us to propose that the post-thickening Miocene extension in the Northern Apennines was driven by the collapse of the previously thickened Northern Apennine orogenic wedge, combined with the progressive eastward retreat of the Adria subduction boundary.