Sedimentary vs. tectonic burial and exhumation along the Apennines (Italy)

Sveva Corrado, Massimiliano Zattin, and Luca Aldega

We review the burial-exhumation history of sedimentary units along the Apennines, focussing on paleo-thermal and thermochronological data derived from organic matter optical analyses, X-ray diffraction of clay-rich sediments, fission-track and (U-Th)/He dating.

In the Northern Apennines, burial conditions and timing of exhumation progressively decreases eastwards from the inner towards the outer zones and through the nappes from the lowermost to the uppermost unit. Apart from large outcrops of metamorphic rocks well exposed in Tuscany, most of the rocks of the Northern Apennines reached only diagenetic conditions. In the Central Apennines paleo-thermal and thermochronological data indicate a substantial low sedimentary and tectonic burial testifying minor amounts of orogenic shortening with a prevailing thick-skinned structural style and scarce exhumation when compared to the Northern and Southern Apennines. In the Southern Apennines, thermal indicators record exhumation of sedimentary units in the axial zone of the chain from depths locally in excess of 4 km (Lagonegro Unit and Monte Alpi structure). Apatite fission-track data indicate that exhumation marks the late tectonic stages (younger than 10 Ma) of chain evolution, probably initiating with the buttressing of the allochthonous wedge against the thickening passive margin of Adria microplate. On the other hand, higher structural units (derived from Apenninic platform deformation) show variable amounts of burial along the strike of the chain (increasing from Lucania to North Calabria border). In Eastern Sicily thermal maturity decreases from hinterland to foreland as a result of less severe thermal evolution and/or tectonic loading apart from the Peloritani Mts. in the hinterland that subdued two different phases of exhumation: the first between 35-20 Ma and the second younger than 15 Ma. Accretionary prism made up of Sub-ligurian unit (namely Sicilidi) in the footwall of the Peloritani Mts. mainly exhumed in Burdigalian times (17-19 Ma) from depths of a few kilometers. Frontal thrust stack derived from late deformation of Mesozoic passive margin successions mainly exhumed in Tortonian-Pliocene times from depths of about three kilometers. Syn-orogenic siliciclastics (mainly thrust-top basins) generally show low thermal maturity testifying scarce burial apart from those at the rear of the chain (on top of the Peloritani Mts) that are thermally imprinted by out-of-sequence reactivation in Serravalian times followed by fast exhumation in extensional regime.