Neotectonics in the Corsica-Sardinia block: relationships between surface processes and lithospheric structure

Abstract

The Corsica-Sardinia block (CSB) is set in the middle of western Mediterranean between two highly stretched lithospheric domains, the Balearic and Tyrrhenian basins, that opened because of the progressive eastward migration of the Apennine front and roll-back of the Neotethys slab. The main tectonic features recorded in the CSB are Oligocene-Miocene strike-slip faults with either NE-SW orientation in Corsica and northern Sardinia and NW-SE orientation in southern Sardinia. Several evidence indicate multiple reactivation of these trans-crustal structures over time. The oldest stage of reactivation is testified by voluminous Pliocene-Quaternary anorogenic volcanic activity localized along the strike-slip faults in northern and central Sardinia. Farther to the south, strike-slip faults reactivated as normal faults during the Quaternary accommodating the deposition of more than 1000 m of continental deposits in the Campidano basin. Finally, in several sites strike-slip faults reactivated as normal or oblique faults offsetting upper Pleistocene to Holocene coastal deposits.In spite of these evidence of recent deformation, the CSB is characterized by vertical aseismic movements in the order of few mm per year and weak seismicity, with earthquakes occurring usually at depth not higher than 10 km and along the main Cenozoic faults. These structures also control the topography of the Corsica and Sardinia islands, supporting a rugged morphology with peaks close to 2000 m in Sardinia and 3000 m in Corsica, deep river incisions and other geomorphic features typical of relief rejuvenation. To investigate the cause of this cryptic neotectonic activity we run a set of Finite Differences numerical models that simulate the CSB as thin elastic plate overlying an inviscid asthenosphere. The structure of the model lithosphere is based on available geological and geophysical dataset and is divided into six compositionally homogeneous layers: air or water, sedimentary or volcanic cover, crystalline middle crust, lower crust, lithospheric mantle, and asthenosphere. In the experiments, we change the density and heat production rate of the crustal layers to fit the measured Bouguer gravity anomaly and surface heat flow.The best fit experiment shows that the CSB crust consists of a relatively low-density lower crust composed by felsic granulites moderately enriched in heat-producing elements, and a standard-density middle crust composed of highly productive granites or migmatites. The results suggest that neotectonic activity can be related to regional uplift driven by a mass deficit localized in the lower crust. In our opinion, the Cenozoic faults accommodate differential vertical displacements of fault-bounded blocks, occasionally triggering low-magnitude earthquakes in the upper crust. This interpretation account also for the peculiar geomorphic features of Sardinia and Corsica, where the landscape is continuously rejuvenated according to the uplift movements.