Neotectonics in southern Sardinia: insights from the Cala Mosca site

Sardinia Island is located in the centre of the western Mediterranean area, and it has been considered tectonically stable since the Early Pleistocene. The marine sequence of Cala Mosca is considered one of the key paleo sea-level markers for the Marine Isotopic Stage (MIS) 5e highstand (116-126 ka), and it has been used as a proxy to calibrate the Mediterranean Pleistocene sea-level curve and to support the tectonic stability of the island. However, a chrono-stratigraphic review demonstrated that the marine sequence consists of two superimposed marine events divided by a composite unconformity surface. Performed luminescence dating related the two events to 137±7, 134±7 ka and 92±6 ka, respectively, and thus associated with highstands during the MIS5e and MIS5c substages. The stratigraphic superimposition of these two highstands, both placed above present sea level (~+5 m), conflicts with the global accepted sea-level curve, according to which the mean position of MIS5e is placed above the present sea level (+5-9 m) whereas the MIS5c is worldwide accepted to be below (-22 m) the modern coastline. Possible hypotheses to explain such a chrono-stratigraphic framework are: i) the MIS5c highstand is still not well constrained in amplitude and elevation compared with the present sea-level positions; ii) the Quaternary Relative Sea Level curve of the Mediterranean is characterized by unrecognized high-frequency sea oscillations; iii) regional and/or local tectonic activity affected the island during the late Pleistocene. Assuming the first two hypotheses are questionless and focusing on the last one, we tentatively explain the stratigraphic setting of Cala Mosca in the context of the fault-bounded blocks asset of southern Sardinia. Two master faults bound a large-scale graben structured into two tectonic basins separated by a structural high where the Cala Mosca site is hosted. Note that low-magnitude earthquakes have been recorded along these faults and high-resolution GPS dataset point out to vertical movement of the blocks up to 1.5 mm/a. Also, geomorphic evidence of a generalized uplift, as inverted relief and deep river incision are widespread in the whole Sardinia block. The absolute vertical movement of the Cala Mosca block will be positive or negative depending on the prevailing uplift or fault slip rates, respectively. After depositing the MIS5e deposits (+5/9 m), the Cala Mosca block have been displaced downward to approach the MIS5c highstand (-20 m), allowing the superimposition of the two marine events and the formation of a composite marine terrace; once formed, it have been uplifted toward the present position. Although the kinematics and dynamics of this differential tectonics are far from being fully understood in this area, the proposed model may explain the controversial occurrence of composite marine terraces considering the worldwide accepted sea-level curve, leading to reconsidering the Sardinia tectonic stability paradigm.