Uplifted Holocene shorelines at Capo Milazzo (NE Sicily, Italy): Evidence of co-seismic and steady-state deformation

Milazzo, the main headland of NE Sicily, supplied evidence of the interplay between abrupt and steady uplift. Field analysis revealed two shorelines at distinct elevations, which were characterized through position and radiometric dating of geomorphologic and biological sea-level markers. The upper shoreline is represented by a notch, marine deposits and barnacle rims. The notch is found at average elevation of w2.1 m above mean sea level (a.m.s.l.) and is filled by a marine deposit containing shells dated between 6.2 and 3.9 ka BP. Locally, the deposit includes pottery of Roman age (w2 ka BP). An intertidal balanid rim associated with the upper shoreline yielded ages between 3.8 and 1.6 ka BP. The lower shoreline involves a notch at w0.8 m a.m.s.l., locally carved in the older Holocene deposit, and a likely coeval, subjacent abrasion platform at 0.5e0.6 m a.m.s.l. Remains of a balanid rim in this lower notch were dated at w1.4 ka BP. Morphological and chronostratigraphical relations indicate that uplift of the upper shoreline occurred abruptly. Sudden displacement of 1.3e1.5 m occurred during a co-seismic event. Age and elevation relationships document that the upper shoreline was active between 6.2 and 1.6 ka, and thus co-seismic uplift must have occurred between 1.6 and w1.4 ka, the bracketed ages of cessation and inception of the upper and lower shorelines, respectively. Using similar reasoning, a younger (w0.8e0.9 m) co-seismic uplift occurred probably shortly after 1.4 ka. These uplifts supplemented a regional, background uplift that operated steadily during the lifespan of the two shorelines. Precise compensation for the glacio-hydro-isostatic sea-level change indicated that this uplift took place at w0.65 mm/y. Thus, cumulative uplift at the studied sites occurred at 1.2e1.5 (locally up to 2.1) mm/y, in close agreement with nearby coastal sites in Sicily and southern Calabria. Analysis suggests the occurrence of a seismogenic source in the Capo Milazzo area, possibly related to transpressive structures recently mapped offshore. The seismogenic potential of this sector of Sicily needs to be re-evaluated.