“Punishment for the Sinner”: Holocene episodic subsidence and steady tectonic motion at ancient Sybaris (Calabria, southern Italy)

Abstract

Although the Calabria region in southern Italy experienced Quaternary uplift, intense Holocene subsidence is documented at ancient Sybaris, one of the most powerful among the Greek colonies collectively known as Magna Greacia. Sybaris flourished along the Ionian Sea coast of north-eastern Calabria since 2.7 ka BP and until the end of the Roman empire, when it went progressively concealed below the ground. Study of the Latest Pleistocene–Holocene evolution of this area involved analysis of the relative sea-level history for individual borehole logs, based on re-calibration of published ages and stipulation of nominal sea-level positions related to each marker. For investigation of the short-term evolution, a novel compilation of geological and archaeological sea-level markers was supported by new radiometric ages. Appraisal of the position of dated markers, when compared to a sea-level curve built on purpose for this coast using an updated glacio-hydro-isostatic model, indicates a locally intense difference in vertical motion between boreholes located away from, or close to, the ancient town. Specifically, whereas data from the basement of the archaeological layers document large subsidence, more distant sites behaved differently. A site ∼2 km to the NW of the settlement was apparently stable throughout the Holocene, and a site ∼6 km to the SE experienced uplift at ∼1.5 mm/y since 0.6 ka BP. In addition, analysis reveals temporal changes in subsidence pattern in the archaeological area. Large (5–6 mm/y) Early Holocene subsidence at Sybaris slowed to ∼1.5 mm/y during the middle part of the Holocene. The slowing-down trend continued during and after historical occupation at ∼0.8 mm/y, a value similar to the long-term (35–40 ka BP) rate established from the deepest borehole samples. These data are interpreted as suggesting that sediment compaction affected the basement of Sybaris during the Early Holocene, and progressively ceased during historical times, when a tectonic signal prevailed. During historical time, the spatial difference in vertical motion is chiefly attributed to differential growth of nearby folds documented by morpho-tectonic studies on-land and seismic profiles analysis in the offshore of Sybaris. Whereas location above a syncline caused tectonic subsidence at Sybaris, the regions to the north and south record stability or uplift that reflects transition to growing anticlines. In the short term (1–10 ka scale), these local tectonic processes prevail over the regional uplift pattern, which is expressed at the 100 ka scale or over.