Late Holocene interplay between coseismic uplift events and interseismic subsidence at Koločep island and Grebeni islets in the Dubrovnik archipelago (southern Adriatic, Croatia)

Abstract

The relative sea-level (RSL) change is a sum of effects of processes acting on local, regional and global scales. By far the most variable and uncertain component of RSL change relates to local, non-linear scale processes, that is to active tectonics. Studying the RSL markers: algal rims, tidal notches and Lithophaga lithophaga borehole upper limits, two elevated palaeoshorelines (today 60–90 cm and 25–40 cm above the mean sea-level) have been distinguished at Koločep island and Grebeni islets. The high-resolution geochronology built on AMS radiocarbon dating of algal rims enabled the distinction of coseismic uplift events from subsidence periods during the last 2600 years. Accordingly, we provide new, field-based reconstruction of palaeoearthquakes, describe spatial patterns of differential uplift and distinguish the main drivers of RSL change.Two successive major earthquakes affected in total ∼5 km of coasts related to the Pelješac–Dubrovnik fault zone, with an uplift amplitude between ∼40 and 80 ± 15 cm per event. The earlier, older events, caused larger displacements (60–80 cm) related to 4th to 6th century AD and 750–1100 AD earthquakes, while the later, younger events, revealed on average lower displacements (40–55 cm) corresponding to the 1520 AD quake and the well-known 1667 Dubrovnik earthquake. The cumulative offsets increase from ∼0.57 ± 0.15 m at Lopud (studied previously) to 1.00–1.25 ± 0.15 m at Koločep and Grebeni. Following the overall uplifting pattern at rates of 0.6–0.8 mm/yr, it was observed that the repeated uplifting events progress generally toward the SW, consistent with SW-migrating deformation in the southern Dinarides (Bennett et al., 2008).Interseismic periods allow further insight into regional and global driving mechanisms of RSL change. The distinction of local coseismic displacements of RSL history together with the previously obtained regional glacio- and hydro-isostatic adjustment estimates of 0.34 mm/yr allow us to approach the global contribution, which seems to be minimal between ∼0 and 1800 cal AD as RSL changed at rates of 0.3 to 0.4 mm/yr. The increased rates of RSL change occurred prior to 0 cal AD, with rates of ∼0.7 mm/yr, and go up to 1.4 mm/yr after 1800 cal AD.This high-resolution study provides new insight into the late Holocene sea-level history, particularly into local tectonic uplift, testifying the ongoing intense tectonic activity related to Late Holocene shortening, well localized along the south-eastern Adriatic coasts, which is consistent with contemporary Adria-Eurasia collision.