ICG2022-36, updated on 20 Jun 2022
10th International Conference on Geomorphology
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Lithophyllum rims as markers of palaeoseismic events and relative sea-level change in the Dubrovnik archipelago, Southern Adriatic, Croatia

Sanja Faivre1, Tatjana Bakran-Petricioli2, Marijan Herak3, Jadranka Barešić4, Davor Horvatić5, and Damir Borković4
Sanja Faivre et al.
  • 1Faculty of Science, Department of Geography, University of Zagreb, Zagreb, Croatia (sfaivre@geog.pmf.hr)
  • 2Faculty of Science, Department of Biology, University of Zagreb, Zagreb, Croatia
  • 3Faculty of Science, Department of Geophysics, University of Zagreb, Zagreb, Croatia
  • 4Ruđer Bošković Institute, Radiocarbon Laboratory, Zagreb, Croatia
  • 5Faculty of Science, Department of Physics, University of Zagreb, Zagreb, Croatia

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 component of RSL change relates to local, non-linear scale processes, that is to active tectonics. The studied islands and islets, Lopud, Koločep and Grebeni are part of Dubrovnik archipelago, situated near the external front of the Dinarides where seismicity attains one of the highest levels in Croatia. The focal mechanisms in the area show that reverse and transpressive faulting prevails.

Studying the RSL markers: algal rims, tidal notches and Lithophaga lithophaga borehole upper limits, we identified one elevated palaeoshoreline at Lopud island (today 25-30 cm above the mean sea-level (MSL)) and two elevated palaeoshorelines at Koločep island and Grebeni islets (60-90 cm and 25-40 cm above MSL). The high-resolution geochronology built on radiocarbon dating of algal rims (77 sea-level index points) enabled the distinction of coseismic uplift events from subsidence periods during the last 3300 years. As a result, we provide new, field-based reconstruction of palaeoearthquakes, describe spatial patterns of differential uplift and distinguish the main drivers of RSL change.

Two major seismically triggered uplift events with displacements between 40 and 80 ± 15 cm per event have been differentiated along investigated sectors of the coast related to the Pelješac-Dubrovnik fault zone pointing to repeated earthquakes of magnitudes exceeding Mw = 6.5. 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 1667 Dubrovnik earthquake. The cumulative offsets increase from around 0.57 ± 0.15 m at Lopud to 1.00-1.25 ± 0.15 m at Koločep and Grebeni.

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 (between 0.30 and 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 around 0.7 mm/yr, and go up to 1.4 mm/yr after 1800 cal AD.

Overall, here we demonstrate the importance of Lithophyllum rims in the studies of RSL change as they make creation of high-resolution geochronology possible. Moreover, this new approach refers to algal rims as a possible tool for constraining palaeoseismic events, allowing to supplement the database of instrumental records and historical observations through field-based evidence. This research was supported by Croatian Science Foundation project HRZZ-IP-2019-04-9445 – Relative sea-level change and climate change along the eastern Adriatic coast – SEALeveL.

How to cite: Faivre, S., Bakran-Petricioli, T., Herak, M., Barešić, J., Horvatić, D., and Borković, D.: Lithophyllum rims as markers of palaeoseismic events and relative sea-level change in the Dubrovnik archipelago, Southern Adriatic, Croatia, 10th International Conference on Geomorphology, Coimbra, Portugal, 12–16 Sep 2022, ICG2022-36, https://doi.org/10.5194/icg2022-36, 2022.