Mourtzas, N.1, Kolaiti E.2, Anzidei M. 3
1 GAIAERGON LtD, Athens, Greece
2GAIAERGON LtD, Athens, Greece, University of Peloponnese, Kalamata, Greece
3 Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy
The island of Crete represents an uplifting sedimentary forearc zone located north of the Hellenic subduction zone in the transition between the African and Eurasian plates.
The Late Holocene history of the relative sea level change (RSL) on the coast of Crete differentiated between the western and eastern part of the island in AD 365 (Pirazzoli, 1986), when during a strong seismic event the island was split along the neotectonic graben of Spili and its northern and southern prolongation. Until then, the sea level throughout the coast of Crete was -1.25 ± 0.05 m lower than today (Mourtzas, 1990; 2012a, b; Mourtzas et al., 2016). With fragmentation, the western part - approximately 100 km long - uplifted by 9.15 ± 0.20 m at its westernmost end and 2.00 ± 0.30 m at its eastern boundary, while rotating towards the SE. This significant tectonic event, that originated the largest earthquake ever occurred in the Mediterranean since the Late Holocene and among one of the largest in the world with the exception of the M = 9.2, 1964, Alaska earthquake (Brocher et al., 2014), seems to have affected the entire Eastern Mediterranean as it was accompanied by a tsunami which devastated the Nile Delta and the ancient towns of west Cyprus and Libya (Stiros, 2001).
During the period of 2600 years that preceded it, between 4200 BP and 1600 BP, the entire island submerged as a single tectonic block. Nine to eleven successive subsidence tectonic episodes gradually submerged the western part of the island. In this time, the total subsidence of the southwest edge of the island reached 1.60 m (Pirazzoli et al., 1982; Mourtzas et al., 2016). The same period, in the central and eastern part of Crete the submerged coastal landforms and the functional height of selected archaeological markers, which are related to the sea level at the time of their construction, revealed and dated five distinct sea level stands at -6.55 ± 0.55 m, -3.95 ± 0.35 m and -2.70 ± 0.15 m (Mourtzas et al., 2016). After the partition of the island and the uplift of its western part, the entire island subsided again, by -0.70 m and -0.55 m during two distinct events, for an amount of 1.25 m.
The deepest sea level stand of central and eastern Crete at -6.55 ± 0.55 m can be identified by the earliest tidal notch of western Crete dated to between 3930 ± 90 BP and 4200 ± 90 BP. The next sea level stand at -3.95 ± 0.35 m has been dated to between 3900 BP and 3600 BP. It is related to the tidal notch of the southwest edge of the island (cape Chrysoskalitissa) that formed at -2.85 m during the initial subsidence and has been dated to 3870 ± 90 BP (Pirazzoli et al., 1982; Mourtzas et al., 2016).
The change to the subsequent sea level is probably associated with the eruption of the Thera volcano, occurred 3600 BP, and its attendant neotectonic upheavals in the area of the southern Aegean. The sea level stand at -2.70 ± 0.15 m of the eastern and central part of the island has been dated to between 3600 BP and 3200 BP (Mourtzas et al., 2016). Comparing it with the corresponding tidal notches of SW Crete that formed at the same depth and have been dated to 3330 ± 80 BP or 3290 ± 70 BP, and considering their lowest limit (3250 BP and 3220 BP, respectively), we conclude that the change to the subsequent sea level stand at -1.25 ± 0.05 m should be attributed to the
destructive co-seismic tectonic events occurred during the middle or by the end of LM IIIB period, about 3250 BP or 3200 BP. The dating of the -1.25 ± 0.05 m sea level was based on the ancient Hellenistic and Roman coastal installations and mainly on the constructional features of the Roman fish tanks found in many locations along the entire coast of central and eastern Crete. Historical sources report a change by 0.75 m in the Hellenistic and Roman sea level during the AD 1604 paroxysmal event that is also confirmed by the submerged Byzantine and Venetian coastal installations throughout the coast of Crete. For an undetermined but significant period during the last 400 years the sea level remained stable, so that to form a sea erosional tidal notch at -0.55 ± 0.05 m, and then rose to its current position.
Here we show and discuss the relative sea level changes in Crete for the late Holocene, from archaeological and geomorphological indicators and sea level predictions.
Fig. 1: RSL change in western and central-eastern Crete during the Late Holocene. Blue line is the RSL change curve as results from beachrock data; magenta line is the RSL change curve estimated from tidal notches data. Orange line is the land subsidence in the NW extremity of Crete
before the AD 365 uplift that clearly shows the coincidence between the higher uplifted tidal
notch and a sea level stand at -1.25 ± 0.05. Cyan line is the predicted sea level curve for Crete, according to the glacio-hydro-isostatic model (Lambeck and Purcell, 2005). Error bars are the time
span (horizontal) and the depth uncertainties (vertical). Historical periods and major catastrophic
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