Tectonic geomorphology of active faults in Eastern Crete (Greece) with slip rates and earthquake history from cosmogenic 36Cl dating of the Lastros and Orno faults

Abstract

In eastern Crete, several normal faults trend both NNE-SSW and E-W. Most of them form impressive bedrock fault scarps indicating ongoing extension in the Hellenic fore-arc. No earthquakes from historical and instrumental catalogues can be directly associated with these faults. Geomorphologic profiles across five different faults in sites well-preserved from erosion, deposition or human intervention revealed displacements of ∼6.9 m–∼30 m. Following a robust site selection at the Lastros and Orno faults, we mapped different categories of weathered horizontal ribbons along the bedrock fault plane based on colour contrast, lichen colonisation and karstic features that affect fault plane roughness, features that altogether are interpreted as earthquake horizons. The data indicate coseismic displacements of 15–35 cm for the last three earthquakes at the Lastros fault and 10–20 cm for the last two earthquakes at the Orno fault. To constrain the age of these earthquakes and to estimate millennial fault slip rates, we determined cosmogenic 36Cl concentrations from vertical profiles composed of 24 samples from the exposed Lastros fault plane and 7 samples of the Orno fault plane. Bayesian Markov Chain Monte Carlo (MCMC) modelling implies that the last three earthquakes at the Lastros fault, occurred within the last 1.8 ± 1.2 kyr (EQ1 = 1.1 ± 1.1 kyr, EQ2 = 1.4 ± 1.1 kyr, EQ3 = 1.8 ± 1.2 kyr). Furthermore, Lastros’ slip rate varied with a rate of 0.6 ± 0.3 mm/yr during the last 1.8 ± 1.2 kyr, preceded by a rate of 1.1 ± 0.4 mm/yr from ∼1.8 to 8 kyr; suggesting periods of earthquake clustering. At the Orno fault, the last two earthquakes occurred within the last ∼3.5 kyr accompanied with a slip rate of 0.16+0.06-0.02 mm/yr during postglacial times. This confirms for the first time based on absolute dating, that both NNE-SSW and E-W trending neighbouring faults in Crete were active during the Holocene.Most of the 13.2 m high Orno fault scarp was exhumed during glacial times, with an apparent slip rate of 0.28+0.12-0.07 mm/yr. The upper part of the Lastros fault scarp (9–12 m) and the apparent age of the four Lastros footwall samples (∼17.9 kyr, ∼26.0 kyr, ∼29.9 kyr, ∼37.4 kyr) underline the long preservation throughout glacial times as well. These findings show that the general hypothesis of predominantly postglacial bedrock fault scarps needs to be regarded with caution at lower latitudes and/or low altitudes (<1000 m), since degradation processes are weaker.