Geomorphic and biological indicators of paleoseismicity and Holocene uplift rate at a coastal normal fault footwall (western Corinth Gulf, Greece)

The westernmost part of the Gulf of Corinth (Greece) is an area of very fast extension and active normal faulting, accompanied by intense coastal uplift and high seismicity. This study presents geomorphic and biological evidence of Holocene coastal uplift at the western extremity of the Gulf, where such evidence was previously unknown. Narrow shore platforms (benches) and rare notches occur mainly on Holocene littoral conglomerates of uplifting small fan deltas. They are perhaps the only primary paleoseismic evidence likely to provide information on earthquake recurrence on coastal faults in the specific part of the Rift, whereas dated marine fauna can provide constraints on the average Holocene coastal uplift rate. The types of geomorphic and biological evidence identified are not ideal, and there are limitations and pitfalls involved in their evaluation. In a first approach, 5 uplifted paleoshorelines may be identified, at 0.4-0.7, 1.0-1.3, 1.4-1.7, 2.0-2.3 and 2.8-3.4 m a.m.s.l., probably forming after 1728 or 2250 Cal. B.P. A most conservative estimate for the average coastal uplift rate during the Late Holocene is 1.6 or 1.9 mm/yr minimum (with different amounts of reservoir correction). Some of the obtained radiocarbon ages of Lithophaga sp. allow for much higher Holocene uplift rates, of the order of 3-4 mm/yr, which cannot be discarded but require further research. That the identified paleoshoreline record corresponds to episodes of coastal uplift only, cannot be demonstrated beyond all doubt. Yet, it appears the most likely interpretation given the geological and active-tectonic context and present knowledge on eustatic sea-level fluctuations in the Mediterranean. Proving that the documented uplifts were abrupt (i.e., arguably coseismic) is equally difficult but rather probable. Five earthquakes in the last ca. 2000 yr on the coastal fault zone that is responsible for the uplift, compare well with historical seismicity and the results of recent on-fault paleoseismological studies at the nearby Eliki fault zone. Exact amounts of coseismic uplift cannot be determined precisely unless the rate of non-seismic uplift of Northern Peloponnesus at the specific part of the Corinth Rift is somehow constrained.