P.J. Rowe

Rates of tectonic uplift in the Corinth and Megara Basins, central Greece

We present evidence of the rates of late Quaternary tectonic uplift that have affected parts of central Greece during this period of active extension and basin formation. Dual lines of evidence indicate upper Pleistocene to Holocene uplift rates of the order of 0.3 mm yr−1 for the Corinth and Megara basins. First, U-series disequilibrium ages of Acropora sp. corals from uplifted marine sediments are used to derive minimum average rates of vertical displacement since deposition. Second, the geometries of cyclical transgressive sequences and of erosional terraces are consistent with the radiometric evidence and the known history of late Quaternary sea level fluctuations. Two types of uplift are distinguished on the basis of structural relationships: (1) fault block rotation about a horizontal axis (tilting) in response to footwall uplift on the active normal fault bounding the Alkyonides Gulf and (2) a more regional uplift which affects the Corinth Basin and Peloponnesos to the south. The results of U-series dating are discussed in the light of initial 234U/238U activity ratios, derived from the coral samples, which are higher than for average marine waters (1.17–1.31 compared to 1.14). These values may be related to a variable freshwater input to the structurally confined Gulf of Corinth or, alternatively, they may reflect a previously higher global 234U/238U activity ratio in marine waters, or post-mortem enrichment in 234U, although the mechanism of the latter is not yet understood.

Normal faulting and crustal deformation, Alkyonides Gulf and Perachora peninsula, eastern Gulf of Corinth rift, Greece

Geophysical, structural, geochronological and geomorphological data indicate that the Psatha, East Alkyonides, Skinos and Pisia faults are Holocene-active structures whereas the status of the West Alkyonides, Strava, Perachora and Loutraki faults is less certain. We see no evidence for significant lateral surface fault growth. New data for late Pleistocene footwall uplift of the Psatha fault are comparable with previously estimated Holocene rates. Pre-Holocene stratigraphic sequences in the Alkyonides Gulf allow calculation of vertical displacement on the Skinos fault of 1.42–1.60 km over a period of >0.6 Ma. Previous palaeoseismological studies indicate comparable displacement rates extrapolated to 0.61–2.20 Ma, whereas extrapolation of previous geodetic data indicate a range of 0.17–0.46 Ma. The latter is too short given the evidence of the stratigraphic record, signifying either that these data may not be representative of longer-term rates, or that significant deformation has taken place elsewhere, for example, on offshore antithetic faults. A case is established for uniform late Quaternary (post-MIS 7) uplift of the Perachora peninsula at rates of c. 0.2–0.3 mm a−1. The lack of regional tilting over Perachora–Corinth–Isthmia is in marked contrast to the situation in the Alkyonides–Megara basins to the east.

The reliability of uranium-series mollusc dates from the western mediterranean basin

Abstract Fossil mollusc shells from western Mediterranean raised beach deposits have been dated using U-series techniques. Sub-sampling has allowed intra-shell isotopic variability to be assessed. Although most of the calculated ages appear plausible and some internal consistency exists, closer scrutiny of the data reveals that the shells have behaved as geochemically open systems and all the ages therefore must be considered suspect. Some previously published U-series mollusc data from the same region may also contain evidence of open system behaviour and are considered equally unreliable. Simple modelling suggests considerable variation in isotopic uptake behaviour both within and between shells.

Discussion on Pleistocene calcified cyanobacterial mounds, Perachora peninsula, central Greece: a controversy of growth and historyGeological Society, London, Special Publications, Vol. 255, 2006, 53–69

J. E. Andrews, M. R. Leeder, C. Portman, P. J. Rowe & J. Smith write: we were very interested to read Kershaw & Guos (2006) analysis of the conflicting views on the growth and history of Pleistocene cyanobacterial mounds (bioherms) from the Perachora peninsula in the eastern Gulf of Corinth rift, central Greece. A new field guide to this classic geological area and these spectacular exposures will soon be available (Leeder et al . 2007) and the issues we debate below have an important bearing on regional uplift rates, the role of faulting, and the environmental conditions required for cyanobacterial calcification. We welcome Kershaw & Guos (2006) fair and balanced account of competing hypotheses for bioherm evolution, and particularly commend and accept their new interpretation of the cavity-dwelling, pendant coralline algae. However, we dispute the more fundamental basis of their interpretations and take this opportunity to discuss them further.   In the vicinity of Cape Heraion (Perachora peninsula) uplifted Pleistocene shorelines of marine isotope stage (MIS) 5a/c, MIS 5e, MIS 7a/c and MIS 7e age have been mapped (Leeder et al . 2005), and where possible U/Th dated using coral aragonite (Vita-Finzi 1993; Dia et al . 1997; Leeder et al . 2005). The upper part of a prominent terrace, at c . +25 to +30 m above modern sea level, is composed of marine bioclastic marls and carbonates of proven MIS 5e age (Vita-Finzi 1993; Leeder et al . 2005). However, directly below these sediments is a horizon of spectacular bioherms, up to 10 m high (Fig. 1) constructed by calcified cyanobacteria ( Rivularia haematites ) and coralline algae ( Lithophyllum pustulatum ). There is controversy regarding the age, stratigraphic relations and environment of formation of these bioherms. They have variously been interpreted as brackish lake deposits forming immediately before the …