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A single-channel seismic reflection survey of the Argolikos Gulf and Idhra Channel casts light on the tectonic history of this extensional basin of which the current subsidence phase is less than 0.5 Ma old. Although the Gulf has very low seismicity, Holocene subsidence on the western boundary faults increases southward from 50 to 100 cm ka −1 . The eastern margin of the Gulf is offset eastward twice. For these offsets we postulate two left-lateral strike-slip faults accommodating a southward increase in the extension rate. These features can be traced on land where at least one of them shows evidence of left lateral movement. The Idhra Channel has a different tectonic pattern of E–W-trending extensional troughs separated by basement ridges. The latter diverge westward and continue into the southern Argolid as Mesozoic limestone ridges separated by ophiolite-floored tectonic valleys. The causes of this discordant pattern remain obscure.

The relationship between the geotectonic zones of the Hellenides orogen is important for understanding its geological evolution. The Parnassus-Ghiona geotectonic zone in central Greece has long been interpreted as thrust over the Vardoussia subzone. On the basis of detailed geological mapping, supported by a stratigraphic study and tectonic observations, no evidence of an overthrust of the Ghiona limestones on the Vardoussia flysch was found.The new data show that the transition of the Ghiona limestones to the Vardoussia flysch is a sedimentary passage and at certain places the base of the flysch is marked by a basal conglomerate formation. The entire deformation of the Ghiona is related to a large-scale anticline of the limestone series, presenting a highly inclined western limb, overlain by the flysch strata, which were deposited in a common basin between Ghiona and Vardoussia mountains. Consequently, questions concerning the geotectonic position of the Vardoussia subzone and the role of the Parnassus-Ghiona zone to the orogenic evolution of the eastern External Hellenides are raised.

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