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.